Functional Group

Summary: An atom or group of atoms that gives a homologous series its characteristic chemical reactions. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-14

A functional group is a specific atom or characteristic group of atoms within an organic molecule that is largely responsible for the molecule’s chemical properties and the reactions it undergoes. Compounds that share the same functional group are classified into the same homologous series and exhibit similar chemical behaviour, regardless of the length of their carbon chain. The key functional groups tested at IGCSE level are: the carbon-carbon double bond (C=C) in alkenes, which undergoes addition reactions such as bromination; the hydroxyl group (—OH) in alcohols, which enables oxidation and esterification; the carboxyl group (—COOH) in carboxylic acids, which makes them weak acids that react with metals, bases, and carbonates; and the ester linkage (—COO—) in esters, which gives them their characteristic fruity smells. Identifying the functional group present in a structural formula allows a chemist to predict the compound’s chemical reactions — for example, any molecule containing the C=C bond will decolourise bromine water in an addition reaction. IGCSE exam success depends on being able to recognise and name these functional groups from displayed or structural formulae and to understand that the functional group, not the length of the carbon chain, determines the homologous series to which a compound belongs.

Key term — IGCSE Chemistry (0620/0971) glossary.