Nucleophilic acyl substitution is an important class of substitution reactions involving a nucleophile and an acyl compound, such as carboxylic acids and their derivatives. In these reactions, the leaving group attached to the acyl group is substituted by a nucleophile. The general mechanism proceeds via two steps.

1. Addition: Nucleophilic attack at the carbonyl carbon, forming a tetrahedral intermediate.
2. Elimination: Departure of the leaving group.

The net outcome of the addition–elimination reaction is the conversion of one acyl derivative into another. The reaction can be carried out in the presence of an acid or a base catalyst depending on the reactivity of the acyl derivative.