Hunsdiecker Reaction

Hunsdiecker Reaction is a chemical reaction that involves the silver salts of carboxylic acid reacting with halogens to form an unstable intermediate which further undergoes decarboxylation thermally leading to the formation of a final product known as alkyl halides.

Hunsdiecker Reaction

This reaction is also called Hunsdiecker–Borodin reaction or Borodin reaction. It is an example of both halogenation and decarboxylation reaction.

A Brief History

Alexander Borodin was the first to demonstrate this type of reaction in the year 1861 with the preparation of methyl bromide using silver acetate. The mechanism was also later applied by Angelo Simonini, a pupil of Austrian- Jewish chemist Adolf Lieben while experimenting with the degradation of fatty acids which included reactions between iodine and silver carboxylates. However, the name of the reaction has been specially kept after German chemist, Heinz Hunsdiecker and his wife Clare Hunsdiecker. They basically improved the reaction and their contributions led to it being the general method to produce organic halide.

Mechanism Of The Reaction

Hunsdiecker Reaction mechanism mainly involves organic radical intermediates where;

  • Formation of the reactive intermediate.
  • The occurrence of decarboxylation to form a diradical pair.
  • Recombination of reactant to form the desired product.

To break down the process further, the reaction starts by heating of silver carboxylate in CCl4 along with bromine. During this, the silver carboxylate is transformed into acyl hypobromite which is mainly due to the presence of bromine. Then precipitation of the stable silver bromide takes place. Consequently, a radical chain reaction takes place involving homolysis of the weaker oxygen-bromine bond. This results in the formation of a bromine atom and the carboxyl radical. This carboxyl radical the decarboxylates resulting in the formation diradical pair of a hydrocarbon radical or an alkyl radical which then recombines to form the desired halide which in this case is an alkyl bromide.

Mechanism Of Hunsdiecker Reaction

Some Variations Of The Reaction

Hunsdiecker reaction also has several variations. There are cases where silver(I) carboxylate is exchanged with thallium(I) carboxylate and the carboxylic acid is reacted or treated with halide ions of bromine, chloride or iodide and lead tetraacetate mainly to effect halogenation and decarboxylation. Similarly, in the preparation of 1-Bromo-3-chlorocyclobutane from 3-chlorocyclobutane carboxylic acid mercury oxide and bromide is used.

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