A homologous series is a group of compounds that differ by a constant unit, generally a methylene (−CH2−) group.
The reactants undergo a homologation when the number of a repeated structural unit in the molecules is increased.
The most common homologation reactions increase the number of methylene (−CH2−) units in saturated chain within the molecule.
[1] For example, the reaction of aldehydes or ketones with diazomethane or methoxymethylenetriphenylphosphine to give the next homologue in the series.
Likewise the chain length can also be reduced: