homologous series

In chemistry, any of a number of series of organic compounds with similar chemical properties in which members can be described by a general formula and ofen differ by a constant relative molecular mass.

Alkanes (paraffins) form such series, which begins with methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), and pentane (C₅H₁₂), each member differing from the previous one by a CH₂ group (or 14 atomic mass units). The general formula for alkanes is C_nH_{2n + 2}.

Subsets of the alkenes (olefins) and alkynes (acetylenes) also form homologous series. With only a single instance of the defining double or triple bond, respectively, they will also gain 14 mass units with every new CH₂ group introduced. Their general formulae are C_nH_2n and C_nH_{2n - 2} respectively, starting at n=2 (ethene and ethyne). However, longer chain alkenes or alkynes may contain several double or triple bonds, requiring different formulae.

Other examples of homologous series include the alcohols, carboxylic acids, and aldehydes. Each of these is defined by the presence of a specific functional group which can be combined with various lengths of hydrocarbon chains.