Beckmann Rearrangement

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.

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.

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

Mechanism of this rearrangement is given below

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

Above two isomers for the unsymmetrical oxime are possible.  When these oximes are rearranged mixture of products are formed and the ratio in which they form is same as the isomer ratio in oxime.

Oximes derived from aldehydes are not good for Beckmann rearrangement because of the poor yields for primary amides.  Tosyl chloride forms oxime tosylate which eliminates the stable tosylate anion.  PCl₅ and SOCl₂ induce rearrangement by converting OH to a better leaving group.

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.