Hell-Volhard-Zelinsky (HVZ) Reaction

$$\text{R-CH}_2\text{-COOH} \xrightarrow{\text{1. X}_2 \, / \, \text{Red Phosphorus}} \text{R-CHX-CO-X} \xrightarrow{\text{2. H}_2\text{O}} \text{R-CHX-COOH}$$

Concept: Selective halogenation specifically replacing a proton at the carbon position adjacent to the carboxylic acid group. Small amounts of red phosphorus react with the halogen to create a phosphorus trihalide, which converts the acid into an acyl halide intermediate that enolizes much faster than the parent acid.

Notation:

• $$\text{X}_2$$ = Strictly limited to elemental chlorine ($$\text{Cl}_2$$) or bromine ($$\text{Br}_2$$);
• $$\alpha\text{-hydrogen}$$ = The unique proton target required for this reaction to proceed.