11.4 Alkanes and Alkyl Radicals

Alkanes are saturated hydrocarbons or paraffins (para means little, affin means affinity). Their general formula is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$$\mathrm{C_nH_{2n + 2}}$ , where 'n' is number of carbon atoms. In case of alkanes 'n' ranges from 1 to 40. In this way, alkanes form the most important homologous series of compounds.

Homologous Series

Organic compounds are divided into groups of compounds having similar chemical properties. Each group is known as a homologous series. Organic compounds of the same homologous series have the following properties in common:

1. All members of a series can be represented by a general formula for example general formulae of alkane, alkenes and alkynes are ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$$\mathrm{C_nH_{2n + 2}}$ , ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}$$\mathrm{C_nH_{2n}}$ and ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}-2}$$\mathrm{C_nH_{2n - 2}}$ , respectively.

2. Successive members of the series differ by one unit of ${-\mathrm{C}\mathrm{H}}_2$$\mathrm{- CH}_{2}$ - and 14 units in their relative molecular mass.

3. They have similar chemical properties (because they contain the same functional group).

4. There is a regular change in their physical properties; the melting and boiling points increase gradually with the increase of molecular masses.

5. They can be prepared by similar general methods.