Hunsdiecker Reaction
Hunsdiecker reaction is also known as Borodin reaction or the Hunsdiecker–Borodin reaction in which silver salts of carboxylic acids react with a halogen (mainly bromine) to produce an organic halide. The product has one fewer carbon atoms than the starting carboxylic acid (lost as carbon dioxide) and a halogen atom is introduced its place. This reaction is an example of both halogenation and decarboxylation reactions.
In Hunsdiecker reaction, chlorine is not used because, the yield of alkyl chloride is very poor due to the reactivity of chlorine. Dichloro and trichloroalkanes may be formed in addition to alkyl chloride.
Mechanism of Hunsdiecker Reaction
Hunsdiecker reactiona takes place in three steps.
1. Formation of silver carboxylate from the carboxylic acid
2. Decarboxylation, which generates a carboxyl radical and an alkyl radical
3. Radical recombination tp forming the desired halogenated product and carbon dioxide.
The 1:1 ratio of silver salt and iodine gives the alkyl iodide. However, an ester is formed when the reaction is carried out with a 2:1 ratio of silver carboxylate and iodine. This is called as Simonini reaction.
The formation of ester is due to the alkyl iodide formed in the Hunsdiecker reaction reacts with the excess silver carboxylate salt and form the ester.
2R—COOAg + I2 → R—CO—OR + 2AgI + CO2
Whta is the role of CCl4 in Hunsdiecker reaction?
Solution:
CCl4 is a nonpolar covalent molecule so, bromine is easily soluble in CCl4 than in water. The reaction takes place in a neutral, nonpolar environment which is provided by CCl4. As a result, it acts as a very good solvent and ensure that the reaction runs smoothly.
Hunsdiecker reaction follows free radical mechanism. Though 1° radical is less stable, the yield of alkyl halide follows 1° > 2° > 3°. Why?
Solution:
This is because, 1° radicals are the most reactive and have the least steric hindrance in the second propagation step (attack on the acyl hypobromite). 2° and 3° radicals can sometimes rearrange or form side products, which lowers the yield of the desired alkyl halide.
