AdiChemistry Home

NAMED ORGANIC REACTIONS

< Bamford-Stevens reaction  Table of contents Beckmann-Exercises >

BECKMANN REARRANGEMENT

* The Beckmann rearrangement is an acid catalyzed rearrangement of an oxime to an N-substituted amide. 

beckmann rearrangement 1-1

* Conc.H2SO4, HCl, PCl5,  PCl3, SOCl2, ZnO, SiO2, PPA (Poly phosphoric acid) etc., are commonly employed in Beckmann rearrangement.

* Mostly applied for ketoximes. 

* Aldoximes are less reactive. 

* Cyclic oximes give lactams (cyclic amides).

 

MECHANISM OF BECKMANN REARRANGEMENT

 * Initially the -OH group of the oxime is protonated. Then 1,2 shift of alkyl group (R1) onto electron deficient nitrogen and the cleavage of N-O bond occurs simultaneously. 

Always the alkyl group which is 'anti' to the -OH group on nitrogen undergoes 1,2 shift which indicates the concerted nature of the beckmann rearrangement.

beckmann rearrangement 1-2

Comment: The migration of hydrogen is seldom observed. Hence the N-unsubstituted amides cannot be obtained by beckmann rearrangement reaction.

 

ILLUSTRATIONS

1) Industrial conversion of cyclohexanone to caprolactam, which is used in the manufacture of Nylon-6, involves Beckmann rearrangement.

 beckmann rearrangement 1-3

 

2) The relative migratory aptitudes of different groups in Beckmann rearrangement is illustrated below.

beckmann rearrangement 1-4

The 1,2 shift  of phenyl group is faster than that of alkyl groups. It is due to formation of phenonium ion. Hence the anti isomer reacts faster than the syn isomer. 

beckmann rearrangement 1-5

Note: Learn how to assign syn and anti descriptors to oximes at Geometrical isomerism page.

3) An Abnormal Beckmann rearrangement occurs when the migrating group departs from the intermediate and thus by furnishing a nitrile. 

beckmann rearrangement 1-6

 

< Bamford-Stevens reaction  Table of contents Beckmann-Exercises >

 

Author: Aditya vardhan Vutturi Google+ profile