Hell–Volhard–Zelinsky reaction template

PBr₃ makes the acyl bromide, which enolises; the enol attacks Br₂ and hydrolysis gives 2-bromopropanoic acid.

Mechanism step by step

α-Bromination of a carboxylic acid through its acyl bromide enol

1. Protonation – PBr₃ has made the acyl bromide; HBr protonates its C=O
2. Enol – Loss of an α-H
3. α-Bromination – The enol attacks Br₂
4. Deprotonation – The α-bromo acyl bromide

Key points

• Carboxylic acids barely enolise; the acyl bromide enolises readily.
• The α-bromo acyl bromide swaps Br for OH with more acid, so PBr₃ is catalytic.
• Water at the end hydrolyses the acyl bromide to the acid (addition–elimination).
• The α-bromo acid is a route to α-amino acids (NH₃) and α-hydroxy acids.

Drawn with 66 atoms, 15 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Hell, Ber. Dtsch. Chem. Ges. 1881, 14, 891 Original paper
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook