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Hell volhard-zelinski reaction

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Hell volhard-zelinski reaction

  1. 1. Hell-Volhard-Zelinsky Reaction HVZ Reaction
  2. 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. 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. 4. THE HELL–VOLHARD– ZELINSKI REACTION Carboxylic acids cannot undergo substitution reactions at the alpha carbon. Why?
  5. 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. 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. 7. Mechanism + 𝛿 + 𝛿 − 𝛿 − + Br + − − + • 𝑃𝐵𝑟3 replaces the carboxylic OH with a Bromide.
  8. 8. Mechanism + Br + Br + ߜ+ 𝛿 − 𝛿 − 𝛿 + + − − + fast and reversible slow • The acyl Bromide then Tautomerize to an enol.
  9. 9. Mechanism • Enol form undergo rapid bromination at the 𝛼 carbon. + 𝛿 − 𝛿 + + +
  10. 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.

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