Reactions of Carboxylic Acids
The treatment of carboxylic acids with bromine and phosphorus tribromide () or a mixture of bromine and phosphorus, yields the α-bromocarboxylic acid. Phosphorus tribromide is often applied only as a catalyst. Thus, if chlorine and are applied, chlorination occurs instead of bromination.
The reaction is known as the Hell-Volhard-Zelinskii reaction (after J. Volhard, C.M. Hell, and N.D. Zelinskii). In the frist reaction step, converts the carboxylic acid into the corresponding acyl bromide. The acyl bromide is in equlibrium with the corresponding enol. A nucleophilic attack of the enol π bond on a bromine molecule yields HBr and an α-bromoacyl bromide, which may be isolated.
Analogous to acyl halides, α-bromoacyl halides may be converted into α-bromoesters, α-bromocarboxamides, and "back" to the α-bromocarboxylic acid. However, their α bromine atom may also be exchanged for another nucleophile in an reaction. The α-cyanocarboxylic acids and α-aminocarboxylic acids are, for example, available in this way.
If catalytic amounts of are applied, the intermediate α-bromoacyl bromide is attacked by another carboxylic acid molecule, yielding the corresponding anhydride. The bromide that is released during anhydride formation, then cleaves the anhydride into an α-bromocarboxylic acid - the product - and an acyl bromide. The acyl bromide reenters the reaction cycle.