Hunsdiecker Reaction

Hunsdiecker Reaction

Bunsdiecker Reaction also known as Borodine-Hunsdiecker reaction. Hunsdiecker et al. (1935) found that silver salts of the carboxylic acids in carbon tetrachloride solution are decomposed by chlorine or bromine to form the alkyl halide, e.g.,  

RCO₂Ag   +   Br    →    RBr   +   CO₂   +   AgBr

The yield of halide is primary > secondary > tertiary, and bromine is generally used, chlorine giving a poorer yield of alkyl chloride. The mechanism is uncertain, but a favorable theory is that the first step is the formation of an acyl hypohalite which then decomposes into free radicals. The details of the later steps are less certain:

RCO₂Ag   +   Br₂       →           RCOOBr   +   AgBr

            RCOOBR       →           Br^.   +   RCOO^.          →          R^.   +   CO₂

R^.   +   Br₂      →          RBr   +   Br^.

R^.   +   RCOOBr         →          RBr   +   RCOO^.,       etc.

Cristol et al. (1963) have obtained very good yields of alkyl bromide by adding bromine to a refluxing carbon tetrachloride solution of the acid in the presence of an excess of red mercuric oxide.

2RCO₂H   +   2Br₂   +   HgO   →    2RBr   +   2CO₂   +   HgBr₂  

 Iodine forms esters with the silver slats; this is known as the Birnbaun-Simonini reaction (1892).

2RCO₂Ag   +    I₂    →    RCO₂R   +   CO₂   +   2AgI

Cristol et al. (1964), however, have shown that acids form alkyl iodides if iodine and red mercuric oxide are used.

On the other hand, Bachman et al. (1963) have shown that aliphatic carboxylic acids may be converted into esters and alkyl halides as follows:

RCO₂H    →   (RCO₂)₄Pb    (X₂)→   RCO₂R   +   2RX

Alkyl chlorides may be prepared without simultaneous ester formation by the action of lead tetra-acetate and lithium chloride on the carboxylic acid in benzene solution. This reaction is applicable to primary, secondary and tertiary acids, and occurs without rearrangement, e.g.,

Me₃CCH₂CO₂H    →    Me₃CCH₂Cl

The reaction is believed to proceed by a free-radical mechanism.