Hell-Volhard-Zelinskii reaction nucleophilic acyl substitution

The Hell-Volhard-Zelinskii reaction is a bit more complex than it looks and actually involves substitution of an acid bromide enol rather than a carboxylic acid enol. The process begins with reaction of the carboxylic acid with PBr3 to form an acid bromide plus HBr (Section 21.4). The HBr then catalyzes enolization of the acid bromide, and the resultant enol reacts with Br2 in an cr-substitution reaction to give an cv-bromo acid bromide. Addition of water hydrolyzes the acid bromide in a nucleophilic acyl substitution reaction and yields the a-bromo carboxylic acid product. 

The first step in the Hell-Volhard-Zelinskii reaction takes place bet PBra and a carboxylic acid to yield an intermediate acid bromide plus] (Section 21.4). The HBr thus formed catalyzes enolization of the acid I mide, and the resultant enol reacts rapidly with Brg in an a-substittttif reaction. Addition of water results in hydrolysis of the a-bromo acid 1 mide (a nucleophilic acyl substitution reaction) and gives the n-bromo< boxylic acid product. 

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