Mechanism Of Hunsdiecker Reaction: carboxylicv acid to Halides

This reaction is also known as Hunsdiecker–Borodin reaction or Borodin reaction in organic chemistry. Hunsdiecker reaction is a chemical reaction involved functional group interconversion of a carboxylic acid (benzoic acid) into  an alkyl halide, usually a bromide or iodide,   involving the loss of one carbon atom, which is  eliminated as CO₂. chemical equation has shown above.

hunsdiecker reaction is given by German husband-and-wife   team of Heinz and Cläre Hunsdiecker in 1942. hunsdiecker reaction is a chemical reaction used for the preparation of alkyl halide. hunsdiecker reaction reagent are halides, and silver oxide.

Mechanism:

Hunsdiecker reaction mechanism involve free radicals. First of all, the silver carboxylate  must be made, which can be prepared by mixing   the carboxylic acid with silver oxide. Removal of  water leads to the isolation of the silver salt.

Silver  carboxylates react with halogens like bromine   to form acyl hypohalites, in this case the acyl  hypobromite. The driving force for the reaction   is provided by the formation of silver bromide,  AgBr, which is highly insoluble and precipitates.  Now, given that oxygen-bromine bonds are quite  weak, these acyl hypobromites tend to break.  

oxygen and bromine are highly  electronegative, and neither atom wants to   give up a pair of electrons, they split the pair evenly, and therefore both radicals will generate. Carboxylate radicals  slowly decarboxylate in the following manner,   thereby producing an alkyl radical. form the alkyl halide.

The reaction can produce   primary, secondary, or tertiary alkyl halides,  and also aryl bromides, via this mechanism.  

Side reaction:

Hunsdiecker Reaction, Side reaction  is electrophilic bromination of the benzene ring,   especially if the aromatic  ring is very electron rich referred to as   the Simonini reaction.

Application and Modification:

• Hunsdiecker Reaction have been many modifications of  this decarboxylative bromination. Some methods application even more toxic salts than silver, like mercury  or lead carboxylates. These alternatives are even   less practical than the original Hunsdiecker  protocol.
• As an example, let’s highlight the  work of Professor Chaozhong Li,   from the Shanghai Institute of Organic  Chemistry

Related Reaction:

1. Sandmeyer Reaction
2. Wohl-Ziegler bromination
3. Balz-Schiemann reaction
4. Tsuji-Wilkinson decarbonylation
5. Finkelstein reaction
6. wurtz reaction
7. Barton decarboxylation
8. Barton–McCombie deoxygenation

References:

1. Simonini Reaction – Major Reference Works
2. Professor Dave Explains
3. Hunsdiecker reaction – Wikipedia