In the first step, a combination of bromine and phosphorus tribromide (catalyst) is used in the Hell–Volhard–Zelinsky reaction to prepare 2-bromopropionic acid,[3] which in the second step is converted to a racemic mixture of the amino acid product by ammonolysis.
[4][5] The reaction is initiated by addition of a catalytic amount of PBr3, after which one molar equivalent of Br2 is added.
The acyl bromide tautomerizes to an enol, which reacts with the Br2 to brominate at the α position.
If an aqueous solution is desirable, a full molar equivalent of PBr3 must be used as the catalytic chain is disrupted.
If little nucleophilic solvent is present, reaction of the α-bromo acyl bromide with the carboxylic acid yields the α-bromo carboxylic acid and regenerates the acyl bromide intermediate.