Organic Chemistry

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Reactions of Alkanes •

Radicals

vitamin C vitamin E

W

e have seen that there are three classes of hydrocarbons:alkanes, which

contain only single bonds; alkenes, which contain double bonds; and

alkynes, which contain triple bonds. We examined the chemistry of alkenes

in Chapter 4 and the chemistry of alkynes in Chapter 6. Now we will take a look at the

chemistry of alkanes.

Alkanesare called saturated hydrocarbonsbecause they are saturated with hy-

drogen. In other words, they do not contain any double or triple bonds. A few exam-

ples of alkanes are shown here. Their nomenclature is discussed in Section 2.2.

Alkanes are widespread both on Earth and on other planets. The atmospheres of

Jupiter, Saturn, Uranus, and Neptune contain large quantities of methane the

smallest alkane, which is an odorless and flammable gas. In fact, the blue colors of

Uranus and Neptune are due to methane in their atmospheres. Alkanes on Earth are

found in natural gas and petroleum, which are formed by the decomposition of plant

and animal material that has been buried for long periods in the Earth’s crust, an en-

vironment with little oxygen. Natural gas and petroleum, therefore, are known as

fossil fuels.

Natural gas consists of about 75% methane. The remaining 25% is composed of

small alkanes such as ethane, propane, and butane. In the 1950s, natural gas replaced

coal as the main energy source for domestic and industrial heating in many parts of the

United States.

Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated

into fractions by distillation. The fraction that boils at the lowest temperature (hydro-

(CH 4 ),

CH 3 CH 2 CH 2 CH 3

butane

CH 2 CH 3

CH 3

CH 3

ethylcyclopentane 4-ethyl-3,3-dimethyldecane trans-1,3-dimethyl-

cyclohexane

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