3.4.1: Structures and Names of Alkanes
- Page ID
- 478505
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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 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).
Consider below the three simplest alkanes—methane (CH4), ethane (C2H6), and propane (C3H8) and they are shown again in Figure \(\PageIndex{1}\). You can observe a pattern in the bonds drawn in these compounds: all carbon atoms have bonds to four other atoms, and every hydrogen atom forms a bond to one other atom. This bonding pattern is observed for carbon and hydrogen for all organic compounds.
![](https://chem.libretexts.org/@api/deki/files/27398/12.1.jpg?revision=1)
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 \(\PageIndex{1}\).
Consider the series in Figure \(\PageIndex{3}\). The sequence starts with C3H8, and a CH2 unit is added in each step moving up the series. The members of this series have properties that vary in a regular and predictable manner. This sequence demonstrates the principle of homology which 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.
![12.3.jpg](https://chem.libretexts.org/@api/deki/files/27400/12.3.jpg?revision=1)
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.