14.16 : הכנת אנהידרידים חומציים

After acid halides, acid anhydrides are the most reactive derivatives of carboxylic acid.

One of the main approaches for preparing acid anhydrides involves the treatment of acid chlorides with the sodium salt of carboxylic acids, where the carboxylate ion acts as a nucleophile.

The reaction mechanism proceeds in two steps. The first step is the nucleophilic attack by the carboxylate ion at the carbonyl carbon of the acid chloride, forming a tetrahedral intermediate.

Next, the carbonyl is re-formed with the loss of the chloride ion as a leaving group to give an acid anhydride as the final product.

This is a useful method to synthesize symmetrical or unsymmetrical anhydrides.

Another approach for preparing symmetrical or unsymmetrical anhydrides involves treating acid chlorides with carboxylic acids in the presence of pyridine.

Additionally, five or six-membered cyclic anhydrides can also be prepared by heating the corresponding dicarboxylic acids.