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25.1: Hydrocarbons

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Lesson Objectives

  • Describe the bonding characteristics of carbon that lead to such a large number of organic compounds.
  • Differentiate between aliphatic and aromatic hydrocarbons.
  • Describe, name, and draw structures for straight-chain and branched alkanes.
  • Differentiate between saturated and unsaturated hydrocarbons.
  • Describe alkenes and alkynes.
  • Describe the characteristics of structural isomers, geometric isomers, and optical isomers.

Vocabulary

  • aliphatic hydrocarbon
  • alkane
  • alkene
  • alkyl group
  • alkyne
  • aromatic hydrocarbon
  • geometric isomer
  • hydrocarbon
  • optical isomer
  • organic compound
  • saturated hydrocarbon
  • structural isomer
  • unsaturated hydrocarbon

Check Your Understanding

Recalling Prior Knowledge

  • How many valence electrons does a carbon atom have?
  • How does VSEPR theory predict the shapes of molecules?

Gasoline and other fuels, plastics, and synthetic fabrics are just a few examples of everyday products made from substances that are derived from petroleum. In this lesson, you will be introduced to organic chemistry and some of the many compounds that contain carbon.

Carbon

At one time in history, it was thought that only living things were capable of synthesizing the carbon-containing compounds present in cells. For that reason, the term organic was applied to those compounds. Eventually, it was proven that carbon-containing compounds could be synthesized from inorganic substances, but the term organic has remained. Currently, most compounds that contain covalent bonds between carbon and at least one other atom are considered organic compounds. However, for historical reasons, some oxides of carbon are considered inorganic, such as the carbonate ion (CO32-) and carbon dioxide (CO2). Organic chemistry is the study of organic compounds.

Organic chemistry is a vast and complex subject. There are over a million known organic compounds, which is far more than the number of inorganic compounds. The reason lies with the uniqueness of carbon’s structure and bonding. Carbon has four valence electrons, so it makes four separate covalent bonds when it forms compounds. Carbon has the ability to make strong single, double, or triple covalent bonds with other carbon atoms, so long chains of carbon or ringed structures can be produced. Carbon also readily makes covalent bonds with other elements, primarily hydrogen, oxygen, nitrogen, the halogens, and several other nonmetals. Illustrated below (Figure below) are the ball-and-stick models of two organic compounds.

(A) Stearic acid is composed of many carbon (black) and hydrogen (white) atoms, along with two oxygen (red) atoms. (B) Methionine is composed of carbon, hydrogen, oxygen, nitrogen (blue), and sulfur (yellow) atoms.

Hydrocarbons

A hydrocarbon is an organic compound that is made up of only carbon and hydrogen. A hydrocarbon is the simplest kind of organic molecule and is the basis for all other more complex organic compounds. Hydrocarbons can be divided into two broad categories. Aliphatic hydrocarbons are hydrocarbons that do not contain any aromatic rings, while aromatic hydrocarbons contain one or more aromatic rings. We will define and discuss aromatic rings in the next lesson. For now, we turn our attention to various types of aliphatic hydrocarbons.

Alkanes

An alkane is a hydrocarbon in which there are only single covalent bonds. The simplest alkane is methane, with the molecular formula CH4. The central carbon atom makes four single covalent bonds to hydrogen atoms.

Methane is the simplest hydrocarbon. It is shown here as (A) a structural formula, (B) a ball-and-stick model, and (C) a space-filling model.

Shown above (Figure above) are three different depictions of the methane molecule, each of which is useful for conveying different types of information. On the left is a structural formula, which was introduced in the chapter Covalent Bonding. A structural formal clearly shows the bonding pattern of the entire molecule. The disadvantage to a structural formula is that it shows a molecule in only two dimensions and does not give an indication of the shape or bond angles. Recall that the methane molecule is tetrahedral, with H-C-H bond angles of 109.5° and sp3 hybridization around the central carbon. In the middle of the figure above (Figure above) is a ball-and-stick model. This version more clearly represents the geometry of the molecule, but it exaggerates the distances between atoms and is much more difficult to draw than the structural formula. On the right is a space-filling model that gives a more accurate representation of the space being occupied by each of the individual atoms. However, it can be difficult to see all of the atoms in larger, more complex molecules when they are presented in this way. For the most part, this text will use structural formulas when discussing organic molecules.

