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.

Methyl bromide and a hydroxyl anion react together to form methanol and a bromide ion. In this substitution reaction the bromine in the reactant has been replaced by a hydroxyl group.

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:

  • Nucleophilic substitution where a nucleophile - or electron-pair donor - reacts with an electron-deficient atom to eject a leaving group.
  • Electrophilic substitution reactions see an electrophile displace a functional group. Hydrogen atoms typically act as the electrophile - but not always.
  • Radical substitution is a substitution reaction that involves free radicals as reactive intermediates.