Organic functional groups are the section of the organic molecule which controls the reactivity of the molecule. For example the presence of multiple bonds between carbon atoms increases the molecule's reactivity. Functional groups undergo characteristic reactions, these occur regardless of the size of the molecule. The reactions of an organic compound are controlled largely by the functional group, or groups, it contains. For further information on each of these groups refer to their main entry in the book. Several functional groups may be contained in any one molecule, giving that molecule the properties of all of the functional groups present.

Molecules or atoms attached to functional groups act as modifiers of the functional group and they alter such characteristics as boiling point and melting point of the substance. The carbon-carbon single bonds and the carbon-hydrogen bonds are the unreactive parts of the molecule. Normally the designating letter R is given to the modifier, the alkyl group, including the carbon it is bonded to. So for example the alcohol has a general formula of ROH. Where two or more alkyl groups are present (e.g. an ether) the symbols R, R', R", etc. are used. These may be the same or different molecules. An alkyl group is a straight chain molecule, but if the modifier is in the form of a ring structure it is more properly called an aryl group. Homologous series are produced for each functional group. These show slight changes in characteristics as increasing numbers of carbon atoms are added as modifier groups.