14.16: Preparation of Acid Anhydrides
One of the methods for preparing symmetrical or unsymmetrical acid anhydrides involves the treatment of acid chlorides with the sodium salt of carboxylic acids. The reaction proceeds via a nucleophilic acyl substitution.
The carboxylate ion acts as a nucleophile that attacks the carbonyl carbon of the acid chloride to form a tetrahedral intermediate. Subsequently, the re-formation of the carbonyl group with the loss of the chloride ion as a leaving group leads to the formation of an acid anhydride as the final product.
An alternative method for preparing symmetrical or unsymmetrical acid anhydrides involves the treatment of acid chlorides with carboxylic acids in the presence of pyridine. The pyridine deprotonates the carboxylic acid and enhances its nucleophilicity.
Notably, symmetric anhydrides can be prepared by heating two carboxylic acid molecules, eliminating one equivalent of water. However, the method is limited to only acetic acid as most other acids would fail to survive the excess heat treatment.
The preparation of five or six-membered cyclic anhydrides involves heating the corresponding dicarboxylic acids.