The Hell-Volhard-Zelinsky reaction makes it possible to halogenate the a-position of carboxylic acids. Phosphorus-catalyzed bromine is used as the reagent. Phosphorus in the presence of bromine generates phosphorus tribromide, which is actually the compound that acts as a catalyst.
![hell-volhard-zelinsky hell-volhard-zelinsky](/images/stories/organica-ii/acidos-carboxilicos/hell-volhard-zelinsky/hell-volhard-zelinsky-01.png)
The Hell-Volhard-Zelinsky mechanism occurs in four stages.
Stage 1. Formation of the alkanoyl bromide
![hell-volhard-zelinsky hell-volhard-zelinsky](/images/stories/organica-ii/acidos-carboxilicos/hell-volhard-zelinsky/hell-volhard-zelinsky-02.png)
Stage 2. Enolization of the alkanoyl halide
![hell-volhard-zelinsky hell-volhard-zelinsky](/images/stories/organica-ii/acidos-carboxilicos/hell-volhard-zelinsky/hell-volhard-zelinsky-03.png)
Stage 3. Halogenation of the position a
![hell-volhard-zelinsky hell-volhard-zelinsky](/images/stories/organica-ii/acidos-carboxilicos/hell-volhard-zelinsky/hell-volhard-zelinsky-04.png)
Stage 4. Exchange
![hell-volhard-zelinsky hell-volhard-zelinsky](/images/stories/organica-ii/acidos-carboxilicos/hell-volhard-zelinsky/hell-volhard-zelinsky-05.png)