Alkanes are an example of a class of hydrocarbons called saturated hydrocarbons. A saturated hydrocarbon is a hydrocarbon that contains no double or triple bonds. It is referred to as saturated because it contains the maximum number of hydrogen atoms that can be added to a given carbon skeleton.

Straight-Chain Alkanes

After methane, the next simplest alkane is called ethane (C2H6). The two carbon atoms in ethane are connected by a single covalent bond, and each carbon is also able to bond with three hydrogen atoms. As more carbon atoms are added to the chain, the alkane series progresses to longer and larger compounds. Structural formulas for ethane, propane (C3H8), and butane (C4H10) are shown below.

These alkanes are called straight-chain alkanes because the carbon atoms are connected to one another in one continuous chain with no branching. Naming and writing structural and molecular formulas for the straight-chain alkanes is straightforward. The name of each alkane consists of a prefix that specifies the number of carbon atoms and the ending –ane. The molecular formula follows the pattern of CnH2n+2 where n is the number of carbons in the chain. Listed below (Table below) are the first ten members of the alkane series.

First Ten Members of the Alkane Series
Name Molecular Formula Condensed Structural Formula Boiling Point (°C)
Methane CH4 CH4 −161.0
Ethane C2H6 CH3CH3 −88.5
Propane C3H8 CH3CH2CH3 −42.0
Butane C4H10 CH3CH2CH2CH3 0.5
Pentane C5H12 CH3CH2CH2CH2CH3 36.0
Hexane C6H14 CH3CH2CH2CH2CH2CH3 68.7
Heptane C7H16 CH3CH2CH2CH2CH2CH2CH3 98.5
Octane C8H18 CH3CH2CH2CH2CH2CH2CH2CH3 125.6
Nonane C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3 150.7
Decane C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 174.1

Note that the table shows a variation of a structural formula called a condensed structural formula. In this formula, the covalent bonds are understood to exist between each carbon and the hydrogens associated with it as well as between carbons. The table above (Table above) also shows that the boiling points of the alkanes steadily increase as the length of the carbon chain increases. This is due to an increase in the strength of the attractive London dispersion forces between molecules, which are the only relevant intermolecular forces for nonpolar hydrocarbons.

Branched Alkanes

Beginning with butane, there is an alternate structure possible that is not a straight chain. The structural formula below shows a four-carbon structure consisting of a three-carbon chain with a –CH3 group attached to the middle carbon.

The official name of this molecule is 2-methylpropane. Note that the molecular formula is still C4H10, which is the same as that of butane. A structural isomer is one of multiple molecules that have the same molecular formula but different structural formulas. Butane and 2-methylpropane are structural isomers. Except for the simplest molecules, nearly all organic molecules have multiple potential isomers, which means that a chemical formula is not sufficient to uniquely identify most organic compounds.

2-methylpropane is an example of a branched alkane. The official IUPAC system of nomenclature for branched alkanes follows a set of steps that will be outlined in the following example.

  1. Find the longest continuous chain of carbon atoms in the molecule. This is called the parent chain. In the example above, the longest chain is eight carbon atoms, so the parent hydrocarbon is octane.
  2. Number the carbon atoms in the parent chain. To do this, start at the end that will give the smallest numbers possible to the carbon atoms where the branches originate. In the example above, the branches are on carbons 3 and 5 when the chain is numbered left-to-right. If it were to be numbered right-to-left, the branches would be on carbons 4 and 6, so the left-to-right order is preferable.
  3. The atoms attached to the parent chain are called substituents. A substituent that is itself a hydrocarbon is called an alkyl group. The names of alkyl groups use the same prefixes as the alkanes, but with a –yl suffix. A 1-carbon alkyl group is a methyl group, a 2-carbon alkyl group is an ethyl group, and so on. The substituents are named by placing the number from the parent carbon chain in front of the name of the substituent. In the current example, we have 3-methyl and 5-ethyl substituents.
  4. Use a prefix to indicate the appearance of more than one of the same substituent in the structural formula. Two of the same group is di-, three is tri-, and four is tetra-. Larger numbers use the same prefixes as those used to name straight-chain alkanes. For example, if methyl groups were attached to both carbons 2 and 3, that part of the name would be 2,3-dimethyl. This rule does not apply to the current structure above.
  5. Different substituents are listed in alphabetical order. Ignore any of the prefixes from rule 4. In the current example, the 5-ethyl- comes before the 3-methyl.
  6. Commas are used to separate multiple numbers. Hyphens are used to separate numbers from the names of various substituents. The parent name comes immediately after the last substituent. There are no blank spaces in the name.

