in HVZ,the presence of alpha hydrogen 2 d carboxylic acid is d main criteria..is dat d only criteria 4 any compound 2 undergo HVZ rxn??

Carboxylic acids do not undergo substitution reactions at the a-carbon because the OH group is much more acidic than the alpha-carbon. However, if a carboxylic acid is treated with PBr₃ and Br₂, then bromination at the alpha-carbon occurs. (Red phosphorus can be used in place of PBr₃ because P and excess Br₂ react to form PBr₃). This halogenations reaction is known as the Hell-Volhard-Zelinski reaction or, more simply, as the HVZ reaction. We will see when we look at the mechanism of the HVZ reaction that alpha-substitution occurs because an acyl bromide, rather than a carboxylic acid, undergoes a-substitution.

In the step of the HVZ reaction, PBr₃ coverts the carboxylic acid into an acyl bromide by a mechanism similar to the one by which PBr₃ converts an alcohol into an alkyl bromide. (In both reactions PBr₃ replaces an OH with a Br). The acyl bromide is in equilibrium with its enol. Bromination of the enol forms the alpha-brominated acyl bromide, which is hydrolyzed to the alpha-brominated carboxylic acid.