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• Hydrocarbons

Unsaturated Hydrocarbons

What are unsaturated hydrocarbons.

Unsaturated hydrocarbons are organic compounds that are entirely made up of carbon and hydrogen atoms and consist of a double or a triple bond between two adjacent carbon atoms. For example, CH 3 CH 2 CH=CH 2 (n-Butylene) & CH 3 CH=CH 2 (Propylene) The difference in the chemical formulae of saturated and unsaturated hydrocarbons is illustrated below.

In the IUPAC nomenclature of unsaturated hydrocarbons, the position of the double or triple bond is either described by a number written before the name of the compound (as in 2,4 pentadiene) or by a number written before the suffix, ‘-ene’ or ‘-yne’ (as in pent-2-ene).

The physical properties of saturated hydrocarbons and unsaturated hydrocarbons are quite similar. These types of hydrocarbons (except aromatic hydrocarbons) are quite reactive and tend to undergo addition reactions with elemental halogens, hydrogen halides, alcohols, and many other compounds.

Table of Contents

Types of unsaturated hydrocarbons, uses of unsaturated hydrocarbons.

Unsaturated hydrocarbons , based on the types of bonds they contain, can be classified into alkenes, alkynes, and aromatic hydrocarbons. The different types of organic compounds that can be classified as unsaturated hydrocarbons are briefly discussed below.

• The hydrocarbons that contain at least one double bond between two adjacent carbon atoms are called alkenes or olefins. The simplest alkene is ethylene (or ethene), given by the chemical formula C 2 H 4 .
• Alkenes containing only one double bond and having no functional groups or substituents attached to them can be generalized to the chemical formula C n H 2n .
• Hydrocarbons containing a minimum of one triple bond between two carbon atoms that are positioned adjacent to each other are referred to as alkynes . The alkyne with the simplest structure is acetylene (systematic IUPAC name: ethyne) with the chemical formula C 2 H 2 .
• The alkynes that have only one carbon-carbon triple bond and have no functional or substituent groups attached to them can be generalized to the chemical formula C n H 2n-2
• Cyclic hydrocarbons that contain at least one double or triple bond between two carbon atoms are also considered to be unsaturated hydrocarbons, one such example being cyclopentene (C 5 H 8 ).
• Although Aromatic Hydrocarbons (ring-shaped hydrocarbons containing delocalised pi electrons) can be considered unsaturated hydrocarbons, but they are generally referred to as aromatic compounds because they are relatively stable and do not share similar properties with other such unsaturated compounds.

In order to check whether a given hydrocarbon is unsaturated, bromine water can be added to it. Should the bromine water become decolourised, the hydrocarbon in question is unsaturated. If a white precipitate is formed, the hydrocarbon sample is phenol or aniline. It can be noted that benzene does not decolourise the bromine water.

Some uses of compounds belonging to the unsaturated hydrocarbon category are listed below.

• Many fruits can be artificially ripened with the help of alkenes.
• Mustard gas, a poisonous gas used in chemical warfare, can be created with the help of alkenes.
• Unsaturated hydrocarbons are extremely useful organic compounds in the manufacturing of plastics.
• LDPE, which is a low-density variation of polyethylene, is used in the manufacturing of grocery bags
• Polystyrene is used in making egg cartons, disposable cups, and other convenient products.
• Industrial chemicals such as alcohol include the usage of alkenes in their manufacturing process.
• Some unsaturated hydrocarbons are used as general anaesthetics.
• Many organic compounds of high industrial importance are manufactured with the help of alkynes.

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Saturated and unsaturated compounds.

Thus, the general properties, types, and uses of unsaturated hydrocarbons are briefly discussed in this article. For more information on unsaturated hydrocarbons and other types of hydrocarbons, such as aromatic hydrocarbons , register with BYJU’S and download the mobile application on your smartphone.

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Chapter 13 Unsaturated and Aromatic Hydrocarbons

Opening essay.

Our modern society is based to a large degree on the chemicals we discuss in this chapter. Most are made from petroleum. In Chapter 12 "Organic Chemistry: Alkanes and Halogenated Hydrocarbons" we noted that alkanes—saturated hydrocarbons—have relatively few important chemical properties other than that they undergo combustion and react with halogens. Unsaturated hydrocarbons—hydrocarbons with double or triple bonds—on the other hand, are quite reactive. In fact, they serve as building blocks for many familiar plastics—polyethylene, vinyl plastics, acrylics—and other important synthetic materials (e.g., alcohols, antifreeze, and detergents). Aromatic hydrocarbons have formulas that can be drawn as cyclic alkenes, making them appear unsaturated, but their structure and properties are generally quite different, so they are not considered to be alkenes. Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics (e.g., polyesters and polystyrene).

The two simplest unsaturated compounds—ethylene (ethene) and acetylene (ethyne)—were once used as anesthetics and were introduced to the medical field in 1924. However, it was discovered that acetylene forms explosive mixtures with air, so its medical use was abandoned in 1925. Ethylene was thought to be safer, but it too was implicated in numerous lethal fires and explosions during anesthesia. Even so, it remained an important anesthetic into the 1960s, when it was replaced by nonflammable anesthetics such as halothane (CHBrClCF 3 ).

Saturated vs Unsaturated Hydrocarbons- 10 Key Differences

Table of Contents

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Saturated Hydrocarbons Definition

Saturated hydrocarbons are the simplest forms of hydrocarbons consisting entirely of single bonds that remain saturated with hydrogen atoms.

• The general formula for acyclic saturated hydrocarbons or alkanes is C n H 2n+2 . The more general formula can also be written as C n H 2n+2(1-r) , where r is the number of rings.
• Saturated hydrocarbons do not have any double or triple bonds between the carbon atoms in the structure, and all carbon atoms make four distinct covalent bonds.
• The term saturated indicates the saturation of hydrogen atoms in the structure which also makes them the simples and least polar organic compounds.
• Saturated hydrocarbons can occur either in a linear form or a branched form, depending on the complexity of the structure.
• These hydrocarbons primarily occur in petroleum products as well as other forms of fossil fuels.
• Saturated hydrocarbons burn with a blue, non-sooty flame which is why these are used as sources of fuel in vehicles and other engines.
• Substitution reaction is the characteristic property of saturated hydrocarbons as they resist other reactions like hydrogenation and oxidative addition.
• Saturated hydrocarbons have a lower concentration of carbon atoms as the number of hydrogen atoms is relatively high as compared to unsaturated hydrocarbons.
• These are also more stable and less reactive and can resist attacks by nucleophiles and electrophiles.
• Saturated hydrocarbons consist of two groups of hydrocarbon; acyclic alkanes and cycloalkanes.
• Some examples of saturated hydrocarbons include methane, butane, propane, cyclohexane, etc.

Unsaturated Hydrocarbons Definition

Unsaturated hydrocarbons are the group of hydrocarbons composed of double or triple bonds between the carbon atoms.

• Unsaturated hydrocarbons with double bonds are called alkene, which has the general formula, C n H 2n , whereas those with triple bonds are called alkynes which have the general formula, C n H 2n-2 .
• The term unsaturated indicates that more hydrogen atoms can be added to the molecules to make them saturated.
• Like saturated hydrocarbons, unsaturated hydrocarbon can also exist in straight-chain linear form as well as branched and aromatic forms.
• All aromatic hydrocarbons are unsaturated hydrocarbons as the aromatic rings are formed by the delocalization of double bonds between multiple carbon atoms.
• Unsaturated hydrocarbons are comparatively more reactive, except aromatic compounds, and undergo multiple addition reactions on multiple bonds.
• Unsaturation of hydrocarbons can be expressed in terms of the degree of unsaturated, which is the measure of the number of π -bonds present in a molecule.
• Unsaturated hydrocarbons have a higher carbon concentration than saturated hydrocarbons as the number of hydrogen atoms is less.
• The combustion of unsaturated hydrocarbons produces a yellow flame as these have a lesser hydrogen content which, in turn, decreases the moisture of the flame.
• Unsaturated hydrocarbons, like most hydrocarbons, are non-polar, and thus, the structures are held together by weak van der Waal’s force of attraction.
• The nomenclature of unsaturated hydrocarbons is different from saturated hydrocarbons as these require the addition of numbers between the prefix to indicate the position of the double or triple bonds.
• Some examples of unsaturated hydrocarbons include ethane, acetylene, benzene, butadiene, etc. 

10 Major Differences (Saturated vs Unsaturated Hydrocarbons)

Examples of Saturated Hydrocarbons

• Methane is the simplest aliphatic hydrocarbon consisting of a single carbon atom singly bonded to four hydrogen atoms.
• It is a saturated hydrocarbon with all four covalent bonds linked to four distinct hydrogen atoms with the molecular formula CH 4 .
• Methane is a simple hydrocarbon that burns with a pale, slightly luminous blue flame. It is quite stable and less flammable than other similar compounds.
• All the four carbon- hydrogen bonds in methane are equivalent to the bond length of 1.09×10 -10 and the bond angle of 109.5°. It is a tetrahedral molecule with three sp 3 hybridized orbitals. 
• Methane is an important fossil fuel commonly used as a form of biogas for cooking. It is also an important member of the greenhouse gases and is considered one of the prime gases involved in global warming.

Examples of Unsaturated Hydrocarbons

• Acetylene or ethyne is a two-carbon unsaturated hydrocarbon with the molecular formula C 2 H 2 .
• It is an alkyne with a triple bond between the carbon atoms and a single covalent bond between each carbon and hydrogen atom.
• The carbon-carbon triple bond results in a linear structure of the molecule as it places all four atoms in the same straight line with a bond angle of 180°.
• One of the triple bonds is a σ -bond, whereas the two other bonds are weaker π -bonds.
• About 20% of the total acetylene occurring in the environment is the result of oxyacetylene gas welding and cutting.
• The carbon-carbon triple bond in acetylene has energy richness which enables it to be used as a suitable substrate for bacteria.
• Gautam AD, Pant M and Adhikari NR (2017). Comprehensive Chemistry Part 2. Heritage Publishers and Distributors. Kathmandu, Nepal.
• National Center for Biotechnology Information. “PubChem Compound Summary for CID 297, Methane”  PubChem ,  https://pubchem.ncbi.nlm.nih.gov/compound/Methane . Accessed 21 February, 2021.
• National Center for Biotechnology Information. PubChem Compound Summary for CID 6326, Acetylene.  https://pubchem.ncbi.nlm.nih.gov/compound/Acetylene . Accessed Apr. 8, 2021.  
• https://pediaa.com/difference-between-saturated-and-unsaturated-hydrocarbons/

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• Unsaturated Hydrocarbons

Discover the intriguing world of unsaturated hydrocarbons through this comprehensive guide. Dive into a detailed explanation of what unsaturated hydrocarbons are and their unique attributes. Explore various types and examples, uncovering the diverse range and classifications of unsaturated hydrocarbons found in our world. You will then gain insight into the practical uses and applications of these substances in both everyday life and industrial contexts. Finally, learn about the vital chemical tests used to analyse these compounds and see how they compare with their saturated counterparts. This is a scientific journey not to be missed by those keen to deepen their chemistry knowledge.

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What is the significance of the term 'unsaturated' in unsaturated hydrocarbons?

How does the food industry utilise unsaturated hydrocarbons, particularly alkenes?

