24E - Organic Chemistry (Mcmurry)

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.

Step-by-Step Solution

Answer

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.

 

 

Step 2: The mechanism of the given reaction

Hence, the mechanism of the equilibrium is