Hell–Volhard–Zelinsky halogenation

The Hell-Volhard - Zelinsky reaction or Zelinsky reaction is a reaction in the name of organic chemistry. Designated in this by Carl Magnus von Hell (1849-1926) (published 1881), Jacob Volhard and Nikolaj Dimitrievic Zelinskij (extension of the reaction 1887) reaction is (usually bromine) replaces a hydrogen atom on the α - carbon atom of a carboxylic acid with a halogen.

Overview reaction

The carboxylic acid is treated with a halogen, and a catalytic amount of a phosphorus halide. This results in a α - halogen carboxylic acid. When bromine is used and a catalytic amount of PBr3, for example, is obtained an α - bromo carboxylic acid (2- bromocarboxylic acid ):

Mechanism

The mechanism is illustrated here with reference to the reaction of a carboxylic acid with phosphorous (III ) bromide and bromine.

The carboxylic acid 1 reacts with phosphorus tribromide ( 2) via an intermediate step to a Carbonsäurebromid 3 This tautomerization to the enol form 4 In the reaction with bromine is formed with elimination of hydrogen bromide an α - Bromcarbonsäurebromid 5 The carbonyl group in 5 is now attacked by a water molecule and re- formation of the C = O double bond 6 and the elimination of hydrogen bromide produced an α - bromo carboxylic acid 7

Applications

From α -halo carboxylic acid, diverse derivatives can be produced:

• α -hydroxy acids by a reaction with sodium hydroxide
• α -amino acids by reaction with ammonia
• Malonic acid derivatives by reaction with sodium cyanide and subsequent hydrolysis of the nitrile