The reaction of metal carboxylate with a halogen, leading to a alkyl halide, carbon dioxide and a metal halide is generally referred to as the Husdiecker reaction. The husband and wife team of Heinz and Clare Hunsdiecker did not discover this reaction, but they popularized its use in organic synthesis. The reaction was first reported in 1861 by Alexander Borodine an Russian chemist, who prepared bromomethane from silver acetate and bromine.
The reaction is performed by adding halogen to the suspension of the acid salt in carbon tetrachloride uder anhydrous conditions. This reaction is equally applicable for preparing bridgehead halides as illustrated.
Hunsdiecker Reaction DefinitionBack to Top
Treatment of silver salt of acid with bromine in presence of carbon tetra chloride to give an alkyl or aryl bromide is known as Hunsdiecker reaction.
If the silver salt of a carboxylic acid is treated with bromine, the salt decarboxylateand a bromoalkane forms. This process is called the Hunsdiecker reaction. The decarboxylation reaction can also be carried ou using the carboxylic acid and mercury(II) oxide.
Hunsdiecker Reaction MechanismBack to Top
In the Hunsdiecker reaction, the silver salt of a carboxylic acid, prepared by treating the acid with silver oxide, is treated with a halogen. Bromine is the usual reagent but iodine may also be used. Carbondioxide is evolved and the corresponding alkyl halide is obtained usually in fair to good yield.
The reaction appears to proceed by a free radical path and may be formulated as follows.
Variations of Hunsdiecker ReactionBack to Top
Suitable substrates for the Hunsdiecker reaction are first of all aliphatic carboxylates. Aromatic carboxylates do not react uniformly. Silver benzoates with electron-withdrawing substituents react to the corresponding bromobenzenes, while electron donating substituents can give rise to formation of products where as aromatic hydrogen is replaced by bromine.
For example, the silver p-methoxybenzoate is converted to 3-bromo-4-methoxybenzoic acid in good yield.
The silver carboxylate employed as starting material can be prepared from the corresponding carboxylic acid and silver oxide. In order to achieve the conversion to an alkyl bromide through decarboxylation in good yield, the silver carboxylate must be sufficiently pure. Bromine is often used as reagent in the Hunsdiecker reaction, though chlorine or iodine may also be employed. As solvent carbon tetrachloride is most often used.
Hunsdiecker Reaction ExamplesBack to Top
The Hunsdiecker reaction is a free radical reaction for the synthesis of an alkyl halide. The starting material comes from the reaction of a silver carboxylate with a solution of a halogen in a solvent such as carbon tetrachloride.
The example of Hunsdiecker reaction is shown below.
Some additional examples are given below.