What difference can the bromine water test detect between saturated and unsaturated hydrocarbons?

What are the three common types of unsaturated hydrocarbons?

What is the key differentiation between saturated and unsaturated hydrocarbons?

What is the process of hydrogenation in the context of hydrocarbons?

Which unsaturated hydrocarbon is extensively used in the plastic production industry?

What are unsaturated hydrocarbons?

What are the two primary types of unsaturated hydrocarbons?

What distinctive features does benzene have as an aromatic hydrocarbon?

What role do unsaturated hydrocarbons, such as alkenes and aromatic hydrocarbons, play in vehicle fuels?

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Understanding Unsaturated Hydrocarbons

Unsaturated hydrocarbons are a significant topic that you ought to understand if you wish to excel in your Chemistry endeavours. Their presence is everywhere, from the fuel you use to power your automobiles to the plastic products that are integral parts of your daily life.

Defining Unsaturated Hydrocarbons: The Unsaturated Hydrocarbons Meaning

So, what exactly are Unsaturated Hydrocarbons?

An Unsaturated Hydrocarbon is a type of hydrocarbon that contains double or triple carbon-carbon bonds. This implies there are fewer hydrogen atoms attached to the carbon chain compared to saturated hydrocarbons.

Interestingly, the term 'Unsaturated' in Unsaturated Hydrocarbons indicates that if additional hydrogen is available, they can react to form a new compound, effectively becoming 'saturated'.

Unpacking the main components of Unsaturated Hydrocarbons

Unsaturated Hydrocarbons are generally composed of Carbon and Hydrogen atoms. However, it's the arrangement of these atoms, specifically the double and triple bonds between carbon atoms, that grant them their unique properties and reactivity. Here, we'll break down their structure into their fundamental components.

Carbon-Carbon Double Bonds: Alkenes are Unsaturated Hydrocarbons that contain at least one carbon-carbon double bond. This bond is denoted by the symbol '=' in chemical structures. For example, Ethene (\(C_2H_4\)) is the simplest alkene.

Carbon-Carbon Triple Bonds: Alkynes, another type of Unsaturated Hydrocarbons, contain at least one carbon-carbon triple bond. This bond is denoted by the symbol '≡' in chemical structures. An example of an alkyne is Ethyne (\(C_2H_2\)), commonly known as acetylene.

Now that you understand the basic components of Unsaturated Hydrocarbons, further study would lead to understanding their reactions, how they are represented in structural formulas, and their uses in various industries. This foundation will also help make later topics like isomers, polymerization, and substitutions easier for you to grasp.

Look around you, and you'll notice Unsaturated Hydrocarbons are all around. For instance, the ethylene gas (\(C_2H_4\)) used to hasten the ripening of fruits is an Alkene, an Unsaturated Hydrocarbon.

Different Types of Unsaturated Hydrocarbons: Examples

In the world of chemistry, unsaturated hydrocarbons are distinguished by double or triple bonds between the carbon atoms. The presence of these extra bonds imbues unsaturated hydrocarbons with their characteristic reactivity. Within this collection, there are three common types, namely, alkenes, alkynes, and aromatic hydrocarbons. Each presents unique properties and applications.

Exploration of diverse Unsaturated Hydrocarbons Examples

Unsaturated hydrocarbons play pivotal roles in many industries, as they are crucial for a myriad of reactions and processes. These hydrocarbons mainly fall into the categories of alkenes, alkynes, and aromatic hydrocarbons. Let's explore these in more detail.

Alkenes: These are a type of unsaturated hydrocarbon where carbon atoms are connected via at least one double bond. This bestows upon them a greater degree of reactivity compared to their saturated counterparts. Some everyday examples of alkenes include:

• Ethene (\(C_2H_4\)) – It's extensively used in the plastic production industry.
• Propene (\(C_3H_6\)) – It undergoes polymerisation to create polypropylene, a common type of plastic.

Alkynes: These unsaturated hydrocarbons stand out due to the presence of a triple bond between carbon atoms. The high energy associated with a triple bond conveys a high level of reactivity. Alkynes include compounds like:

• Ethyne (\(C_2H_2\)) – Famously known as acetylene, it's used for welding and metal cutting due to its ability to produce a very hot flame.
• Propyne (\(C_3H_4\)) – Less common than acetylene but used in organic synthesis.

Aromatic Hydrocarbons: Present as cyclic compounds with alternating double and single carbon-carbon bonds. Their stability and aromaticity make them a fascinating study. Examples include:

• Benzene (\(C_6H_6\)) – It's a starting material in the production of a vast number of chemicals including plastics, resins, synthetic fibres , rubber, dyes, detergents, pharmaceuticals, and explosives.
• Toluene (\(C_7H_8\)) – It serves as a solvent and a precursor to other chemical compounds.

Classification of Unsaturated Hydrocarbons

Differentiating between the various types of unsaturated hydrocarbons is essential to comprehending their unique properties and applications. Here, the classification is largely based on the number and nature of carbon-carbon bonds.

The simplest form of classification breaks down unsaturated hydrocarbons into the three following types:

• Alkenes: Contains at least one carbon-carbon double bond, denoted by a '=', in their molecular structure.
• Alkynes: Contains at least one carbon-carbon triple bond, represented by '≡' in chemical formulas.
• Aromatic Hydrocarbons: Formed by cyclic arrangements of carbon atoms, alternating between single and double bonds. This arrangement generates a distinctive ring-like pattern, granting them ' aromaticity ' or unique stability.

To fully appreciate the roles of these unsaturated hydrocarbons, you must delve into their chemical reactions and interactions, their physical and chemical properties, and, importantly, their significant industrial applications. Mastering these intricate details will undoubtedly make other areas of chemistry, such as organic synthesis, polymer chemistry, and even biochemistry, much easier to comprehend.

Practical Uses: Unsaturated Hydrocarbons Applications

Unsaturated hydrocarbons, owing to their reactivity and unique properties, find extensive use across diverse sectors. From the plastics sitting on your desk to the fuels that power your vehicle, you interact with products derived from these chemical compounds daily. Nonetheless, the study of unsaturated hydrocarbons isn't merely confined to academic intrigue — their practical applications are sweeping and transcend diverse sectors.

How we utilise Unsaturated Hydrocarbons: Applications in everyday life

Given the significance of unsaturated hydrocarbons, a broader understanding of their applications enriches your comprehension of how chemistry links with our everyday lives. Here, the emphasis will be on the practical applications of unsaturated hydrocarbons, demonstrating that their realm spans beyond textbooks and laboratory experiments.

For starters, consider the fuel being consumed by vehicles. Unsaturated hydrocarbons, especially aromatic hydrocarbons, play a crucial role as constituents of petrol and diesel. Some common examples include toulene and xylene . They are essential due to their energy content and the fact that they burn cleaner than their counterparts, contributing to fewer harmful emissions.

Unsaturated hydrocarbons also form the backbone of the plastic industry. The versatility of these compounds allows them to be manipulated into various forms, making them the ideal raw material for plastic production. For instance, polyethylene and polypropylene , two types of plastics ubiquitous in everyday life, originate from alkenes — a type of unsaturated hydrocarbon.

Furthermore, these hydrocarbons play an indispensable role in the synthesis of many industrial chemicals. Alkenes such as ethene and propene, for instance, serve as feedstock in the synthesis of a variety of chemicals including alcohols , alkyl halides , and detergents , thanks to their reactivity and accessibility.

Lastly, unsaturated hydrocarbons, particularly alkenes, feature prominently in the food industry. Ripening of fruits is stimulated by ethene (also known as ethylene ), a gas produced by some fruit species. The commercial cultivation of fruits exploits this characteristic, employing synthesized ethylene to hasten the ripening process, ensuring that fruits reach the market at their peak.

Industrial applications of Unsaturated Hydrocarbons

Delving further into the industrial domain, it becomes evident that unsaturated hydrocarbons are pivotal to manifold operations. Their intrinsic reactivity and ease of manipulation make them indispensable to a host of chemical processes.

The role of unsaturated hydrocarbons in the production of plastics is an example of their industrial significance. Consider, for example, polyvinyl chloride (PVC), a durable plastic derived from ethene (an alkene). PVC is widely used in the construction industry for products such as pipes, window frames, and roofing sheets due to its resistance to environmental elements.

Other alkenes, like propene, act as the foundation for the production of polypropylene , another common plastic. Its high tensile strength and resistance to chemical degradation make it suitable for a variety of applications, ranging from packaging materials to automotive parts.

In the realm of energy production, unsaturated hydrocarbons, particularly aromatic hydrocarbons like benzene, toluene, and xylene, are essential components of petrol . The high calorific values of these hydrocarbons make them ideal for use as fuel, emitting significant amounts of energy when burned.

Further, the reactivity of unsaturated hydrocarbons becomes a boon in the realm of chemical synthesis . Whether in creating dyes, detergents, or synthetic fibres , their use as feedstock is prolific.

Ultimately, the industrial applications of unsaturated hydrocarbons are as diverse as the compounds themselves. Whether it's serving as the backbone for polymeric materials, fuelling vehicles, or acting as a feedstock for countless industrial processes, their practical significance is vast and truly remarkable.

Analysing Unsaturated Hydrocarbons: The Chemical Test

A critical element in chemistry is confirming the identity of compounds, and unsaturated hydrocarbons are no exception. Since these molecules have double or triple bonds, they exhibit certain behaviour under specific conditions. As such, there are dedicated chemical tests to confirm the presence of unsaturated hydrocarbons in a compound, chiefly among these, the bromine water test.

Conducting a Chemical Test for Unsaturated Hydrocarbons

Unsaturated hydrocarbons, due to their unique features, undergo certain reactions that provide reliable confirmation of their existence. These tests play crucial roles in laboratories, industrial setups, and educational institutions where the identification of compounds is necessary.

An effective technique to detect the presence of unsaturated hydrocarbons is the bromine water test, also known as the bromine test. The premise of this test hinges on the reaction between bromine and unsaturated hydrocarbons. Unsaturated hydrocarbons, upon reacting with bromine water, will add bromine across their multiple bonds, thus decolourising bromine water's original brown colour.

To carry out this test, you introduce bromine water to the sample suspected of containing unsaturated hydrocarbons. On adding the bromine water – which is characteristically brown due to the presence of elemental bromine (Br 2 ) – the liquid should lose its colour if unsaturated hydrocarbons are present. This is because the double or triple bonds characteristic of unsaturated hydrocarbons will break and react with the bromine atoms, resulting in a halogenated compound and water.

The chemical equation for this reaction is:

\[ RCH=CH2 + Br2 -> RCHBr-CH2Br \]

Where \(RCH=CH2\) represents an alkene (an unsaturated hydrocarbon), \(Br2\) is bromine, and \(RCHBr-CH2Br\) is the halogenated compound formed.

In comparison, saturated hydrocarbons, which do not have these reactive sites (double or triple bonds), will not react with bromine water, and thus the brown colour of the bromine water will remain unchanged.

The bromine test is a striking example of a chemical test for unsaturated hydrocarbons due to its visual impact. The clear change – or lack thereof – delivers a powerful, understandable message even to individuals who might not have extensive chemical knowledge.

