Hell-Volhardt-Zelinsky Reaction Definition:
The Hell-Volhardt-Zelinsky [HVZ] reaction is a means of converting a carboxylic acid to a brominated carboxylic acid (the bromine ends up on the α carbon of the carboxylic acid).
Hell-Volhardt-Zelinsky Reaction Explained:
The Hell-Volhardt-Zelinsky reaction is a reaction used to convert a carboxylic acid with α hydrogen into an α-halo carboxylic acid. The reaction is initiated by addition of a catalytic amount of PBr3, after which one molar equivalent of Br2 is added. Because the highly corrosive PBr3 is difficult to handle, it is often generated in the reaction flask. This is achieved by the addition of a little elemental phosphorus to the mixture of starting materials, it is converted into PBr3 instantaneously by the bromine present.
PHASE 1. Conversion of carboxylic acid to acyl bromide. The hydroxide is a bad leaving group but when oxygen attacks phosphorus it becomes a good leaving group. Actually, phosphorus tribromide works by turning the hydroxy group into a better leaving group. Because the P-O bond is formed, the P-Br bond is broken and the bromide ion is expelled. But in the next step, this bromide ion attacks the carbonyl carbon, π bond breaks, and the