Question

In the Hell–Volhard–Zelinskii reaction, only a catalytic amount of PBr₃ is necessary because of the equilibrium below. Review the mechanism for the reaction of a carboxylic acid with thionyl chloride and propose a mechanism for the equilibrium.

Short answer

The lone pair of carboxylic acid oxygen attacks the acid bromide's carbonyl carbon. This attack yields an intermediate that eliminates a bromide ion. Then the bromide ion again attacks the intermediate and produces another intermediate. Lastly, the intermediate eliminates an acyl bromide and forms the desired products.

Step-by-step solution

Step 1:HVZ reaction

The lone pair of carboxylic acid oxygen attacks the acid bromide's carbonyl carbon. This attack yields an intermediate that eliminates a bromide ion. Then the bromide ion again attacks the intermediate and produces another intermediate. Lastly, the intermediate eliminates an acyl bromide and forms the desired products.

The mechanism of the given reaction

Hence, the mechanism of the equilibrium is