Hell volhard-zelinski reaction

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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 ( ), 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 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 +       + Br +     + replaces the carboxylic OH with a Bromide.  

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

9. Mechanism Enol form undergo rapid bromination at the carbon.   +     + +

10. Reaction Conditions