The Hunsdiecker Reaction is an organic reaction in which the silver salt of a carboxylic acid reacts with a halogen to form an alkyl halide. It is a type of decarboxylative halogenation because the carboxyl group is removed as carbon dioxide during the reaction. As a result, the alkyl halide product has one fewer carbon atom than the starting carboxylic acid derivative. ^[1–4]

Alkyl halides are useful intermediates in organic chemistry because they can undergo many reactions, including substitution and elimination reactions.

General Equation

To predict the product of a Hunsdiecker Reaction, remove the carboxyl carbon as carbon dioxide and attach the halogen to the remaining alkyl group. The general equation for this reaction is: ^[1,3]

RCOOAg + X₂ → RX + CO₂ + AgX

Where:

RCOOAg: Silver salt of a carboxylic acid

X: Halogens, such as Cl, Br, or I

X₂: Halogen molecule

RX: Alkyl halide product

CO₂: Carbon dioxide

AgX: Silver halide

Bromine is the most commonly used halogen in this reaction. 

Examples

1. Silver propionate contains three carbon atoms: CH₃CH₂COOAg. When it reacts with bromine, the carboxyl carbon is lost as carbon dioxide. The remaining two-carbon ethyl group then forms ethyl bromide.

CH₃CH₂COOAg + Br₂ → CH₃CH₂Br + CO₂ + AgBr

2. Silver acetate reacts with bromine to form methyl bromide.

CH₃COOAg + Br₂ → CH₃Br + CO₂ + AgBr

3. Silver butanoate reacts with bromine to form 1-bromopropane.

CH₃CH₂CH₂COOAg + Br₂ → CH₃CH₂CH₂Br + CO₂ + AgBr

Mechanism

The Hunsdiecker Reaction proceeds through a radical mechanism. ^[1–3]

Step 1: Formation of Acyl Hypobromite

The silver carboxylate reacts with bromine to form an acyl hypobromite intermediate. Silver bromide is also produced.

RCOOAg + Br₂ → RCOOBr + AgBr

Step 2: Homolytic Cleavage

The acyl hypobromite undergoes homolytic cleavage. In this process, a bond breaks in such a way that each atom takes one electron, forming radical species.

RCOOBr → RCOO· + Br·

Step 3: Loss of Carbon Dioxide

The carboxyl radical is unstable and loses carbon dioxide. It produces an alkyl radical with one fewer carbon atom than the original carboxylate.

RCOO· → R· + CO₂

Step 4: Formation of Alkyl Halide

The alkyl radical then reacts with a bromine radical or abstracts bromine from another bromine-containing species, forming the alkyl bromide.

R· + Br· → RBr

Therefore, the final product is an alkyl halide, not an acid bromide.

Limitations

The classic Hunsdiecker Reaction requires silver carboxylates, which can be costly and sometimes inconvenient to prepare. It also does not give equally good results with all substrates. Carboxylic acids that form complex or sterically hindered silver salts may produce lower yields. Since the reaction proceeds through radical intermediates, substrates containing double bonds or other reactive functional groups may also undergo side reactions. ^[3]