Understanding the outcomes of the chemical test for Unsaturated Hydrocarbons

Performing the bromine test presents two main outcomes that provide insight into the compound's nature. However, understanding the principals behind these results is fundamental for interpreting and applying this knowledge accurately.

If the sample contains unsaturated hydrocarbons, the bromine water will lose its brown colour upon addition of the sample. This occurs because the bromine atoms add across the double bond of the unsaturated hydrocarbon, breaking the multiple bonds, and forming a halogenated compound. As a result, the bromine molecules in the water diminish, thus eliminating the brown colour they lend to the water. The disappearance of the brown colour is evidence of the presence of unsaturated hydrocarbons in the mixture.

Alternatively, if the sample comprises saturated hydrocarbons, then the bromine water will retain its brown colour even after the addition of the sample. The absence of double or triple bonds in these compounds means they do not react with the bromine, leaving the bromine concentration in the water unchanged, and thereby maintaining its brown colour. This outcome, i.e., the persistent brown colour, signifies the absence of unsaturated hydrocarbons in the mixture.

While easy to conduct and interpret, it's necessary to consider the bromine test's limitations. This test does not differentiate between alkenes (double bonds) and alkynes (triple bonds). Whether the unsaturated hydrocarbon is an alkene or an alkyne , the bromine will add across the multiple bonds producing a decolourised solution. Consequently, additional tests might be necessary to further characterise the compound.

A clear understanding of these outcomes coupled with thorough knowledge of the reasons behind these results, enables a better comprehension of unsaturated hydrocarbons, their differentiated features, and how, through simple tests, science allows us to dissect, determine, and deliver precise information about the seemingly complex world of chemistry.

In summary, the decolourisation of bromine water indicates the presence of unsaturated hydrocarbons, whereas if the colour remains unchanged, unsaturated hydrocarbons are likely absent. The results of this chemical test afford valuable insights into the molecular structure of the studied compounds, and the ability to distinguish between saturated and unsaturated hydrocarbons forms one of the cornerstones of organic chemistry.

Comparing Saturated and Unsaturated Hydrocarbons

Hydrocarbons, the organic compounds made up of hydrogen and carbon atoms, can be broadly classified into two categories: saturated and unsaturated hydrocarbons. Each comes with characteristic properties that determine their chemical behaviour, reactivity, and their role in various practical applications.

Highlighting the Difference Between Saturated and Unsaturated Hydrocarbons

When comparing saturated and unsaturated hydrocarbons , the differentiation primarily revolves around their structure and the type of bonding between the carbon atoms.

Saturated hydrocarbons, also known as alkanes, contain single bonds between carbon atoms and have the maximum number of hydrogen atoms possible. The formula for an alkane is \(C_nH_{2n+2}\), where \(n\) defines the number of carbon atoms.

On the other hand, unsaturated hydrocarbons can be alkenes, which carry at least one carbon-carbon double bond (\(C_nH_{2n}\)) or alkynes, which have at least one carbon-carbon triple bond (\(C_nH_{2n-2}\)). These compounds have fewer than the maximum possible number of hydrogen atoms, hence the term 'unsaturated'. They are more reactive than their saturated counterparts due to the presence of double or triple bonds.

Here is a concise comparison focusing on their core differences:

• Saturated hydrocarbons contain single C-C bonds, whereas unsaturated hydrocarbons carry double or triple C-C bonds.
• Saturated hydrocarbons maximum number of hydrogen atoms; unsaturated hydrocarbons, carrying fewer hydrogen atoms, do not.
• Saturated hydrocarbons are generally less reactive, while unsaturated hydrocarbons are more reactive due to the presence of multiple bonds.

These differing structures and characteristics of saturated and unsaturated hydrocarbons mean they have different chemical reactivity and thus, different roles in practical applications like fuel production, polymers, and more.

The presence of multiple bonds in unsaturated hydrocarbons alters their reactivity. This is because the pi electrons (electrons in a double bond) are readily available and thus, more susceptible to attack by electrophiles. In contrast, the single bonds in saturated hydrocarbons contain sigma electrons, which are not as easily accessible and therefore make saturated compounds less reactive.

How the Hydrogenation of Unsaturated Hydrocarbons bridges the gap

Understanding the strong contrast between saturated and unsaturated hydrocarbons leads us naturally to the concept of hydrogenation, a process that effectively 'bridges the gap' by converting unsaturated hydrocarbons into saturated ones.

Hydrogenation is a chemical reaction that adds hydrogen (H2) to an unsaturated hydrocarbon, breaking the double or triple carbon bonds and converting it into a saturated hydrocarbon. It's carried out in the presence of a catalyst, often palladium, platinum or nickel, and under specific heat/pressure conditions.

This reaction can be represented generally as follows:

\[ RCH=CH2 + H2 -> RCH2-CH3 \]

This equation illustrates that an alkene (an unsaturated hydrocarbon, herein \(RCH=CH2\)) reacts with hydrogen (H2), under certain conditions, to produce an alkane (a saturated hydrocarbon, herein \(RCH2-CH3\)) which has no multiple bonds and has the maximum number of hydrogen atoms.

The hydrogenation process is widely applied in the industrial production of substances, such as margarine from vegetable oil, or the conversion of alkenes to alkanes in petroleum refining. The fact the unsaturated hydrocarbons can undergo hydrogenation and essentially turn into saturated hydrocarbons is significant, as it allows the reactivity of these compounds to be manipulated according to requirements.

The hydrogenation process offers evidence that although saturated and unsaturated hydrocarbons differ remarkably, they can be inter-converted via chemical reactions, thus establishing their intrinsic link within the fascinating sphere of hydrocarbon chemistry.

Unsaturated Hydrocarbons - Key takeaways

• Unsaturated Hydrocarbons Meaning: These are hydrocarbons that contain double or triple bonds between carbon atoms, including alkenes, alkynes, and aromatic hydrocarbons.
• Examples of Unsaturated Hydrocarbons: Ethene and propene (Alkenes), Ethyne and propyne (Alkynes), Benzene and Toluene (Aromatic Hydrocarbons).
• Unsaturated Hydrocarbons Applications: Wide-ranging applications in various sectors, including fuel (petrol and diesel), plastic production, synthesis of industrial chemicals, and in the food industry for ripening fruits.
• Chemical Test for Unsaturated Hydrocarbons: The bromine water test is commonly used. In this test, bromine water decolorizes when mixed with a sample containing unsaturated hydrocarbons due to the addition of bromine across the multiple bonds of the hydrocarbons.
• Difference between Saturated and Unsaturated Hydrocarbons: Saturated hydrocarbons have single bonds between carbon atoms, while unsaturated hydrocarbons have double or triple bonds. This gives unsaturated hydrocarbons a greater degree of reactivity and applications in various industries.

Flashcards in Unsaturated Hydrocarbons 249

The term 'unsaturated' indicates that these hydrocarbons can react with additional hydrogen to form a new compound, effectively becoming 'saturated'.

The food industry employs unsaturated hydrocarbons, notably alkenes like ethylene, to stimulate fruit ripening. The synthesis of ethylene is used commercially to hasten the ripening process, ensuring fruits reach markets at their peak.

The bromine water test can distinguish between saturated and unsaturated hydrocarbons by the change in colour. Unsaturated hydrocarbons decolourise bromine water, whereas saturated ones leave the colour unchanged.

The three common types of unsaturated hydrocarbons are alkenes, alkynes, and aromatic hydrocarbons.

Saturated hydrocarbons, also known as alkanes, contain single bonds between carbon atoms and have the maximum number of hydrogen atoms. In contrast, unsaturated hydrocarbons either possess double or triple bonds between carbon atoms and have fewer than the maximum possible number of hydrogen atoms.

Hydrogenation is a chemical reaction that adds hydrogen to an unsaturated hydrocarbon, breaking the double or triple carbon bonds and converting it into a saturated hydrocarbon. This reaction is usually catalysed by palladium, platinum or nickel and under specific heat/pressure conditions.

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Home » Science » Chemistry » Organic Chemistry » Difference Between Saturated and Unsaturated Hydrocarbons

Difference Between Saturated and Unsaturated Hydrocarbons

Main difference – saturated vs unsaturated hydrocarbons.

Hydrocarbons are organic compounds that contain only carbon and hydrogen atoms. Hydrocarbons are considered as parent compounds of many organic compounds. The main chain of hydrocarbons is made by the C-C bonds, and hydrogen atoms are attached to carbon atoms in the main chain. Based on the presence of single or multiple bonds between carbon atoms, hydrocarbons are classified into two groups namely; saturated hydrocarbons and unsaturated hydrocarbons. The main difference between saturated and unsaturated hydrocarbon is that saturated hydrocarbons contain only single covalent bonds between carbon atoms, whereas unsaturated hydrocarbons contain at least one double or triple covalent bond in the main chain . Saturated and unsaturated hydrocarbons show different characteristics because of these structural differences.

Key Areas Covered

1. What are Saturated Hydrocarbons       – Definition, Structure, Types, Properties 2. What are Unsaturated Hydrocarbons       – Definition, Structure, Types, Properties 3. What is the difference between Saturated and Unsaturated Hydrocarbons       – Comparison of Key Differences

Key Terms: Hydrocarbons, Saturated Hydrocarbons, Unsaturated Hydrocarbons, Covalent Bonds, Alkanes, Alkenes, Alkynes, Aromatic Hydrocarbons

What are Saturated Hydrocarbons

Hydrocarbons in which all carbon atoms are bonded to other atoms by single covalent bonds are called saturated hydrocarbons. Thus, saturated hydrocarbons do not contain any multiple bonds, including double or triple covalent bonds. In these compounds, each carbon atom is bonded directly to four other atoms. Hence, all carbon atoms are fully occupied by making four bonds. This is why these compounds are called saturated hydrocarbons. Saturated hydrocarbons are the simplest and the least polar organic natural products. Examples of saturated hydrocarbons include alkanes and cycloalkane families of hydrocarbons.

The simplest form of saturated hydrocarbons includes methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ) etc. Saturated hydrocarbons burn and give a blue, non-sooty flame in air. Because of the flammability of saturated hydrocarbons that ultimately release a lot of energy, saturated hydrocarbons are often used as a fuel source of vehicle and airplane engines. Well known LPG or cooking gas is also a saturated hydrocarbon called butane (C 4 H 10 ).  The combustion of alkanes with air will result in carbon dioxide gas, water vapor, heat, and light. Hydrocarbons are usually obtained from fossilized plant and animal matter. Once they are obtained as crude oil , the process called distillation is used to separate various products according to their mass. This whole process is called refining of crude oil.

Figure 1: Ethane

What are Unsaturated Hydrocarbons

Unsaturated hydrocarbons are the hydrocarbons that contain at least one carbon-carbon double or triple bond in their carbon chain or ring. These compounds have similar physical properties to those of saturated hydrocarbons. However, their chemical properties are much different from saturated hydrocarbons mainly due to the presence of multiple bonds. Usually, chemical reactions initiate from locations where multiple bonds are present in the carbon chain. Hence, the reactivity of unsaturated hydrocarbon increases with a number of multiple bonds present in the main chain.