The correct name for the above structure according to the IUPAC system is 5-ethyl-3-methyloctane.

If the name of a branched alkane is known, it is relatively easy to draw the structural formula. First, use the parent hydrocarbon name to draw the longest continuous carbon chain. Then, number the carbons and add the substituents onto the correct carbon atoms. Finally, add in hydrogen atoms so that each carbon has four bonds.

Sample Problem 25.1: Branched Alkanes

Name the following branched alkane.

Step 1: Plan the problem.

Follow the steps in the text. The parent chain is the longest continuous carbon chain. Number the carbons in the parent chain and label each substituent with the number of the carbon to which it is attached. Alphabetize the substituents, and string together the full name of the compound.

Step 2: Solve.

Note that there is not a substituent on the left-most carbon. Instead, the longest continuous chain happens to contain a bend when drawn in this form. This is most often done either to save space or to trick students on an exam. Because these molecules are not shown three-dimensionally, this “bend” is not really a necessary part of the structure. The longest continuous parent chain is ten carbons in length. It should be numbered right-to-left because that will give the substituents smaller numerical values. There are two methyl groups, both on carbon 4, and a propyl group on carbon 5. The IUPAC name is 4,4-dimethyl-5-propyldecane.

Practice Problems
  1. Name the following molecule.
  1. Draw the structural formula for 3,3-diethyl-2,6-dimethylnonane.

Alkenes

An alkene is a hydrocarbon with one or more carbon-carbon double covalent bonds. The simplest alkene is composed of two carbon atoms and is called ethene. Each carbon is bonded to two hydrogen atoms in addition to the double bond between them.

Each carbon atom is sp2 hybridized and exhibits a trigonal planar geometry. All the atoms of the molecule lay in one plane. Like the alkane series, the names of alkenes are based on the number of carbon atoms in the parent chain. Propene (C3H6) has three carbons total, while butene (C4H8) has four. The general formula for an alkene with one double bond is CnH2n. Alkenes are called unsaturated hydrocarbons. An unsaturated hydrocarbon is a hydrocarbon that contains less than the maximum number of hydrogen atoms that can possibly bond with the number of carbon atoms present.

Starting with butene, there are multiple structural isomers based on where in the chain the double bond occurs. This is illustrated by the condensed structural formulas of 1-butene and 2-butene.

The number in the name of the alkene refers to the lowest numbered carbon in the chain that is part of the double bond.

A different type of isomerism is exhibited by the two different molecules that would both be called 2-butene. Geometric isomers are isomers in which all of the atoms are still bonded to the same bonding partners, but the arrangement of atoms in space is different. Geometric isomers are also referred to as stereoisomers. The double bond in an alkene is not free to rotate because of the nature of the pi bond. Therefore, there are two different ways to construct the 2-butene molecule. Shown below (Figure below) are the two geometric isomers, called cis-2-butene and trans-2-butene.

The geometric isomers of 2-butene are called cis and trans.

The cis isomer has the two single hydrogen atoms on the same side of the double bond, while the trans isomer has them on opposite sides. In both molecules, each atom has all of the same bonding partners. In order for geometric isomers to exist, there must be a rigid structure in the molecule to prevent free rotation around a bond. If the double bond in an alkene were capable of rotating, the two geometric isomers above would not be different molecules because the structures would be free to interconvert. In addition, the two carbon atoms must each be attached to two different groups for there to be geometric isomers. Propene (Figure below) has no geometric isomers because one of the carbon atoms is bonded to two identical hydrogen atoms.

