17.1 Hydrocarbons

Learning Objectives

1. Identify alkanes, alkenes, alkynes, and aromatic compounds.
2. List some properties of hydrocarbons.

The simplest organic compounds are those composed of only two elements: carbon and hydrogen. These compounds are called hydrocarbons. Hydrocarbons themselves are separated into two types: aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons are hydrocarbons based on chains of C atoms. There are three types of aliphatic hydrocarbons. Alkanes are aliphatic hydrocarbons with only single covalent bonds. Alkenes are aliphatic hydrocarbons that contain at least one C–C double bond, and alkynes are aliphatic hydrocarbons that contain a C–C triple bond. Occasionally, we find an aliphatic hydrocarbon with a ring of C atoms; these hydrocarbons are called cycloalkanes (or cycloalkenes or cycloalkynes).

Aromatic hydrocarbons, such as benzene,are flat-ring systems that contain continuously overlapping p orbitals.Electrons in the benzene ring have special energetic properties that give benzene physical and chemical properties that are markedly different from alkanes. Originally, the term aromatic was used to describe this class of compounds because they were particularly fragrant. However, in modern chemistry the term aromatic denotes the presence of a very stable ring that imparts different and unique properties to a molecule.

The simplest alkanes have their C atoms bonded in a straight chain; these are called normal alkanes. They are named according to the number of C atoms in the chain. The smallest alkane is methane:

To make four covalent bonds, the C atom bonds to four H atoms, making the molecular formula for methane CH₄. The two-dimensional diagram for methane is misleading, however; the four covalent bonds that the C atom makes are oriented three-dimensionally toward the corners of a tetrahedron. A better representation of the methane molecule is shown in Figure 17.1 “Three-Dimensional Representation of Methane.”

The next-largest alkane has two C atoms that are covalently bonded to each other. For each C atom to make four covalent bonds, each C atom must be bonded to three H atoms. The resulting molecule, whose formula is C₂H₆, is ethane:

Propane has a backbone of three C atoms surrounded by H atoms. You should be able to verify that the molecular formula for propane is C₃H₈:

The diagrams we have seen so far representing alkanes are fairly simiple Lewis structures. However, as molecules get larger, the Lewis structures become more and more complex. One way around this is to use a condensed structure,which lists the formula of each C atom in the backbone of the molecule. For example, the condensed structure for ethane is CH₃CH₃, while it is CH₃CH₂CH₃ for propane. Table 17.1 “The First 10 Alkanes” gives the molecular formulas, the condensed structural formulas, and the names of the first 10 alkanes.

Because alkanes have the maximum number of H atoms possible according to the rules of covalent bonds, alkanes are also referred to as saturated hydrocarbons.

Alkenes have a C–C double bond. Because they have less than the maximum number of H atoms possible, they are called unsaturated hydrocarbons. The smallest alkene—ethene—has two C atoms and is also known by its common name, ethylene:

The next largest alkene—propene—has three C atoms with a C–C double bond between two of the C atoms. It is also known as propylene:

What do you notice about the names of alkanes and alkenes? The names of alkenes are the same as their corresponding alkanes except that the suffix (ending) is –ene, rather than –ane. Using a stem known as the parent chain to indicate the number of C atoms in a molecule and an ending to represent the type of organic compound is common in organic chemistry, as we shall see.

With the introduction of the next alkene, butene, we begin to see a major issue with organic molecules: choices. With four C atoms, the C–C double bond can go between the first and second C atoms or between the second and third C atoms:

(A double bond between the third and fourth C atoms is the same as having it between the first and second C atoms, only flipped over.) The rules of naming in organic chemistry require that these two substances have different names. The first molecule is named but-1-ene, while the second molecule is named but-2-ene. The number between the parent-chain name and suffix is known as a locant, and indicates on which carbon the double bond originates. The lowest possible number is used to number a feature in a molecule; hence, calling the second molecule but-3-ene would be incorrect. Numbers are common parts of organic chemical names because they indicate which C atom in a chain contains a distinguishing feature. When the double bond (or other functional group) is located on the first carbon, it is common practice for some authors to leave out the locant. For example, if  butene were written without a locant, you should assume it refers to but-1-ene, not but-2-ene.

The compounds but-1-ene and but-2-ene have different physical and chemical properties, even though they have the same molecular formula—C₄H₈. Different molecules with the same molecular formula are called isomers. Isomers are common in organic chemistry and contribute to its complexity.

Alkynes, with a C–C triple bond, are named similarly to alkenes except their names end in –yne. The smallest alkyne is ethyne, which is also known as acetylene:

Propyne has this structure:

With butyne, we need to start numbering the position of the triple bond, just as we did with alkenes:

Benzene is an aromatic compound composed of six C atoms in a ring, with alternating single and double C–C bonds:

The alternating single and double C–C bonds give the benzene ring a special stability, and it does not react like an alkene as might be expected.

As fundamental as hydrocarbons are to organic chemistry, their properties and chemical reactions are rather mundane. Most hydrocarbons are nonpolar because of the close electronegativities of C and H atoms. As such, they dissolve only sparingly in H₂O and other polar solvents. Small hydrocarbons, such as methane and ethane, are gases at room temperature, while larger hydrocarbons, such as hexane and octane, are liquids. Even larger hydrocarbons, like hentriacontane (C₃₁H₆₄), are solids at room temperature and have a soft, waxy consistency.

Hydrocarbons are rather unreactive, but they do participate in some classic chemical reactions. One common reaction is substitution with a halogen atom by combining a hydrocarbon with an elemental halogen. Light is sometimes used to promote the reaction, such as this one between methane and chlorine:

Halogens can also react with alkenes and alkynes, but the reaction is different. In these cases, the halogen molecules react with the C–C double or triple bond and attach onto each C atom involved in the multiple bonds. This reaction is called an addition reaction. One example is

The reaction conditions are usually mild; in many cases, the halogen reacts spontaneously with an alkene or an alkyne.

Hydrogen can also be added across a multiple bond; this reaction is called a hydrogenation reaction. In this case, however, the reaction conditions may not be mild; high pressures of H₂ gas may be necessary. A platinum or palladium catalyst is usually employed to get the reaction to proceed at a reasonable pace:

By far the most common reaction of hydrocarbons is combustion, which is the combination of a hydrocarbon with O₂ to make CO₂ and H₂O. The combustion of hydrocarbons is accompanied by a release of energy and is a primary source of energy production in our society (Figure 17.2 “Combustion”). The combustion reaction for gasoline, for example, which can be represented by C₈H₁₈, is as follows:

2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O + ~5060 kJ