Types of Unsaturated Hydrocarbons

There are three types of unsaturated hydrocarbons, namely; (a) alkenes , which contain one or more double bond (C=C), (b) alkynes , which contains one or more triple bonds (C≡C), and (c) aromatic hydrocarbons , which consist of a delocalized bonding resulting in a six-membered carbon ring. Examples of alkenes include ethene, propene, butene, etc. Acetylene, propyne, butyne are some examples for alkynes. Benzene, toluene, aniline are some common examples of aromatic hydrocarbons. The simplest form of unsaturated hydrocarbon is ethylene, which is important as a plant hormone that triggers the ripening of fruits.

Figure 2: Some Alkynes

Saturated Hydrocarbons:   Saturated hydrocarbons are hydrocarbons with only single covalent bonds.

Unsaturated Hydrocarbons: Unsaturated hydrocarbons are hydrocarbons with multiple covalent bonds (double and triple bonds).

Saturated Hydrocarbons: Alkanes are saturated hydrocarbons.

Unsaturated Hydrocarbons:  Alkenes, alkynes, and aromatic hydrocarbons are types of unsaturated hydrocarbons.

Reactivity 

Saturated Hydrocarbons: Saturated hydrocarbons are less reactive.

Unsaturated Hydrocarbons: Unsaturated hydrocarbons are more reactive.

Burn in air

Saturated Hydrocarbons:  Burning saturated hydrocarbons result in a blue, non-sooty flame.

Unsaturated Hydrocarbons: Burning unsaturated hydrocarbons result in a yellow, sooty flame.

Amount of Carbon and Hydrogen 

Saturated Hydrocarbons: Saturated hydrocarbons have a less amount of carbon and high amount of hydrogen.

Unsaturated Hydrocarbons: Unsaturated hydrocarbons have a high amount of carbon and less amount of hydrogen

Saturated Hydrocarbons:  These are usually obtained from fossilized plant and animal materials.

Unsaturated Hydrocarbons:   These are mainly obtained from plants (plant pigments, waxes, proteins, vegetable oils etc.)

The difference between saturated and unsaturated hydrocarbons depends on the types of bonds they contain.  Saturated hydrocarbons contain only single covalent bonds whereas unsaturated hydrocarbons contain at least one or more double or triple carbon-carbon bond. Hence, unsaturated hydrocarbons are more reactive than saturated hydrocarbons. Saturated hydrocarbons include the alkanes, whereas unsaturated hydrocarbons include alkene, alkynes and aromatic hydrocarbons.

References: 1. Cseke, Leland J., Ara Kirakosyan, Peter B. Kaufman, Sara Warber, James A. Duke, and Harry L. Brielmann. Natural products from plants. Boca Raton, FL: CRC Press, 2006. Print 2. Singh, Lakmir, and Manjit Kaur. Science for Tenth Class Part 2 Physics. N.p.: S. Chand, 2016. Print. 3. Stoker, H. Stephen. General, organic, and biological chemistry. 6th ed. N.p.: Cengage Learning, 2012. Print.

Image Courtesy: 1. “Ethane-2D” (Public Domain) via Commons Wikimedia 2. “Alkyne General Formulae V” By Jü – Own work (CC0) via Commons Wikimedia

About the Author: Yashoda

Yashoda has been a freelance writer in the field of biology for about four years. He is an expert in conducting research related to polymer chemistry and nano-technology. He holds a B.Sc. (Hons) degree in Applied Science and a Master of Science degree in Industrial Chemistry.

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Module 20: Organic Chemistry

Hydrocarbons, learning outcomes.

• Explain the importance of hydrocarbons and the reason for their diversity
• Name saturated and unsaturated hydrocarbons, and molecules derived from them
• Describe the reactions characteristic of saturated and unsaturated hydrocarbons
• Identify structural and geometric isomers of hydrocarbons

The largest database [1] of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. The number of potential organic compounds has been estimated [2]  at 10 60 —an astronomically high number. The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities.

The simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. Even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. In addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. Many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have been prepared in the laboratory. We use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. The familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. We can distinguish several types of hydrocarbons by differences in the bonding between carbon atoms. This leads to differences in geometries and in the hybridization of the carbon orbitals.

Alkanes , or saturated hydrocarbons ,  contain only single covalent bonds between carbon atoms. Each of the carbon atoms in an alkane has sp 3 hybrid orbitals and is bonded to four other atoms, each of which is either carbon or hydrogen. The Lewis structures and models of methane, ethane, and pentane are illustrated in Figure 1. Carbon chains are usually drawn as straight lines in Lewis structures, but one has to remember that Lewis structures are not intended to indicate the geometry of molecules. Notice that the carbon atoms in the structural models (the ball-and-stick and space-filling models) of the pentane molecule do not lie in a straight line. Because of the sp 3 hybridization, the bond angles in carbon chains are close to 109.5°, giving such chains in an alkane a zigzag shape.

The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas ). Instead of the usual format for chemical formulas in which each element symbol appears just once, a condensed formula is written to suggest the bonding in the molecule. These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. Condensed structural formulas for ethane and pentane are shown at the bottom of Figure 1, and several additional examples are provided in the exercises at the end of this chapter.

Figure 1. Pictured are the Lewis structures, ball-and-stick models, and space-filling models for molecules of methane, ethane, and pentane.

A common method used by organic chemists to simplify the drawings of larger molecules is to use a skeletal structure (also called a line-angle structure). In this type of structure, carbon atoms are not symbolized with a C, but represented by each end of a line or bend in a line. Hydrogen atoms are not drawn if they are attached to a carbon. Other atoms besides carbon and hydrogen are represented by their elemental symbols. Figure 2 shows three different ways to draw the same structure.

Figure 2. The same structure can be represented three different ways: an expanded formula, a condensed formula, and a skeletal structure.

Example 1:  Drawing Skeletal Structures

Draw the skeletal structures for these two molecules:

Each carbon atom is converted into the end of a line or the place where lines intersect. All hydrogen atoms attached to the carbon atoms are left out of the structure (although we still need to recognize they are there):

Check Your Learning

Example 2:  Interpreting Skeletal Structures

Identify the chemical formula of the molecule represented here:

There are eight places where lines intersect or end, meaning that there are eight carbon atoms in the molecule. Since we know that carbon atoms tend to make four bonds, each carbon atom will have the number of hydrogen atoms that are required for four bonds. This compound contains 16 hydrogen atoms for a molecular formula of C 8 H 16 .

Location of the hydrogen atoms:

All alkanes are composed of carbon and hydrogen atoms, and have similar bonds, structures, and formulas; noncyclic alkanes all have a formula of  C n H 2n+2 . The number of carbon atoms present in an alkane has no limit. Greater numbers of atoms in the molecules will lead to stronger intermolecular attractions (dispersion forces) and correspondingly different physical properties of the molecules. Properties such as melting point and boiling point (Table 1) usually change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change.

Hydrocarbons with the same formula, including alkanes, can have different structures. For example, two alkanes have the formula C 4 H 10 : They are called n -butane and 2-methylpropane (or isobutane), and have the following Lewis structures:

The compounds n -butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules. The n -butane molecule contains an unbranched chain ,  meaning that no carbon atom is bonded to more than two other carbon atoms. We use the term normal , or the prefix n , to refer to a chain of carbon atoms without branching. The compound 2–methylpropane has a branched chain (the carbon atom in the center of the Lewis structure is bonded to three other carbon atoms)

Identifying isomers from Lewis structures is not as easy as it looks. Lewis structures that look different may actually represent the same isomers. For example, the three structures in Figure 3 all represent the same molecule, n -butane, and hence are not different isomers. They are identical because each contains an unbranched chain of four carbon atoms.

Figure 3. These three representations of the structure of n-butane are not isomers because they all contain the same arrangement of atoms and bonds.

The Basics of Organic Nomenclature: Naming Alkanes

The International Union of Pure and Applied Chemistry ( IUPAC ) has devised a system of nomenclature that begins with the names of the alkanes and can be adjusted from there to account for more complicated structures. The nomenclature for alkanes is based on two rules:

• To name an alkane, first identify the longest chain of carbon atoms in its structure. A two-carbon chain is called ethane; a three-carbon chain, propane; and a four-carbon chain, butane. Longer chains are named as follows: pentane (five-carbon chain), hexane (6), heptane (7), octane (8), nonane (9), and decane (10). These prefixes can be seen in the names of the alkanes described in Table 1.
• Add prefixes to the name of the longest chain to indicate the positions and names of substituents . Substituents are branches or functional groups that replace hydrogen atoms on a chain. The position of a substituent or branch is identified by the number of the carbon atom it is bonded to in the chain. We number the carbon atoms in the chain by counting from the end of the chain nearest the substituents. Multiple substituents are named individually and placed in alphabetical order at the front of the name.

When more than one substituent is present, either on the same carbon atom or on different carbon atoms, the substituents are listed alphabetically. Because the carbon atom numbering begins at the end closest to a substituent, the longest chain of carbon atoms is numbered in such a way as to produce the lowest number for the substituents. The ending – o replaces – ide at the end of the name of an electronegative substituent (in ionic compounds, the negatively charged ion ends with -ide like chloride; in organic compounds, such atoms are treated as substituents and the -o ending is used). The number of substituents of the same type is indicated by the prefixes di- (two), tri- (three), tetra- (four), etc. (for example, difluoro- indicates two fluoride substituents).

Example 3:  Naming Halogen-substituted Alkanes

Name the molecule whose structure is shown here:

The four-carbon chain is numbered from the end with the chlorine atom. This puts the substituents on positions 1 and 2 (numbering from the other end would put the substituents on positions 3 and 4). Four carbon atoms means that the base name of this compound will be butane. The bromine at position 2 will be described by adding 2-bromo-; this will come at the beginning of the name, since bromo- comes before chloro- alphabetically. The chlorine at position 1 will be described by adding 1-chloro-, resulting in the name of the molecule being 2-bromo-1-chlorobutane.

Name the following molecule:

3,3-dibromo-2-iodopentane

We call a substituent that contains one less hydrogen than the corresponding alkane an alkyl group. The name of an alkyl group is obtained by dropping the suffix -ane of the alkane name and adding – yl :

The open bonds in the methyl and ethyl groups indicate that these alkyl groups are bonded to another atom.

Example 4: Naming Substituted Alkanes

The longest carbon chain runs horizontally across the page and contains six carbon atoms (this makes the base of the name hexane, but we will also need to incorporate the name of the branch). In this case, we want to number from right to left (as shown by the red numbers) so the branch is connected to carbon 3 (imagine the numbers from left to right—this would put the branch on carbon 4, violating our rules). The branch attached to position 3 of our chain contains two carbon atoms (numbered in blue)—so we take our name for two carbons eth- and attach -yl at the end to signify we are describing a branch. Putting all the pieces together, this molecule is 3-ethylhexane.

