A homologous series in organic chemistry is a group of organic compounds (compounds that contain C atoms) that differ from each other by one methylene (CH2) group. For example, methane, ethane, and propane are part of a homologous series. The only difference among these molecules is that they have different numbers of CH2 groups.
Each member of a homologous series is called a homologue, which can also be spelled ''homolog.'' For example, methane and ethane are homologues and belong to the same homologous series. They differ from each other by one CH2 group. The formula of methane is CH4 and the formula of ethane is C2H6.
The presence of a functional group such as alcohol decides the properties of the carbon compound, regardless of the length of the carbon chain. For example, the chemical properties of CH3OH, C2H5OH, C3H7OH and C4H9OH are all very similar.
The general formula for alkenes is CnH2n, where n = 2, 3, 4.
The general formula for alkanes is CnH2n+2 where n = 2, 3, 4.
The general formula for alkynes is CnH(2n-2) where n = 2, 3, 4.