Propene.

The physical and chemical properties of geometric isomers are generally different. For example, cis-2-butene is slightly more polar than trans-2-butene, so it also has a slightly higher boiling point. Heat or irradiation with light can be used to bring about the conversion of one geometric isomer to another. The input of energy must be large enough to break the pi bond between the two carbon atoms, which is weaker than the sigma bond. At that point, the now single bond is free to rotate, and the isomers can interconvert.

Molecules with multiple double bonds are also quite common. Shown below (Figure below) is a four-carbon chain with double bonds between carbons 1 and 2 and between carbons 3 and 4. This molecule is called 1,3-butadiene.

1,3-butadiene.

Alkynes

An alkyne is a hydrocarbon with one or more carbon-carbon triple covalent bonds. The simplest alkyne consists of two carbon atoms and is called ethyne (Figure below).

Ethyne.

The ethyne molecule is linear, and each carbon atom is sp hybridized. The general formula of an alkyne with one triple bond is CnH2n−2. Alkynes are also unsaturated hydrocarbons. As with alkenes, alkynes display structural isomerism beginning with 1-butyne and 2-butyne. However, simple alkynes do not have geometric isomers because each carbon involved in the triple bond is bound to only one additional group.

Optical Isomers

In addition to structural and geometric isomers, there is a third type of isomerism. Optical isomers are molecules that are non-superimposable mirror images of each other. Most optical isomers have at least one atom, often carbon, that is bonded to four different atoms or groups of atoms. Envision a mirror between the two molecules pictured below (Figure below). The molecules are mirror images of one another, yet it is impossible to rotate one of the molecules and superimpose it on top of the other. Your left and right hand are essentially mirror images of one another, but they also cannot be superimposed.

The carbon atom is bonded to four different atoms in a molecule of CHClBrF. The molecules shown above are non-superimposable mirror images, also called optical isomers.

Lesson Summary

  • Organic chemistry is the chemistry of carbon-containing compounds. Carbon has the unique ability to make long chains and ring structures, resulting in a virtually limitless number of organic compounds.
  • Hydrocarbons can be classified as aliphatic or aromatic, depending on the absence or presence of an aromatic ring.
  • Alkanes are saturated hydrocarbons. The carbon atoms in an alkane may be arranged in a straight chain, or they may be branched. Alkanes can be named by following a systematic method developed by the IUPAC.
  • Alkenes and alkynes are unsaturated hydrocarbons that contain a double or triple covalent bond.
  • Isomers are molecules that consist of the same atoms in different arrangements. Isomers may be structural, geometric, or optical.

Lesson Review Questions

Reviewing Concepts

  1. How many covalent bonds do carbon atoms form in compounds, and why?
  2. What do the terms saturated and unsaturated mean when applied to hydrocarbons?
  3. Give the general formulas for alkanes, alkenes, and alkynes.
  4. Why do alkenes display geometric isomerism, but alkanes do not?
  5. What condition must be met for a given molecule to have an optical isomer?

Problems

  1. An alkane has 13 carbon atoms. What is its molecular formula?
  2. Name the following hydrocarbons.
    1. CH3(CH2)3CH3
    2. CH3CH2CH2CH=CHCH2CH3
    3. CH≡CCH3
  3. Name the following branched alkanes.
  4. Draw structural formulas for the following compounds.
    1. cis-2-pentene
    2. 1-hexyne
    3. 2,2,3,6-tetramethyloctane
    4. 2,4-heptadiene
  5. Draw and name all of the possible isomers for C6H14.

Further Reading / Supplemental Links

Points to Consider

Carbon is capable of making ringed structures called cyclic hydrocarbons. Benzene is an unusually stable type of cyclic hydrocarbon that is referred to as an aromatic ring.

  • What are cycloalkanes, and how are they named?
  • What is the system for naming aromatic compounds with substituents attached to a benzene ring?

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