Beckmann Rearrangement

Beckmann Rearrangement

Session Objectives

By the end of this session, students will be able to:

• Steps involved in Beckmann rearrangement

• Mechanism of Beckmann rearrangement

PREPARATION OF BENZANILIDE FROM BENZOPHENONE (BECKMANN REARRANGEMENT)

• Aldehydes and ketones react with hydroxyl amines in the presence of sodium hydroxide to give an Oxime of low melting point (alpha or syn aldoxime). 

• It rapidly rearranges to give an isomeric Oxime of higher melting point by acids-(beta or anti aldoxime).

PREPARATION OF BENZANILIDE FROM BENZOPHENONE (BECKMANN REARRANGEMENT)

• Oximes of ketones undergo Beckmann rearrangement to substituted amides under the influence of variety of acidic reagents e.g HCOOH, PCl5, H2SO4, SOCl2, etc.

• In this rearrangement, the migrating group is always anti- to –OH group and thus this reaction is stereo specific.

• Here benzophenone oxime in presence of PCl5 rearranges to benzanilide.

Step involved in Beckmann Rearrangement

Reaction: Step-I: Preparation of benzophenone oxime

Preparation of benzophenone oxime

Step-II: Preparation of benzanilide from benzophenone oxime

Preparation of benzanilide from benzophenone oxime

Mechanism of Beckmann Rearrangement

MECHANISM: - Step-I: Preparation of benzophenone oxime

Preparation of benzophenone oxime

Step-II: Preparation of benzanilide:

Preparation of benzanilide

Summary

• Aldehydes and ketones react with hydroxyl amines in the presence of sodium hydroxide to give an Oxime of low melting point (alpha or syn aldoxime). 

• This is stable in alkali but rapidly rearranges to give an isomeric Oxime of higher melting point by acids-(beta or anti aldoxime).

• Oximes of ketones undergo Beckmann rearrangement to substituted amides under the influence of variety of acidic reagents e.g HCOOH, PCl5, H2SO4, SOCl2, etc.

• In this rearrangement, the migrating group is always anti- to –OH group and thus this reaction is stereo specific.

• Here benzophenone oxime in presence of PCl5 rearranges to benzanilide.

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