4-propyloctane

Some hydrocarbons can form more than one type of alkyl group when the hydrogen atoms that would be removed have different “environments” in the molecule. This diversity of possible alkyl groups can be identified in the following way: The four hydrogen atoms in a methane molecule are equivalent; they all have the same environment. They are equivalent because each is bonded to a carbon atom (the same carbon atom) that is bonded to three hydrogen atoms. (It may be easier to see the equivalency in the ball and stick models in Figure 1. Removal of any one of the four hydrogen atoms from methane forms a methyl group. Likewise, the six hydrogen atoms in ethane are equivalent (Figure 1) and removing any one of these hydrogen atoms produces an ethyl group. Each of the six hydrogen atoms is bonded to a carbon atom that is bonded to two other hydrogen atoms and a carbon atom. However, in both propane and 2–methylpropane, there are hydrogen atoms in two different environments, distinguished by the adjacent atoms or groups of atoms:

Each of the six equivalent hydrogen atoms of the first type in propane and each of the nine equivalent hydrogen atoms of that type in 2-methylpropane (all shown in black) are bonded to a carbon atom that is bonded to only one other carbon atom. The two purple hydrogen atoms in propane are of a second type. They differ from the six hydrogen atoms of the first type in that they are bonded to a carbon atom bonded to two other carbon atoms. The green hydrogen atom in 2-methylpropane differs from the other nine hydrogen atoms in that molecule and from the purple hydrogen atoms in propane. The green hydrogen atom in 2-methylpropane is bonded to a carbon atom bonded to three other carbon atoms. Two different alkyl groups can be formed from each of these molecules, depending on which hydrogen atom is removed. The names and structures of these and several other alkyl groups are listed in Figure 4.

Figure 4. This listing gives the names and formulas for various alkyl groups formed by the removal of hydrogen atoms from different locations.

Note that alkyl groups do not exist as stable independent entities. They are always a part of some larger molecule. The location of an alkyl group on a hydrocarbon chain is indicated in the same way as any other substituent:

Alkanes are relatively stable molecules, but heat or light will activate reactions that involve the breaking of C–H or C–C single bonds. Combustion is one such reaction:

[latex]{\text{CH}}_{4}\left(g\right)+2{\text{O}}_{2}\left(g\right)\rightarrow{\text{CO}}_{2}\left(g\right)+2{\text{H}}_{2}\text{O}\left(\text{g}\right)[/latex]

Alkanes burn in the presence of oxygen, a highly exothermic oxidation-reduction reaction that produces carbon dioxide and water. As a consequence, alkanes are excellent fuels. For example, methane, CH 4 , is the principal component of natural gas. Butane, C 4 H 10 , used in camping stoves and lighters is an alkane. Gasoline is a liquid mixture of continuous- and branched-chain alkanes, each containing from five to nine carbon atoms, plus various additives to improve its performance as a fuel. Kerosene, diesel oil, and fuel oil are primarily mixtures of alkanes with higher molecular masses. The main source of these liquid alkane fuels is crude oil, a complex mixture that is separated by fractional distillation. Fractional distillation takes advantage of differences in the boiling points of the components of the mixture (see Figure 5). You may recall that boiling point is a function of intermolecular interactions, which was discussed in the chapter on solutions and colloids.

Figure 5. In a column for the fractional distillation of crude oil, oil heated to about 425 °C in the furnace vaporizes when it enters the base of the tower. The vapors rise through bubble caps in a series of trays in the tower. As the vapors gradually cool, fractions of higher, then of lower, boiling points condense to liquids and are drawn off. (credit left: modification of work by Luigi Chiesa)

In a substitution reaction , another typical reaction of alkanes, one or more of the alkane’s hydrogen atoms is replaced with a different atom or group of atoms. No carbon-carbon bonds are broken in these reactions, and the hybridization of the carbon atoms does not change. For example, the reaction between ethane and molecular chlorine depicted here is a substitution reaction:

The C–Cl portion of the chloroethane molecule is an example of a functional group , the part or moiety of a molecule that imparts a specific chemical reactivity. The types of functional groups present in an organic molecule are major determinants of its chemical properties and are used as a means of classifying organic compounds as detailed in the remaining sections of this chapter.

Want more practice naming alkanes? Watch this brief video tutorial to review the nomenclature process.

You can view the transcript for “Naming simple alkanes | Organic chemistry | Khan Academy” here (opens in new window) .

Organic compounds that contain one or more double or triple bonds between carbon atoms are described as unsaturated. You have likely heard of unsaturated fats. These are complex organic molecules with long chains of carbon atoms, which contain at least one double bond between carbon atoms. Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes . Carbon atoms linked by a double bond are bound together by two bonds, one σ bond and one π bond. Double and triple bonds give rise to a different geometry around the carbon atom that participates in them, leading to important differences in molecular shape and properties. The differing geometries are responsible for the different properties of unsaturated versus saturated fats.

Ethene, C 2 H 4 , is the simplest alkene. Each carbon atom in ethene, commonly called ethylene, has a trigonal planar structure. The second member of the series is propene (propylene) (Figure 6); the butene isomers follow in the series. Four carbon atoms in the chain of butene allows for the formation of isomers based on the position of the double bond, as well as a new form of isomerism.

Figure 6. Expanded structures, ball-and-stick structures, and space-filling models for the alkenes ethene, propene, and 1-butene are shown.

Ethylene (the common industrial name for ethene) is a basic raw material in the production of polyethylene and other important compounds. Over 135 million tons of ethylene were produced worldwide in 2010 for use in the polymer, petrochemical, and plastic industries. Ethylene is produced industrially in a process called cracking, in which the long hydrocarbon chains in a petroleum mixture are broken into smaller molecules.

Everyday Life: Recycling Plastics

Polymers (from Greek words poly meaning “many” and mer meaning “parts”) are large molecules made up of repeating units, referred to as monomers. Polymers can be natural (starch is a polymer of sugar residues and proteins are polymers of amino acids) or synthetic [like polyethylene, polyvinyl chloride (PVC), and polystyrene]. The variety of structures of polymers translates into a broad range of properties and uses that make them integral parts of our everyday lives. Adding functional groups to the structure of a polymer can result in significantly different properties (see Chemistry in Everyday Life: Kevlar, later in this chapter).

An example of a polymerization reaction is shown in Figure 7. The monomer ethylene (C 2 H 4 ) is a gas at room temperature, but when polymerized, using a transition metal catalyst, it is transformed into a solid material made up of long chains of –CH 2 – units called polyethylene. Polyethylene is a commodity plastic used primarily for packaging (bags and films).

Figure 7. The reaction for the polymerization of ethylene to polyethylene is shown.

Polyethylene is a member of one subset of synthetic polymers classified as plastics. Plastics are synthetic organic solids that can be molded; they are typically organic polymers with high molecular masses. Most of the monomers that go into common plastics (ethylene, propylene, vinyl chloride, styrene, and ethylene terephthalate) are derived from petrochemicals and are not very biodegradable, making them candidate materials for recycling. Recycling plastics helps minimize the need for using more of the petrochemical supplies and also minimizes the environmental damage caused by throwing away these nonbiodegradable materials.

Plastic recycling is the process of recovering waste, scrap, or used plastics, and reprocessing the material into useful products. For example, polyethylene terephthalate (soft drink bottles) can be melted down and used for plastic furniture, in carpets, or for other applications. Other plastics, like polyethylene (bags) and polypropylene (cups, plastic food containers), can be recycled or reprocessed to be used again. Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code (see Figure 8). These operations have been in effect since the 1970s and have made the production of some plastics among the most efficient industrial operations today.

Figure 8. Each type of recyclable plastic is imprinted with a code for easy identification.

The name of an alkene is derived from the name of the alkane with the same number of carbon atoms. The presence of the double bond is signified by replacing the suffix -ane with the suffix -ene . The location of the double bond is identified by naming the smaller of the numbers of the carbon atoms participating in the double bond:

Isomers of Alkenes

Molecules of 1-butene and 2-butene are structural isomers; the arrangement of the atoms in these two molecules differs. As an example of arrangement differences, the first carbon atom in 1-butene is bonded to two hydrogen atoms; the first carbon atom in 2-butene is bonded to three hydrogen atoms.

The compound 2-butene and some other alkenes also form a second type of isomer called a geometric isomer. In a set of geometric isomers, the same types of atoms are attached to each other in the same order, but the geometries of the two molecules differ. Geometric isomers of alkenes differ in the orientation of the groups on either side of a [latex]\text{C}=\text{C}[/latex] bond.

Carbon atoms are free to rotate around a single bond but not around a double bond; a double bond is rigid. This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides. When structures of butene are drawn with 120° bond angles around the sp 2 -hybridized carbon atoms participating in the double bond, the isomers are apparent. The 2-butene isomer in which the two methyl groups are on the same side is called a cis -isomer; the one in which the two methyl groups are on opposite sides is called a trans -isomer (Figure 9). The different geometries produce different physical properties, such as boiling point, that may make separation of the isomers possible:

Figure 9. These molecular models show the structural and geometric isomers of butene.

Alkenes are much more reactive than alkanes because the [latex]\text{C}=\text{C}[/latex] moiety is a reactive functional group. A π bond, being a weaker bond, is disrupted much more easily than a σ bond. Thus, alkenes undergo a characteristic reaction in which the π bond is broken and replaced by two σ bonds. This reaction is called an addition reaction. The hybridization of the carbon atoms in the double bond in an alkene changes from sp 2 to sp 3 during an addition reaction. For example, halogens add to the double bond in an alkene instead of replacing hydrogen, as occurs in an alkane:

You can view the transcript for “Naming alkenes examples | Alkenes and Alkynes | Organic chemistry | Khan Academy” here (opens in new window) .

Example 5: Alkene Reactivity and Naming

Provide the IUPAC names for the reactant and product of the halogenation reaction shown here:

The reactant is a five-carbon chain that contains a carbon-carbon double bond, so the base name will be pentene. We begin counting at the end of the chain closest to the double bond—in this case, from the left—the double bond spans carbons 2 and 3, so the name becomes 2-pentene. Since there are two carbon-containing groups attached to the two carbon atoms in the double bond—and they are on the same side of the double bond—this molecule is the cis- isomer, making the name of the starting alkene cis -2-pentene. The product of the halogenation reaction will have two chlorine atoms attached to the carbon atoms that were a part of the carbon-carbon double bond:

This molecule is now a substituted alkane and will be named as such. The base of the name will be pentane. We will count from the end that numbers the carbon atoms where the chlorine atoms are attached as 2 and 3, making the name of the product 2,3-dichloropentane.

Provide names for the reactant and product of the reaction shown:

reactant: trans-3-hexene, product: 3,4-dichlorohexane

Hydrocarbon molecules with one or more triple bonds are called alkynes ; they make up another series of unsaturated hydrocarbons. Two carbon atoms joined by a triple bond are bound together by one σ bond and two π bonds. The sp -hybridized carbons involved in the triple bond have bond angles of 180°, giving these types of bonds a linear, rod-like shape.

The simplest member of the alkyne series is ethyne, C 2 H 2 , commonly called acetylene. The Lewis structure for ethyne, a linear molecule, is:

The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. For example, [latex]{\text{CH}}_{3}{\text{CH}}_{2}\text{C}\equiv \text{CH}[/latex] is called 1-butyne.

Example 6: Structure of Alkynes

Describe the geometry and hybridization of the carbon atoms in the following molecule:

Identify the hybridization and bond angles at the carbon atoms in the molecule shown:

Chemically, the alkynes are similar to the alkenes. Since the [latex]\text{C}\equiv \text{C}[/latex] functional group has two π bonds, alkynes typically react even more readily, and react with twice as much reagent in addition reactions. The reaction of acetylene with bromine is a typical example:

Acetylene and the other alkynes also burn readily. An acetylene torch takes advantage of the high heat of combustion for acetylene.

You can view the transcript for “Naming Alkynes – IUPAC Nomenclature” here (opens in new window) .

