Beckmann Rearrangement

About Beckmann Rearrangement 

The acid-catalyzed conversion of ketoximes to N-substittued amides is known as Beckmann rearrangement. The reaction is catalyzed by acidic reagents like H₂SO₄, SOCl₂, SO₃, PCl₅, P₂O₅, C₆H₅SO₂Cl etc. The reaction involves the migration of a group from carbon to the electron deficient nitrogen.The migration of the groups does not depend on the migratory aptitude but upon the orientation of the group is relation to the -OH group. It is found that the migrating group is always anti (trans) to the hydroxyl group. Thus the reaction is stereospecific.

Reaction Mechanism of Beckmann Rearrangement

The mechanism of the above reaction has been outlined as

In strong acids, the reaction proceeds with the protonation of the OH group of the oxime with subsequent loss of water to yield(A) with electron deficient nitrogen. The migration of alkyl group then gives a carbocation, which is attacked by water molecule followed by the loss of proton gives the amide.
The migrating group retains its configuration and hence the migrating group does not become completely free during the migration, otherwise the reaction cannot be stereospecific. Thus the migration and breaking of N-O bond may be concerted (taking place in one step).For example,

Solved Example of Beckmann Rearrangement 

Questions based on Beckmann rearrangement 

List of Name Reaction of Organic Chemistry consist of detail Reaction Mechanism of all name reactions of Organic Chemistry.