Hell-Volhard-Zelinsky reaction
Carboxylic acids with alpha hydrogen react with chlorine or bromine in the presence of red phosphorus to give alpha-halo acids.
Ethanoic acidCH₃COOHCarboxylic acid - Step 1
(i) Cl₂, red P (ii) H₂O
Chloroethanoic acidClCH₂COOHCarboxylic acid
How it works
- Red phosphorus and the halogen form PX₃, which converts the acid to the acyl halide.
- The acyl halide enolises and is halogenated at the alpha carbon.
- Water hydrolyses the acyl halide to the alpha-halo acid.
What it makes
Alpha-halo acids such as chloroethanoic acid.
Where it fails
The acid needs at least one alpha hydrogen.
Class 12, Chapter 8 Aldehydes, Ketones and Carboxylic Acids
Hell-Volhard-Zelinsky reaction: quick answers
What is the role of red phosphorus?
It forms PX3 with the halogen, which makes the acyl halide that is halogenated.
Where does halogenation occur?
At the alpha carbon.
Can methanoic acid or benzoic acid undergo HVZ?
No, they have no alpha hydrogen.