Reactions of Carboxylic Acids
Analogous to the Kolbe electrolysis reaction, the Hunsdiecker reaction (after Heinz Hunsdiecker, born in 1904) is a decarboxylation with a radical intermediate. However, it is not an electrochemical reaction.
The Hunsdiecker reaction takes place when a silver carboxylate is heated in in the presence of bromine. In the initial step of the Hunsdiecker reaction, the silver carboxylate is converted into an acyl hypobromite through the action of bromine. The main driving force of this reaction step is the precipitation of the extremely poorly soluble and stable silver bromide. Subsequently, a radical chain reaction occurs. The chain initiation is the homolytic cleavage of the relatively weak oxygen-bromine bond. This yields a carboxyl radical and a bromine atom. Similar to the Kolbe electrolysis reaction, the carboxyl radical decarboxylates spontaneously. This is the first propagation step of the chain reaction. The resulting alkyl radical, or hydrocarbon radical, abstracts a bromine atom from a further acyl hypobromite molecule. In this second propagation step, an alkyl bromide - the product - is formed and a carboxyl radical is recovered, which then once again acts as a starting product of the first propagation step. Silver carboxylates, required as starting products for the Hunsdiecker reaction, may be obtained through conversion of the corresponding carboxylic acid with silver oxide.
There are several variations of the Hunsdiecker reaction that are known, in which silver(I) carboxylate is, for instance, exchanged for thallium(I) carboxylate, or in which the carboxylic acid is directly treated with a combination of lead tetraacetate and halide ions (chloride, bromide, or iodide). Through the application of these reagents carboxylic acids are converted into alkyl chlorides and alkyl iodides, as well.