Aromatic Hydrocarbons

Benzene, C 6 H 6 , is the simplest member of a large family of hydrocarbons, called aromatic hydrocarbons . These compounds contain ring structures and exhibit bonding that must be described using the resonance hybrid concept of valence bond theory or the delocalization concept of molecular orbital theory. (To review these concepts, refer to the earlier chapters on chemical bonding). The resonance structures for benzene, C 6 H 6 , are:

Valence bond theory describes the benzene molecule and other planar aromatic hydrocarbon molecules as hexagonal rings of sp 2 -hybridized carbon atoms with the unhybridized p orbital of each carbon atom perpendicular to the plane of the ring. Three valence electrons in the sp 2 hybrid orbitals of each carbon atom and the valence electron of each hydrogen atom form the framework of σ bonds in the benzene molecule. The fourth valence electron of each carbon atom is shared with an adjacent carbon atom in their unhybridized p orbitals to yield the π bonds.

Figure 10. This condensed formula shows the unique bonding structure of benzene.

Benzene does not, however, exhibit the characteristics typical of an alkene. Each of the six bonds between its carbon atoms is equivalent and exhibits properties that are intermediate between those of a C–C single bond and a [latex]\text{C}=\text{C}[/latex] double bond. To represent this unique bonding, structural formulas for benzene and its derivatives are typically drawn with single bonds between the carbon atoms and a circle within the ring as shown in Figure 10.

There are many derivatives of benzene. The hydrogen atoms can be replaced by many different substituents. Aromatic compounds more readily undergo substitution reactions than addition reactions; replacement of one of the hydrogen atoms with another substituent will leave the delocalized double bonds intact. The following are typical examples of substituted benzene derivatives:

Toluene and xylene are important solvents and raw materials in the chemical industry. Styrene is used to produce the polymer polystyrene.

Example 7: Structure of Aromatic Hydrocarbons

One possible isomer created by a substitution reaction that replaces a hydrogen atom attached to the aromatic ring of toluene with a chlorine atom is shown here. Draw two other possible isomers in which the chlorine atom replaces a different hydrogen atom attached to the aromatic ring:

Since the six-carbon ring with alternating double bonds is necessary for the molecule to be classified as aromatic, appropriate isomers can be produced only by changing the positions of the chloro-substituent relative to the methyl-substituent:

Draw three isomers of a six-membered aromatic ring compound substituted with two bromines.

Key Concepts and Summary

Strong, stable bonds between carbon atoms produce complex molecules containing chains, branches, and rings. The chemistry of these compounds is called organic chemistry. Hydrocarbons are organic compounds composed of only carbon and hydrogen. The alkanes are saturated hydrocarbons—that is, hydrocarbons that contain only single bonds. Alkenes contain one or more carbon-carbon double bonds. Alkynes contain one or more carbon-carbon triple bonds. Aromatic hydrocarbons contain ring structures with delocalized π electron systems.

• What is the difference between the hybridization of carbon atoms’ valence orbitals in saturated and unsaturated hydrocarbons?
• On a microscopic level, how does the reaction of bromine with a saturated hydrocarbon differ from its reaction with an unsaturated hydrocarbon? How are they similar?
• On a microscopic level, how does the reaction of bromine with an alkene differ from its reaction with an alkyne? How are they similar?
• Explain why unbranched alkenes can form geometric isomers while unbranched alkanes cannot. Does this explanation involve the macroscopic domain or the microscopic domain?

• How does the carbon-atom hybridization change when polyethylene is prepared from ethylene?
• 3-methylpentane
• cis -3-hexene
• 4-methyl-1-pentene
• 4-methyl-2-pentyne
• 3-methylhexane
• trans -3-heptene
• 4-methyl-1-hexene
• 3,4-dimethyl-1-pentyne
• CH 3 CH 2 CBr 2 CH 3
• (CH 3 ) 3 CCl

• [latex]{\text{CH}}_{3}{\text{CH}}_{2}\text{C}\equiv {\text{CH CH}}_{3}{\text{CH}}_{2}\text{C}\equiv \text{CH}[/latex]

• [latex]{\left({\text{CH}}_{3}\right)}_{2}{\text{CHCH}}_{2}\text{CH}={\text{CH}}_{2}[/latex]
• (CH 3 ) 2 CHF
• CH 3 CHClCHClCH 3

• [latex]{\text{CH}}_{3}{\text{CH}}_{2}\text{CH}={\text{CHCH}}_{3}[/latex]

• [latex]{\left({\text{CH}}_{3}\right)}_{3}{\text{CCH}}_{2}\text{C}\equiv \text{CH}[/latex]
• Butane is used as a fuel in disposable lighters. Write the Lewis structure for each isomer of butane.
• Write Lewis structures and name the five structural isomers of hexane.
• Write Lewis structures for the cis–trans isomers of [latex]{\text{CH}}_{3}\text{CH}=\text{CHCl.}[/latex]
• Write structures for the three isomers of the aromatic hydrocarbon xylene, C 6 H 4 (CH 3 ) 2 .

• What is the IUPAC name for the compound?
• Name the other isomers that contain a five-carbon chain with three methyl substituents.
• Write Lewis structures and IUPAC names for the alkyne isomers of C 4 H 6 .
• Write Lewis structures and IUPAC names for all isomers of C 4 H 9 Cl.
• Name and write the structures of all isomers of the propyl and butyl alkyl groups.
• Write the structures for all the isomers of the –C 5 H 11 alkyl group.
• cis -1-chloro-2-bromoethene
• trans – 6 -ethyl-7-methyl-2-octene
• Benzene is one of the compounds used as an octane enhancer in unleaded gasoline. It is manufactured by the catalytic conversion of acetylene to benzene: [latex]3{\text{C}}_{2}{\text{H}}_{2}\rightarrow{\text{C}}_{6}{\text{H}}_{6}[/latex]. Draw Lewis structures for these compounds, with resonance structures as appropriate, and determine the hybridization of the carbon atoms in each.
• Teflon is prepared by the polymerization of tetrafluoroethylene. Write the equation that describes the polymerization using Lewis symbols.
• 1 mol of 1-butyne reacts with 2 mol of iodine.
• Pentane is burned in air.
• 2-butene reacts with chlorine.
• benzene burns in air.
• What mass of 2-bromopropane could be prepared from 25.5 g of propene? Assume a 100% yield of product.
• What is the empirical formula of the compound of silver and carbon?
• The production of acetylene on addition of HCl to the compound of silver and carbon suggests that the carbon is present as the acetylide ion, [latex]{\text{C}}_{2}{}^{2-}[/latex] . Write the formula of the compound showing the acetylide ion.
• Ethylene can be produced by the pyrolysis of ethane: [latex]{\text{C}}_{2}{\text{H}}_{6}\rightarrow{\text{C}}_{2}{\text{H}}_{4}+{\text{H}}_{2}[/latex]. How many kilograms of ethylene is produced by the pyrolysis of [latex]1.000\times {10}^{3}\text{kg}[/latex] of ethane, assuming a 100.0% yield?

1. There are several sets of answers; one is:

3. Both reactions result in bromine being incorporated into the structure of the product. The difference is the way in which that incorporation takes place. In the saturated hydrocarbon, an existing C–H bond is broken, and a bond between the C and the Br can then be formed. In the unsaturated hydrocarbon, the only bond broken in the hydrocarbon is the π bond whose electrons can be used to form a bond to one of the bromine atoms in Br 2 (the electrons from the Br–Br bond form the other C–Br bond on the other carbon that was part of the π bond in the starting unsaturated hydrocarbon).

5. Unbranched alkanes have free rotation about the C–C bonds, yielding all orientations of the substituents about these bonds equivalent, interchangeable by rotation. In the unbranched alkenes, the inability to rotate about the C=C bond results in fixed (unchanging) substituent orientations, thus permitting different isomers. Since these concepts pertain to phenomena at the molecular level, this explanation involves the microscopic domain.

7. They are the same compound because each is a saturated hydrocarbon containing an unbranched chain of six carbon atoms.

9.  The Lewis structures and molecular formulas are as follows:

11. The IUPAC names are as follows:

• 2,2-dibromobutane
• 2-chloro-2-methylpropane
• 2-methylbutane
• 4-fluoro-4-methyl-1-octyne
• trans -1-chloropropene
• 5-methyl-1-pentene

17. The answers are as follows:

• 2,2,4-trimethylpentane

In acetylene, the bonding uses sp hybrids on carbon atoms and s orbitals on hydrogen atoms. In benzene, the carbon atoms are sp 2 hybridized.

25.  The balanced equations are as follows:

27. [latex]{\text{C}}_{3}{\text{H}}_{7}\text{Br mass}=22.5\text{g}\times \frac{1\text{mol}{\text{C}}_{3}{\text{H}}_{6}}{48.081\text{g}{\text{C}}_{3}{\text{H}}_{6}}\times \frac{1\text{mol}{\text{C}}_{3}{\text{H}}_{7}\text{Br}}{1\text{mol}{\text{C}}_{3}{\text{H}}_{6}}\times \frac{122.993\text{g}{\text{C}}_{3}{\text{H}}_{7}\text{Br}}{1\text{mol}{\text{C}}_{3}{\text{H}}_{7}\text{Br}}=65.2\text{g}[/latex]

29. [latex]1{\text{C}}_{2}{\text{H}}_{6}\rightarrow 1{\text{C}}_{2}{\text{H}}_{4}[/latex]

[latex]\begin{array}{rcl}\text{mass of ethylene}&=&1\times{10}^{3}\text{kg}\times\frac{1\text{mol}}{30.07\text{g}}\times \frac{28.05\text{g}}{1\text{mol}}\\{}&=& 9.328\times{10}^{2}\text{kg}\end{array}[/latex]

alkane: molecule consisting of only carbon and hydrogen atoms connected by single (σ) bonds

alkene: molecule consisting of carbon and hydrogen containing at least one carbon-carbon double bond

alkyl group: substituent, consisting of an alkane missing one hydrogen atom, attached to a larger structure

alkyne: molecule consisting of carbon and hydrogen containing at least one carbon-carbon triple bond

aromatic hydrocarbon: cyclic molecule consisting of carbon and hydrogen with delocalized alternating carbon-carbon single and double bonds, resulting in enhanced stability

functional group: part of an organic molecule that imparts a specific chemical reactivity to the molecule

organic compound: natural or synthetic compound that contains carbon

saturated hydrocarbon: molecule containing carbon and hydrogen that has only single bonds between carbon atoms

skeletal structure: shorthand method of drawing organic molecules in which carbon atoms are represented by the ends of lines and bends in between lines, and hydrogen atoms attached to the carbon atoms are not shown (but are understood to be present by the context of the structure)

substituent: branch or functional group that replaces hydrogen atoms in a larger hydrocarbon chain

substitution reaction: reaction in which one atom replaces another in a molecule

• This is the Beilstein database, now available through the Reaxys site ( http://www.elsevier.com/online-tools/reaxys ). ↵
• Peplow, Mark. “Organic Synthesis: The Robo-Chemist,” Nature 512 (2014): 20–2. ↵
• Physical properties for C 4 H 10 and heavier molecules are those of the normal isomer, n-butane, n-pentane, etc. ↵
• STP indicates a temperature of 0 °C and a pressure of 1 atm. ↵
• Chemistry 2e. Provided by : OpenStax. Located at : https://openstax.org/ . License : CC BY: Attribution . License Terms : Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction
• Naming simple alkanes | Organic chemistry | Khan Academy. Authored by : Khan Academy. Located at : https://youtu.be/NRFPvLp3r3g . License : Other . License Terms : Standard YouTube License
• Naming alkenes examples | Alkenes and Alkynes | Organic chemistry | Khan Academy. Authored by : Khan Academy. Located at : https://youtu.be/KWv5PaoHwPA . License : Other . License Terms : Standard YouTube License
• Naming Alkynes - IUPAC Nomenclature. Authored by : The Organic Chemistry Tutor. Located at : https://youtu.be/hzEfO_ViCz4 . License : Other . License Terms : Standard YouTube License

Hydrocarbons: An Introduction to Structure, Physico-Chemical Properties and Natural Occurrence

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Hydrocarbons are among the most abundant organic compound classes in the biogeosphere. They are formed directly by living organisms as biosynthetic products or through geological transformation of biomass in sedimentary systems. This article provides an introduction to the structural variability of hydrocarbons and their occurrence in natural environments. Besides saturated, unsaturated and aromatic hydrocarbons also selected types of functionalized organic compounds which play key roles in biogeochemical processes are discussed. For each compound type reactivity and important reaction types with a special focus on mechanisms relevant in biochemical transformations are presented. Bio- and geomacromolecules and their role in the formation of fossil fuels are briefly introduced. Important physico-chemical parameters are discussed in relation to the structural characteristics of the presented compound classes.

