Hell–Volhard–Zelinsky reaction: practice and mechanism
PBr₃ converts the acid into an acid bromide, which enolises easily. The enol is brominated at the α-carbon; hydrolysis gives the α-bromo carboxylic acid.
Example
Carboxylic acids themselves hardly enolise. PBr₃ turns them into the acyl bromide, which enolises easily and is brominated at the α carbon. Water then gives the α-bromo carboxylic acid.
Mechanism step by step
- 1
PBr₃ converts the carboxylic acid into the acid bromide R–CH₂–COBr.
- 2
The acid bromide enolises much more easily than the acid; the enol attacks Br₂: α-bromo acid bromide.
- 3
Water hydrolyses the acid bromide to the α-bromo carboxylic acid.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- The acid bromide is only an intermediate. After bromination at the α-carbon, water hydrolyses it back to the carboxylic acid.
- One equivalent of Br₂ replaces exactly one α-H. A second Br would need additional Br₂.
Often confused with: α-Bromination of ketones (acidic) · Acyl chloride from carboxylic acid
Frequently asked questions
Why does the acid itself not react?
Carboxylic acids barely enolise because the OH group stabilises the carbonyl group by resonance. The acid bromide enolises easily.
What are α-bromo acids used for?
With NH₃ they give α-amino acids (SN2), with OH⁻ α-hydroxy acids.