• This Reaction is named after three chemists, The German chemists
Carl Magnus Von Hell and Jacob Volhard and the Russian Chemist
• So this reaction is known as Hell-Volhard-Zelinsky reaction or HVZ
• Von Hell synthesize the Paraffin hydrocarbon (𝐶60 𝐻122), which is the
highest alkane known at that time.
• Volhard synthesize Sacrosine, guanidine and creatine.
When a carboxylic acid is treated with PBr3 and Br2 ,
the alpha carbon can be brominated. This halogenation reaction is
called the Hell–Volhard–Zelinski reaction or, more simply, the HVZ
A base will remove a proton from the OH group instead
of from the alpha carbon, since the OH group is more acidic.
Carboxylic acids cannot undergo substitution
reactions at the alpha carbon. Why?
𝛼 − substitution occurs because an acyl bromide,
rather than a carboxylic acid, is the compound that undergoes alpha
• PBr3 converts the carboxylic acid into an acyl bromide , which is in
equilibrium with its enol.
• Bromination of the enol forms a protonated alpha brominated acyl
bromide, which is hydrolyzed to a alpha brominated carboxylic acid.
• 𝑃𝐵𝑟3 replaces the carboxylic OH with a Bromide.
fast and reversible
• The acyl Bromide then Tautomerize to an enol.
• Enol form undergo rapid bromination at the 𝛼 carbon.
Reaction conditions are relatively harsh, involving high temperatures (usually above 100
degree C) and extended reaction times.
Conducting the reaction at too high a temperature may result in the elimination of hydrogen
halide from the product resulting in the formation of α,β-unsaturated carboxylic acids.
Usually less than one equivalent of P or PX3 catalyst is needed.