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12: Organic Chemistry – Alkanes and Halogenated Hydrocarbons

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12.2: Structures and Names of Alkanes

Last updatedSep 28, 2020

12.1: Organic Chemistry

12.3: Branched-Chain Alkanes

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Learning Objectives

To identify and name simple (straight-chain) alkanes given formulas and write formulas for straight-chain alkanes given their names.

We begin our study of

organic chemistry

with the hydrocarbons, the simplest organic compounds, which are composed of carbon and hydrogen atoms only. As we noted, there are several different kinds of hydrocarbons. They are distinguished by the types of bonding between carbon atoms and the properties that result from that bonding. Hydrocarbons with only carbon-to-carbon single bonds (C-C) and existing as a continuous chain of carbon atoms also bonded to hydrogen atoms are called

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alkanes (or saturated hydrocarbons)

. Saturated, in this case, means that each carbon atom is bonded to four other atoms (hydrogen or carbon)—the most possible; there are no double or

triple bonds

in the molecules.

The word saturated has the same meaning for hydrocarbons as it does for the dietary

fats

and oils: the

molecule

has no carbon-to-carbon double bonds (C=C).

We previously introduced the three simplest alkanes—methane (CH4), ethane (C2H6), and propane (C3H8) and they are shown again in Figure 12.2.112.2.1.

Figure 12.2.112.2.1: The Three Simplest Alkanes

The flat representations shown do not accurately portray bond angles or molecular geometry. Methane has a tetrahedral shape that chemists often portray with wedges indicating bonds coming out toward you and dashed lines indicating bonds that go back away from you. An ordinary solid line indicates a bond in the plane of the page. Recall that the VSEPR

theory

correctly predicts a tetrahedral shape for the methane

molecule

(Figure 12.2.212.2.2).

Figure 12.2.212.2.2: The Tetrahedral Methane

Molecule

Methane (CH4), ethane (C2H6), and propane (C3H8) are the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH2

unit

. The first 10 members of this series are given in Table 12.2.112.2.1.Table 12.2.112.2.1: The First 10 Straight-Chain AlkanesmethaneCH4CH4—ethaneC2H6CH3CH3—propaneC3H8CH3CH2CH3—butaneC4H10CH3CH2CH2CH32pentaneC5H12CH3CH2CH2CH2CH33hexaneC6H14CH3CH2CH2CH2CH2CH35heptaneC7H16CH3CH2CH2CH2CH2CH2CH39octaneC8H18CH3CH2CH2CH2CH2CH2CH2CH318nonaneC9H20CH3CH2CH2CH2CH2CH2CH2CH2CH335decaneC10H22CH3CH2CH2CH2CH2CH2CH2CH2CH2CH375

Consider the series in Figure 12.2.312.2.3. The sequence starts with C3H8, and a CH2

unit

is added in each step moving up the series. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH2 group) is called a

homologous series

. The members of such a series, called homologs, have properties that vary in a regular and predictable manner. The principle of homology gives organization to

organic chemistry

in much the same way that the

periodic table

gives organization to

inorganic chemistry

. Instead of a bewildering array of individual carbon compounds, we can study a few members of a

homologous series

and from them deduce some of the properties of other compounds in the series.

Figure 12.2.312.2.3: Members of a

Homologous Series

. Each succeeding formula incorporates one carbon atom and two hydrogen atoms more than the previous formula.

The principle of homology allows us to write a general formula for alkanes: CnH2n + 2. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. For example, an alkane with eight carbon atoms has the molecular formula C8H(2 × 8) + 2 = C8H18.

Key Takeaway

Simple alkanes exist as a

homologous series

, in which adjacent members differ by a CH2

unit

.

Explanation:

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