General characteristics of any homologous series

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• All the members have the same general formula, e.g. alkanes are all CnH2n þ 2 and alcohols CnH2n þ 1OH.
• Each member differs from its neighbour by the same increment of CH2.
• All have similar chemical properties.
• There is a progression of physical properties, e.g. melting points, which increase as the number of carbon atoms increases.
• All the higher members exhibit isomerism.

The thousands of alkanes exist because of the possible arrangement of the carbon atoms either in long chains or in shorter chains but with branching side arms. Some of the values of n can be as high as 200, or even run into thousands.

When two compounds in any homologous series have the same molecular formula but a different arrangement of atoms or groups of atoms, there is a possibility of a further type of isomerism called ‘positional’ or ‘structural’ isomerism. The bigger the molecule, the greater the possibilities of isomerism. Some theoretically possible
isomers might not actually exist, due to their impossibly contorted shapes.