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Why in Hell-Volhard-Zelinsky (HVZ) reaction the acid is first converted to acid bromide by using $\ce{PBr3}$ and then alpha halogenation is carried out?

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  • $\begingroup$ How do you think the alpha-bromine is incorporated into the acyl bromide? Homework??? $\endgroup$ – user55119 Feb 14 '18 at 15:23
  • $\begingroup$ Google is your friend! $\endgroup$ – Waylander Feb 14 '18 at 15:38
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The $\alpha$ halogenation occurs through the enol form.

The formation of the acyl halide is key because carboxylic acids do not form enols readily since carboxylic acid proton is removed before the $\alpha$ hydrogen. Acyl halides lack the carboxylic acid hydrogen.

The phosphorus halides form the acyl halides readily because phosphorus has a huge affinity for oxygen. Sometimes, phosphorus tribromide is not enough to form the acyl halide but, since there is also bromine in the media, phosphorus pentabromide is formed which can form acyl halides even of solid acids.

The halogenation is performed by the $\ce{X2}$, either bromine or chlorine. When $\ce{PBr3}$ and $\ce{Cl2}$ is used, the alpha-chloro acid is obtained and not the bromo acid.

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  • $\begingroup$ This was a question where AJ needed to make some effort in offering a solution. $\endgroup$ – user55119 Feb 14 '18 at 19:21
  • $\begingroup$ @user55119 Understood. However, I hope it will serve others as well. $\endgroup$ – Raoul Kessels Feb 15 '18 at 13:42

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