The Hell-Volhard-Zelinsky (HVZ) reaction allows for the bromination of the alpha carbon of a carboxylic acid when treated with PBr3 and Br2, by first converting the acid to an acyl bromide. The reaction requires harsh conditions, including high temperatures and extended times, to favor the desired substitution. The process involves the formation of an enol that rapidly undergoes bromination before hydrolysis yields an alpha-brominated carboxylic acid.

1. Hell-Volhard-Zelinsky
Reaction
HVZ Reaction

2. Introduction
• This Reaction is named after three chemists, The German chemists
Carl Magnus Von Hell and Jacob Volhard and the Russian Chemist
Nikolay Zelinsky.
• So this reaction is known as Hell-Volhard-Zelinsky reaction or HVZ
reaction.
• Von Hell synthesize the Paraffin hydrocarbon (𝐶60 𝐻122), which is the
highest alkane known at that time.
• Volhard synthesize Sacrosine, guanidine and creatine.

3. Defination
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
reaction .
A base will remove a proton from the OH group instead
of from the alpha carbon, since the OH group is more acidic.

4. THE HELL–VOLHARD–
ZELINSKI REACTION
Carboxylic acids cannot undergo substitution
reactions at the alpha carbon. Why?

5. Hell-Volhard-Zelinsky Reaction
𝛼 − substitution occurs because an acyl bromide,
rather than a carboxylic acid, is the compound that undergoes alpha
substitution.

6. Hell-Volhard-Zelinsky Reaction
• 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.

7. Mechanism
+
𝛿 +
𝛿 −
𝛿 −
+
Br
+
−
−
+
• 𝑃𝐵𝑟3 replaces the carboxylic OH with a Bromide.

8. Mechanism
+
Br
+ Br
+
ߜ+
𝛿 −
𝛿 −
𝛿 +
+ −
−
+
fast and reversible
slow
• The acyl Bromide then Tautomerize to an enol.

9. Mechanism
• Enol form undergo rapid bromination at the 𝛼 carbon.
+
𝛿 −
𝛿 +
+
+

10. Reaction Conditions
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.