15.11:
α-Bromination of Carboxylic Acids: Hell–Volhard–Zelinski Reaction
Hell–Volhard–Zelinsky reaction is a method to convert carboxylic acids bearing α hydrogen to α-brominated carboxylic acids in the presence of a mixture of phosphorus tribromide and bromine, followed by hydrolysis.
Phosphorus tribromide acts as a catalyst—it is either added directly or generated in situ by the reaction of red phosphorus with bromine.
The reaction begins with the carboxylic acid and the catalyst interacting to generate the acid bromide and HBr.
HBr then catalyzes the enolization of acid bromide to its enol form, both of which exist in equilibrium.
The enol form acts as a nucleophile that undergoes an α substitution reaction with bromine to form an α-bromo acid bromide.
This active intermediate, upon treatment with water, undergoes hydrolysis to produce the desired α-brominated acid.
If instead of water, alcohols or amines are used, the intermediate is converted to its respective α-brominated acid derivative.
15.11:
α-Bromination of Carboxylic Acids: Hell–Volhard–Zelinski Reaction
The method to achieve α-brominated carboxylic acids using a mixture of phosphorus tribromide and bromine is known as the Hell–Volhard–Zelinski reaction. The reaction is catalyzed by phosphorus tribromide, which can be used directly or produced in situ from red phosphorus and bromine. The mechanism comprises PBr3 catalyzed conversion of acid to acid bromide and hydrogen bromide. The acid bromide enolizes to its enol form in the presence of HBr. The nucleophilic enol attacks the bromine molecule to give an α-bromo acid bromide. The resulting molecule, upon subsequent hydrolysis, yields the desired α-bromo acid.