We began our study of organic chemistry in Chapter 4 “Covalent Bonding and Simple Molecular Compounds” 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 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.

We introduced the three simplest alkanes—methane (CH₄), ethane (C₂H₆), and propane (C₃H₈)—in Chapter 4 “Covalent Bonding and Simple Molecular Compounds”, Section 4.6 “Introduction to Organic Chemistry”. They are shown again in Figure 12.1 “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. (For more information about the shape of molecules, see Chapter 4 “Covalent Bonding and Simple Molecular Compounds”, Section 4.5 “Characteristics of Molecules”.) An ordinary solid line indicates a bond in the plane of the page.

Figure 12.1 The Three Simplest Alkanes

Recall from Chapter 4 “Covalent Bonding and Simple Molecular Compounds”, Section 4.5 “Characteristics of Molecules” that the VSEPR theory correctly predicts a tetrahedral shape for the methane molecule (Figure 12.2 “The Tetrahedral Methane Molecule”).

Figure 12.2 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 “The First 10 Straight-Chain Alkanes”.

Consider the series in Figure 12.3 “Members of a Homologous Series”. 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 12.3 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: C_nH_{2n + 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.