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Test for Unsaturation ( CIE A Level Chemistry )

Revision note.

Head of Science

Test for Unsaturation

• Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)
• Br 2 (aq) is an orange or yellow solution, called bromine water  and this is the halogen most commonly used
• The unknown compound is shaken with the bromine water
• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The decolourisation of bromine water by an unsaturated compound as a result of an addition reaction

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76 Hydrocarbons

Learning objectives.

• Explain the importance of hydrocarbons and the reason for their diversity
• Name saturated and unsaturated hydrocarbons, and molecules derived from them
• Describe the reactions characteristic of saturated and unsaturated hydrocarbons
• Identify structural and geometric isomers of hydrocarbons

The largest database 1 of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. The number of potential organic compounds has been estimated 2 at 10 60 —an astronomically high number. The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities.

The simplest organic compounds  contain only the elements carbon and hydrogen, and are called hydrocarbons. Even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. In addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. Many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have been prepared in the laboratory. We use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. The familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. We can distinguish several types of hydrocarbons by differences in the bonding between carbon atoms. This leads to differences in geometries and in the hybridization of the carbon orbitals.

Alkanes , or saturated hydrocarbons , contain only single covalent bonds between carbon atoms. Each of the carbon atoms in an alkane has sp 3 hybrid orbitals and is bonded to four other atoms, each of which is either carbon or hydrogen. The Lewis structures and models of methane, ethane, and pentane are illustrated in the figure below. Carbon chains are usually drawn as straight lines in Lewis structures, but one has to remember that Lewis structures are not intended to indicate the geometry of molecules. Notice that the carbon atoms in the structural models (the ball-and-stick and space-filling models) of the pentane molecule do not lie in a straight line. Because of the sp 3 hybridization, the bond angles in carbon chains are close to 109.5°, giving such chains in an alkane a zigzag shape.

The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas ). Instead of the usual format for chemical formulas in which each element symbol appears just once, a condensed formula is written to suggest the bonding in the molecule. These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. Condensed structural formulas for ethane and pentane are shown at the bottom of the figure below, and several additional examples are provided in the exercises at the end of this chapter.

A common method used by organic chemists to simplify the drawings of larger molecules is to use a skeletal structure (also called a line-angle structure). In this type of structure, carbon atoms are not symbolized with a C, but represented by each end of a line or bend in a line. Hydrogen atoms are not drawn if they are attached to a carbon. Other atoms besides carbon and hydrogen are represented by their elemental symbols. The following figure shows three different ways to draw the same structure.

Solution There are eight places where lines intersect or end, meaning that there are eight carbon atoms in the molecule. Since we know that carbon atoms tend to make four bonds, each carbon atom will have the number of hydrogen atoms that are required for four bonds. This compound contains 16 hydrogen atoms for a molecular formula of [latex]\text{C}_8\text{H}_{16}[/latex].

Location of the hydrogen atoms:

All alkanes are composed of carbon and hydrogen atoms, and have similar bonds, structures, and formulas; noncyclic alkanes all have a formula of [latex]\text{C}_\text{n}\text{H}_\text{2n+2}[/latex]. The number of carbon atoms present in an alkane has no limit. Greater numbers of atoms in the molecules will lead to stronger intermolecular attractions (dispersion forces) and correspondingly different physical properties of the molecules. Properties such as melting point and boiling point usually change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change.

The compounds n -butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules. The n -butane molecule contains an unbranched chain , meaning that no carbon atom is bonded to more than two other carbon atoms. We use the term normal , or the prefix n , to refer to a chain of carbon atoms without branching. The compound 2–methylpropane has a branched chain (the carbon atom in the center of the Lewis structure is bonded to three other carbon atoms)

Identifying isomers from Lewis structures is not as easy as it looks. Lewis structures that look different may actually represent the same isomers. For example, the three structures in the figure below all represent the same molecule, n -butane, and hence are not different isomers. They are identical because each contains an unbranched chain of four carbon atoms.

The Basics of Organic Nomenclature: Naming Alkanes

The International Union of Pure and Applied Chemistry ( IUPAC ) has devised a system of nomenclature that begins with the names of the alkanes and can be adjusted from there to account for more complicated structures. The nomenclature for alkanes is based on two rules:

• To name an alkane, first identify the longest chain of carbon atoms in its structure. A two-carbon chain is called ethane; a three-carbon chain, propane; and a four-carbon chain, butane. Longer chains are named as follows: pentane (five-carbon chain), hexane (6), heptane (7), octane (8), nonane (9), and decane (10). These prefixes can be seen in the names of the alkanes described in the figure below.

When more than one substituent is present, either on the same carbon atom or on different carbon atoms, the substituents are listed alphabetically. Because the carbon atom numbering begins at the end closest to a substituent, the longest chain of carbon atoms is numbered in such a way as to produce the lowest number for the substituents. The ending -o replaces -ide at the end of the name of an electronegative substituent (in ionic compounds, the negatively charged ion ends with -ide like chloride; in organic compounds, such atoms are treated as substituents and the -o ending is used). The number of substituents of the same type is indicated by the prefixes di- (two), tri- (three), tetra- (four), and so on (for example, difluoro- indicates two fluoride substituents).

The four-carbon chain is numbered from the end with the chlorine atom. This puts the substituents on positions 1 and 2 (numbering from the other end would put the substituents on positions 3 and 4). Four carbon atoms means that the base name of this compound will be butane. The bromine at position 2 will be described by adding 2-bromo-; this will come at the beginning of the name, since bromo- comes before chloro- alphabetically. The chlorine at position 1 will be described by adding 1-chloro-, resulting in the name of the molecule being 2-bromo-1-chlorobutane.

The open bonds in the methyl and ethyl groups indicate that these alkyl groups are bonded to another atom.

Solution The longest carbon chain runs horizontally across the page and contains six carbon atoms (this makes the base of the name hexane, but we will also need to incorporate the name of the branch). In this case, we want to number from right to left (as shown by the blue numbers) so the branch is connected to carbon 3 (imagine the numbers from left to right—this would put the branch on carbon 4, violating our rules). The branch attached to position 3 of our chain contains two carbon atoms (numbered in red)—so we take our name for two carbons eth- and attach -yl at the end to signify we are describing a branch. Putting all the pieces together, this molecule is 3-ethylhexane.

Each of the six equivalent hydrogen atoms of the first type in propane and each of the nine equivalent hydrogen atoms of that type in 2-methylpropane (all shown in black) are bonded to a carbon atom that is bonded to only one other carbon atom. The two purple hydrogen atoms in propane are of a second type. They differ from the six hydrogen atoms of the first type in that they are bonded to a carbon atom bonded to two other carbon atoms. The green hydrogen atom in 2-methylpropane differs from the other nine hydrogen atoms in that molecule and from the purple hydrogen atoms in propane. The green hydrogen atom in 2-methylpropane is bonded to a carbon atom bonded to three other carbon atoms. Two different alkyl groups can be formed from each of these molecules, depending on which hydrogen atom is removed. The names and structures of these and several other alkyl groups are listed in the following table.

Alkanes are relatively stable molecules, but heat or light will activate reactions that involve the breaking of C–H or C–C single bonds. Combustion is one such reaction:

[latex]\text{CH}_4 (g) + \text{2O}_2 (g) \rightarrow \text{CO}_2 (g) + \text{2H}_2\text{O} (g)[/latex] Alkanes burn in the presence of oxygen, a highly exothermic oxidation-reduction reaction that produces carbon dioxide and water. As a consequence, alkanes are excellent fuels. For example, methane, [latex]\text{CH}_4[/latex], is the principal component of natural gas. Butane, [latex]\text{C}_4\text{H}_{10}[/latex], used in camping stoves and lighters is an alkane. Gasoline is a liquid mixture of continuous- and branched-chain alkanes, each containing from five to nine carbon atoms, plus various additives to improve its performance as a fuel. Kerosene, diesel oil, and fuel oil are primarily mixtures of alkanes with higher molecular masses. The main source of these liquid alkane fuels is crude oil, a complex mixture that is separated by fractional distillation. Fractional distillation takes advantage of differences in the boiling points of the components of the mixture. You may recall that boiling point is a function of intermolecular interactions, which was discussed in the chapter on solutions and colloids.

The C–Cl portion of the chloroethane molecule is an example of a functional group , the part or moiety of a molecule that imparts a specific chemical reactivity. The types of functional groups present in an organic molecule are major determinants of its chemical properties and are used as a means of classifying organic compounds as detailed in the remaining sections of this chapter.

Organic compounds that contain one or more double or triple bonds between carbon atoms are described as unsaturated. You have likely heard of unsaturated fats. These are complex organic molecules with long chains of carbon atoms, which contain at least one double bond between carbon atoms. Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes . Carbon atoms linked by a double bond are bound together by two bonds, one σ bond and one π bond. Double and triple bonds give rise to a different geometry around the carbon atom that participates in them, leading to important differences in molecular shape and properties. The differing geometries are responsible for the different properties of unsaturated versus saturated fats.

Ethene, [latex]\text{C}_2\text{H}_4[/latex], is the simplest alkene. Each carbon atom in ethene, commonly called ethylene, has a trigonal planar structure. The second member of the series is propene (propylene); the butene isomers follow in the series. Four carbon atoms in the chain of butene allows for the formation of isomers based on the position of the double bond, as well as a new form of isomerism.

Ethylene (the common industrial name for ethene) is a basic raw material in the production of polyethylene and other important compounds. Over 135 million tons of ethylene were produced worldwide in 2010 for use in the polymer, petrochemical, and plastic industries. Ethylene is produced industrially in a process called cracking, in which the long hydrocarbon chains in a petroleum mixture are broken into smaller molecules.

