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Hell-Volhard-Zelinsky Reaction

March 4, 2013

Hell-Volhard-Zelinsky Reaction is an organic reaction involves the halogenation of carboxylic acid at the α-carbon atom of the carboxylic group. This reaction is named after three chemist German chemists Carl Magnus von Hell, Jacob Volhard and Russian chemist Nikolay Dimitrievich Zelinsky.

In this reaction when bromine is treated with catalytic amount of phosphorus leads to selective α-bromination of carboxylic acid. This reaction is carried out in the absence of halogen carrier because this reaction is carried forward by catalytic amount of PBr3.

Hell-Volhard-Zelinsky Reaction

Mechanism of the reaction

  • In the first step bromine reacts with phosphorous to form phosphorous tribromide.
  • In the next step carboxylic acid is converted to acyl bromide.
  • This acyl bromide undergo Keto-Enol tautomerism and exist in enol form.
  • Enol form undergo rapid bromination at the α-carbon.

Hell-Volhard-Zelinsky Reaction machanism

Since the mono brominated compound is less nucleophilic the reaction stops at this stage. This acyl intermediate can undergo exchange of bromine with unreacted carboxylic acid which allows the reaction to completion.

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