Hunsdiecker Reaction: Synthesis of Alkyl Halides
The Hunsdiecker Reaction (also known as the Borodine-Hunsdiecker reaction) is a chemical reaction used to prepare alkyl halides (typically bromides or chlorides) from the silver salts of carboxylic acids via decarboxylation.
1. General Reaction
The silver salt of a carboxylic acid reacts with a halogen (Bromine or Chlorine) in the presence of Carbon Tetrachloride ($CCl_4$) to form an alkyl halide with one less carbon atom.
Conditions:
- Reactant: Silver carboxylate ($R-COOAg$).
- Reagent: Bromine ($Br_2$) or Chlorine ($Cl_2$).
- Solvent: Carbon Tetrachloride ($CCl_4$).
- Reactivity Order: $Br_2 > Cl_2 > I_2$.
2. Detailed Mechanism
The reaction proceeds via a Free Radical Mechanism.
Step 1: Formation of Acyl Hypohalite
The silver salt reacts with bromine to form an unstable acyl hypobromite intermediate.
Step 2: Homolytic Cleavage
The $O-Br$ bond undergoes homolysis to form a carboxyl free radical and a bromine radical.
Step 3: Decarboxylation
The unstable carboxyl radical loses $CO_2$ to form an alkyl free radical.
Step 4: Propagation
The alkyl radical attacks another molecule of acyl hypobromite to form the alkyl bromide and regenerate the carboxyl radical.
3. Examples
A. Synthesis of Methyl Bromide
Reaction of Silver Acetate with Bromine.
$$ CH_3COOAg + Br_2 \xrightarrow{CCl_4} CH_3Br + CO_2 + AgBr $$B. Synthesis of Ethyl Bromide
Reaction of Silver Propionate with Bromine.
$$ C_2H_5COOAg + Br_2 \xrightarrow{CCl_4} C_2H_5Br + CO_2 + AgBr $$4. Exception: The Simonini Reaction
What about Iodine?
When Iodine ($I_2$) is used instead of Bromine or Chlorine, the reaction does not yield alkyl iodides as the major product. Instead, it forms an Ester. This is known as the Simonini Reaction.
$$ 2R-COOAg + I_2 \rightarrow R-COO-R \text{ (Ester)} + 2AgI + CO_2 $$
5. Summary Table
| Halogen Used | Product | Reaction Name |
|---|---|---|
| $Cl_2$ | Alkyl Chloride | Hunsdiecker Reaction (Poor Yield) |
| $Br_2$ | Alkyl Bromide | Hunsdiecker Reaction (Good Yield) |
| $I_2$ | Ester ($RCOOR$) | Simonini Reaction |
Hunsdiecker Reaction Quiz
Test your understanding of decarboxylation. 10 MCQs with explanations.
No comments:
Post a Comment