Chemistry in Everyday Life

Recycling Plastics Polymers (from Greek words poly meaning “many” and mer meaning “parts”) are large molecules made up of repeating units, referred to as monomers. Polymers can be natural (starch is a polymer of sugar residues and proteins are polymers of amino acids) or synthetic [like polyethylene, polyvinyl chloride (PVC), and polystyrene]. The variety of structures of polymers translates into a broad range of properties and uses that make them integral parts of our everyday lives. Adding functional groups to the structure of a polymer can result in significantly different properties (see the discussion about Kevlar later in this chapter).

An example of a polymerization reaction is shown in the figure below. The monomer ethylene [latex]\text{(C}_2\text{H}_4)[/latex] is a gas at room temperature, but when polymerized, using a transition metal catalyst, it is transformed into a solid material made up of long chains of [latex]\text{–CH}_2–[/latex] units called polyethylene. Polyethylene is a commodity plastic used primarily for packaging (bags and films).

Polyethylene is a member of one subset of synthetic polymers classified as plastics. Plastics are synthetic organic solids that can be molded; they are typically organic polymers with high molecular masses. Most of the monomers that go into common plastics (ethylene, propylene, vinyl chloride, styrene, and ethylene terephthalate) are derived from petrochemicals and are not very biodegradable, making them candidate materials for recycling. Recycling plastics helps minimize the need for using more of the petrochemical supplies and also minimizes the environmental damage caused by throwing away these nonbiodegradable materials.

Plastic recycling is the process of recovering waste, scrap, or used plastics, and reprocessing the material into useful products. For example, polyethylene terephthalate (soft drink bottles) can be melted down and used for plastic furniture, in carpets, or for other applications. Other plastics, like polyethylene (bags) and polypropylene (cups, plastic food containers), can be recycled or reprocessed to be used again. Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code. These operations have been in effect since the 1970s and have made the production of some plastics among the most efficient industrial operations today.

Isomers of Alkenes

Molecules of 1-butene and 2-butene are structural isomers; the arrangement of the atoms in these two molecules differs. As an example of arrangement differences, the first carbon atom in 1-butene is bonded to two hydrogen atoms; the first carbon atom in 2-butene is bonded to three hydrogen atoms.

The compound 2-butene and some other alkenes also form a second type of isomer called a geometric isomer. In a set of geometric isomers, the same types of atoms are attached to each other in the same order, but the geometries of the two molecules differ. Geometric isomers of alkenes differ in the orientation of the groups on either side of a [latex]\text{C = C}[/latex]  bond.

Carbon atoms are free to rotate around a single bond but not around a double bond; a double bond is rigid. This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides. When structures of butene are drawn with 120° bond angles around the sp 2 -hybridized carbon atoms participating in the double bond, the isomers are apparent. The 2-butene isomer in which the two methyl groups are on the same side is called a cis -isomer; the one in which the two methyl groups are on opposite sides is called a trans -isomer. The different geometries produce different physical properties, such as boiling point, that may make separation of the isomers possible:

Alkene Reactivity and Naming

This molecule is now a substituted alkane and will be named as such. The base of the name will be pentane. We will count from the end that numbers the carbon atoms where the chlorine atoms are attached as 2 and 3, making the name of the product 2,3-dichloropentane.

The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. For example, [latex]\text{CH}_3\text{CH}_2\text{C} \equiv \text{CH}[/latex] is called 1-butyne.

Solution Carbon atoms 1 and 4 have four single bonds and are thus tetrahedral with sp 3 hybridization. Carbon atoms 2 and 3 are involved in the triple bond, so they have linear geometries and would be classified as sp hybrids.

Acetylene and the other alkynes also burn readily. An acetylene torch takes advantage of the high heat of combustion for acetylene.

Aromatic Hydrocarbons

Valence bond theory describes the benzene molecule and other planar aromatic hydrocarbon molecules as hexagonal rings of sp 2 -hybridized carbon atoms with the unhybridized p orbital of each carbon atom perpendicular to the plane of the ring. Three valence electrons in the sp 2 hybrid orbitals of each carbon atom and the valence electron of each hydrogen atom form the framework of σ bonds in the benzene molecule. The fourth valence electron of each carbon atom is shared with an adjacent carbon atom in their unhybridized p orbitals to yield the π bonds. Benzene does not, however, exhibit the characteristics typical of an alkene. Each of the six bonds between its carbon atoms is equivalent and exhibits properties that are intermediate between those of a [latex]\text{C–C}[/latex] single bond and a [latex]C = C[/latex] double bond. To represent this unique bonding, structural formulas for benzene and its derivatives are typically drawn with single bonds between the carbon atoms and a circle within the ring as shown in the following figure.

Toluene and xylene are important solvents and raw materials in the chemical industry. Styrene is used to produce the polymer polystyrene.

Check Your Learning Draw three isomers of a six-membered aromatic ring compound substituted with two bromines.

Key Concepts and Summary

Strong, stable bonds between carbon atoms produce complex molecules containing chains, branches, and rings. The chemistry of these compounds is called organic chemistry. Hydrocarbons are organic compounds composed of only carbon and hydrogen. The alkanes are saturated hydrocarbons—that is, hydrocarbons that contain only single bonds. Alkenes contain one or more carbon-carbon double bonds. Alkynes contain one or more carbon-carbon triple bonds. Aromatic hydrocarbons contain ring structures with delocalized π electron systems.

END OF CHAPTER EXERCISES

• What is the difference between the hybridization of carbon atoms’ valence orbitals in saturated and unsaturated hydrocarbons?
• On a microscopic level, how does the reaction of bromine with a saturated hydrocarbon differ from its reaction with an unsaturated hydrocarbon? How are they similar? Both reactions result in bromine being incorporated into the structure of the product. The difference is the way in which that incorporation takes place. In the saturated hydrocarbon, an existing C–H bond is broken, and a bond between the C and the Br can then be formed. In the unsaturated hydrocarbon, the only bond broken in the hydrocarbon is the π bond whose electrons can be used to form a bond to one of the bromine atoms in Br 2 (the electrons from the Br–Br bond form the other C–Br bond on the other carbon that was part of the π bond in the starting unsaturated hydrocarbon).
• On a microscopic level, how does the reaction of bromine with an alkene differ from its reaction with an alkyne? How are they similar?
• Explain why unbranched alkenes can form geometric isomers while unbranched alkanes cannot. Does this explanation involve the macroscopic domain or the microscopic domain? Unbranched alkanes have free rotation about the C–C bonds, yielding all orientations of the substituents about these bonds equivalent, interchangeable by rotation. In the unbranched alkenes, the inability to rotate about the C=C bond results in fixed (unchanging) substituent orientations, thus permitting different isomers. Since these concepts pertain to phenomena at the molecular level, this explanation involves the microscopic domain.

• How does the carbon-atom hybridization change when polyethylene is prepared from ethylene?

• Write the chemical formula, condensed formula, and Lewis structure for each of the following hydrocarbons: (a) heptane (b) 3-methylhexane (c) trans -3-heptene (d) 4-methyl-1-hexene (e) 2-heptyne (f) 3,4-dimethyl-1-pentyne

• Write Lewis structures and name the five structural isomers of hexane.

• Write structures for the three isomers of the aromatic hydrocarbon xylene, [latex]\text{C}_6\text{H}_4\text{(CH}_3)_2[/latex].

• Write Lewis structures and IUPAC names for the alkyne isomers of [latex]\text{C}_4\text{H}_6[/latex].

• Name and write the structures of all isomers of the propyl and butyl alkyl groups.

• Write Lewis structures and describe the molecular geometry at each carbon atom in the following compounds: (a) cis -3-hexene   (b) cis -1-chloro-2-bromoethene   (c) 2-pentyne   (d) trans – 6 -ethyl-7-methyl-2-octene

• Teflon is prepared by the polymerization of tetrafluoroethylene. Write the equation that describes the polymerization using Lewis symbols.

• Write two complete, balanced equations for each of the following reactions, one using condensed formulas and one using Lewis structures.  (a) 2-butene reacts with chlorine.  (b) benzene burns in air.
• What mass of 2-bromopropane could be prepared from 25.5 g of propene? Assume a 100% yield of product. 65.2 g
• Acetylene is a very weak acid; however, it will react with moist silver(I) oxide and form water and a compound composed of silver and carbon. Addition of a solution of [latex]\text{HCl}[/latex] to a 0.2352-g sample of the compound of silver and carbon produced acetylene and 0.2822 g of [latex]\text{AgCl}[/latex].  (a) What is the empirical formula of the compound of silver and carbon?  (b) The production of acetylene on addition of [latex]\text{HCl}[/latex] to the compound of silver and carbon suggests that the carbon is present as the acetylide ion, [latex]\text{C}_2 ^{2-[/latex]. Write the formula of the compound showing the acetylide ion.
• Ethylene can be produced by the pyrolysis of ethane:  [latex]\text{C}_2\text{H}_6 \rightarrow \text{C}_2\text{H}_4 + \text{H}_2[/latex] How many kilograms of ethylene is produced by the pyrolysis of 1.000 × 10 3 kg of ethane, assuming a 100.0% yield? 9.328 × 10 2 kg
• 1 This is the Beilstein database, now available through the Reaxys site (www.elsevier.com/online-tools/reaxys).
• 2 Peplow, Mark. “Organic Synthesis: The Robo-Chemist,” Nature 512 (2014): 20–2.
• 3 Physical properties for C 4 H 10 and heavier molecules are those of the normal isomer , n -butane, n -pentane, etc.
• 4 STP indicates a temperature of 0 °C and a pressure of 1 atm.

This chapter is an adaptation of the chapter “ Hydrocarbons ” in Chemistry: Atoms First 2e by OpenStax and is licensed under a CC BY 4.0 license.

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natural or synthetic compound that contains carbon

molecule consisting of only carbon and hydrogen atoms connected by single (σ) bonds

molecule containing carbon and hydrogen that has only single bonds between carbon atoms

shorthand method of drawing organic molecules in which carbon atoms are represented by the ends of lines and bends in between lines, and hydrogen atoms attached to the carbon atoms are not shown (but are understood to be present by the context of the structure)

branch or functional group that replaces hydrogen atoms in a larger hydrocarbon chain

substituent, consisting of an alkane missing one hydrogen atom, attached to a larger structure

reaction in which one atom replaces another in a molecule

part of an organic molecule that imparts a specific chemical reactivity to the molecule

molecule consisting of carbon and hydrogen containing at least one carbon-carbon double bond

reaction in which a double carbon-carbon bond forms a single carbon-carbon bond by the addition of a reactant. Typical reaction for an alkene.

molecule consisting of carbon and hydrogen containing at least one carbon-carbon triple bond

cyclic molecule consisting of carbon and hydrogen with delocalized alternating carbon-carbon single and double bonds, resulting in enhanced stability

element in group 15

inner transition metal in the top of the bottom two rows of the periodic table

similar to internal radiation therapy, but chemical rather than radioactive substances are introduced into the body to kill cancer cells

radiation delivered by a machine outside the body

(also, brachytherapy) radiation from a radioactive substance introduced into the body to kill cancer cells

use of high-energy radiation to damage the DNA of cancer cells, which kills them or keeps them from dividing

(also, radioactive label) radioisotope used to track or follow a substance by monitoring its radioactive emissions

(also, noble gas) element in group 18

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