Substitution reactions

The simple replacement of one atom or group of atoms in a molecule by a second atom or group of atoms is called a substitution reaction. An illustrative example is the conversion of benzyl bromide to benzyl alcohol, using a solution of sodium hydroxide in water.

In this reaction the bromine atom of the benzyl bromide has been replaced by the hydroxyl group of the sodium hydroxide. The displaced bromine atom joins with the sodium ion to form the inorganic by-product sodium bromide, but the focus in organic reactions is always on the changes that occur to the organic molecules. Substitution reactions can also lead to the formation of cyclic compounds, as in the production of a cyclic ether from a di-functional compound containing both a halide atom and a hydroxyl group.