Enols Hell-Volhard-Zelinsky reaction

The bromine is introduced onto the a-carbon by treating the carboxylic acid with Br2 and a catalytic amount of PBr, in a process known as the Hell-Volhard-Zelinsky reaction. This reaction proceeds through an enol intermediate. Because carboxylic acids form enols only with difficulty, a catalytic amount of PBr3 is added to form a small amount of the acyl bromide, which enolizes more readily than the acid. Addition of bromine to the enol produces an a-bromoacyl bromide (see Section 20.2). This reacts with a molecule of the carboxylic acid in a process that exchanges the Br and OH groups to form the... 

Show all of the steps in the mechanism for the acid-catalyzed enolization of the acyl bromide in the Hell-Volhard-Zelinsky reaction ... 

Hell-Volhard-Zelinsky reaction. a-Haloge-nation of carboxylic acids with halogen and phosphorus, presumably involving the enol form of the intermediate acyl halide. 

This method of a bromination of carboxylic acids is called the Hell-Volhard-Zelinsky reaction. This reaction is sometimes carried out by using a small amount of phosphorus instead of phosphorus trichloride. Phosphorus reacts with bromine to yield phosphorus tribromide as the active catalyst under these conditions. The catalyst, PCI3 or PBr3, acts by converting some of the carboxylic acid to the corresponding acyl halide, which has a higher enol content than the parent carboxylic acid. 

Because the enol content of carboxylic acids is much lower than that of aldehydes and ketones, carboxylic acids do not react with halogens to give a-halo carboxylic acids. However, bromination does occur in the presence of a catalytic amount of PBt3. The reaction also occurs in the presence of a small amount of phosphorus, which reacts with bromine to generate PBr3 under the reaction conditions. Either variation of the method is called the Hell—Volhard—Zelinsky reaction. 

The carboxylic acid itself is not brominated in the Hell— Volhard—Zelinsky reaction. Rather, a small amount of the carboxylic acid is converted into an acid bromide, which has a substantially higher concentration of the enol tautomer than the carboxylic acid. 

The Hell-Volhard-Zelinsky reaction converts carboxyiic acids to a-halo acids or a-haloesters, v/a the enol of an acyl halide. 

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