The Hell-Volhard-Zelinsky halogenation reaction was named after three chemists, Carl Magnus von Hell, Jacob Volhard, and Nikolay Zelinsky. This reaction is unlike other halogenation reactions because it takes place in the absence of a halogen carrier, instead, it is initiated by a catalytic amount of PBr3, after which one molar equivalent of Br2 is added.1
The PBr3 replaces the carboxylic OH with a Br, making a carboxylic acid bromide. The acyl bromide can be tautomerized to an enol, which will react with the Br2 to brominates a seond time at the α position.1 A nucleophilic acyl substitution reaction occurs when the hydrolysis of the α-bromo acyl bromide occurs spontaneously in neutral to acid aqueous solution and yields the α-bromo carboxylic acid. If little nucleophilic solvent is present, the reaction of the α-bromo acyl bromide with the carboxylic acid yields the α-bromo carboxylic acid product and creates the acyl bromide intermediate.1 Below is an example of the scheme of the Hell-Volhard-Zelinksy halogenation reaction.
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