Oxidative Decarboxylation of Carboxylic Acids: Hunsdiecker Reaction
A reaction in which the silver salt of a carboxylic acid, prepared by treatment of the acid with silver oxide, is reacted with a halogen, usually bromine, is referred to as the Hunsdiecker reaction. In this reaction, the carboxyl function is substituted by bromine to form an alkyl bromide. Concomitantly, silver bromide precipitates and carbon dioxide is lost.
Alkyl bromides are obtained in moderate to good yields (65-85%).
The reaction is believed to follow a radical mechanism. In the initial, non-radical step, an intermediate acyl hypobromite is formed.
In the second step, cleavage of the hypobromite to a carboxyl radical (RCOO ·) occurs initiating the chain reaction.
The chain is propagated in the third and fourth step.
After losing carbon dioxide, the generated alkyl radical reacts with a new acyl hypochlorite molecule. Termination of the chain reaction can occur by a combination of two radicals, for example the bromine radical from step two and the alkyl radical generated in step three.
The key step in the Hunsdiecker reaction is the formation of the labile RCOO · radical which cleaves to an alkyl radical with generation of carbon dioxide.