A substitution reaction is any reaction in which an atom, ion or functional group in a molecule is substituted by another atom, ion or group. An example is the reaction in which the bromine atom in a bromomethane molecule is displaced by a hydroxide ion, forming methanol.
Figure 1: Example of a substitution reaction
Organic compounds with good leaving groups, such as alkyl halides, serve as excellent starting materials for substitution reactions.
Depending on the type of atom or group that acts as a substituent, substitution reactions generally fall into three classes: