➠The conversion of aldoxime and ketoxime to the corresponding amides in acidic medium is known as Beckmann rearrangement.
➠It is specially important in the industrial production of Caprolactum which is used as a monomer for the polymerization to a polyamide, for the production of synthetic fibre.
➠The stereochemical outcome of this rearrangement is predictable. The 'R group' anti to the leaving group on nitrogen will migrate. If the oxime isomerise under the reaction conditions, a mixture of two possible amides is obtained.
➠The hydrgen atom never migrates in this reaction, hence this method can never be used for the synthesis of an unsubstituted amide. Hence Beckmann rearrangement is used for the synthesis of N-substituted amides.
➠E.g.
➠Mechanism:-
➠In this rearrangement, any group whether the alkyl group or an aryl group anti to the leaving group will migrate with the electron pair. Hence, it doesn't depend upon the migratory aptitude.
➠Rearrangement is strictly intramolecular. Hence, no crossover products are obtained that means migrating group doesn't become free completely during migration.
E.g.
➠Evidence:-
➠In Beckmann rearrangement, the migration of a group towards the nitrogen atom is an intramolecular SN2. Hence, if the migrating group has a chiral center, then it will retain its chirality during the migration.
E.g.
➠Drawbacks of Beckmann rearrangement:-
➥Anti-syn isomerization of an oxime:- Some reaction conditions can lead to the anti-syn isomerization of an oxime in a faster than the Beckmann rearrangement. Hence, mixture of products is obtained. Many protic acids in low concentration allow the oxime isomerization which leads to the formation of a mixture of products. The reagents that have been found to be least lightly to cause isomerization are Al2O3, H2SO4, etc. Whereas PTSA in low concentration can lead to the oxime isomerization. E.g.
➥Beckmann Fragmentation:- Fragmentation reaction occurs if one the oxime substituent can give a very stable carbonation and that carbonation is separated at the intermediate stage. Migrating group will remain as a carbonation during migration if they are stable anion. The fragmentation are favoured by the reagents like PCl5, SOCl2 and strong acids whereas PTSCl in pyridine or in aqueous alkaline favours the rearrangement over fragmentation. Fragmentation mainly occurs in rigid systems or hindered molecules. E.g.
➠The hydrgen atom never migrates in this reaction, hence this method can never be used for the synthesis of an unsubstituted amide. Hence Beckmann rearrangement is used for the synthesis of N-substituted amides.
➠E.g.
➠Rearrangement is strictly intramolecular. Hence, no crossover products are obtained that means migrating group doesn't become free completely during migration.
E.g.
E.g.
➥Anti-syn isomerization of an oxime:- Some reaction conditions can lead to the anti-syn isomerization of an oxime in a faster than the Beckmann rearrangement. Hence, mixture of products is obtained. Many protic acids in low concentration allow the oxime isomerization which leads to the formation of a mixture of products. The reagents that have been found to be least lightly to cause isomerization are Al2O3, H2SO4, etc. Whereas PTSA in low concentration can lead to the oxime isomerization. E.g.
End
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