Beckmann rearrangement

Theory and Defination :

The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is an acid-catalyzed rearrangement of an oxime to an amide. Most commonly used catalysts are Conc.H2SO4, HCl, PCl5, PCl3, SOCl2, ZnO, SiO2, PPA (Poly phosphoric acid). Aldoximes are less reactive than ketoximes. Cyclic oximes yield lactams.

General Reaction :

Mechanism :

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.

Where is it used?
This reagent is useful in ring enlargement of cyclic ketone. A very good example is the industrial conversion of cyclohexanone to caprolactam, which is used in the manufacture of Nylon-6, involves Beckmann rearrangement.

In Cyclic ring the mechanism will as shown below ;

Also relative migratory aptitude comes in place when there are two different groups as shown below.

Also relative migratory aptitude comes in place when there are three different groups as shown below

Certain ketoximes (oximes of alpha-diketones, alpha-keto acids, alpha-dialkylamino ketones, alpha-hydroxy ketones, beta-keto ethers) can be converted to nitriles by the action of proton or Lewis acids via fragmentation reactions, which are considered side reactions, often these are called as ‘abnormal’ or ‘second order’ Beckmann rearrangements.

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Application and Example :

1) Bromodimethylsulfonium Bromide-ZnCl2: A Mild and Efficient Catalytic System

2) Au/Ag-Cocatalyzed Aldoximes to Amides Rearrangement under Solvent- and Acid-Free Conditions