Beckmann rearrangement reaction, examples, mechanism, and its application in organic chemistry have been discussed here. Beckmann rearrangement reaction was first reported by Beckmann in 1886.
Beckmann rearrangement definition
Oximes undergo a rearrangement reaction to give amide in presence of an acid catalyst. Such reaction is known as Beckmann rearrangement. In general, acid-catalyzed isomerization of oximes to amides is known as Beckmann rearrangement.
How is oxime prepared? Actually, the oximes are generally prepared by reacting ketone with hydroxylamine. The formed oxime then undergoes a Backmann reaction to give amide.
Let’s see some general examples of such reactions.
Beckmann rearrangement reaction mechanism
Let’s see how the Beckmann rearrangement reaction proceeds. The mechanism of Beckmann rearrangement completes in the following steps.
Step 1: Protonation of oxime
The protonation at the oxygen atom of the oxime gives oxonium cation.
Step 2: Migration of an alkyl group
The alkyl group trans to the leaving group migrates followed by the elimination of a water molecule to form nitrilium cation.
Step 3: Hydrolysis of nitrilium cation
The nitrilium cation is hydrolyzed in a basic medium to give amide as the final product.
Note: Beckmann rearrangement also takes place in presence of PCl5. Let’s see how the reaction takes place in presence of PCl5.
Abnormal beckmann rearrangement
Certain oximes having quaternary carbon anti to the hydroxyl group undergo rearrangement to give nitriles instead of amides. Such reaction is termed as abnormal Beckmann rearrangement or Beckmann fragmentation.
The alkyl group of the nitrilium cation intermediate detaches to give nitrile as a product instead of amide.
The mechanism of abnormal Beckmann rearrangement completes in the following steps:
Benzophenone oxime beckmann rearrangement
When benzophenone oxime undergoes a Beckmann rearrangement reaction, N-phenyl benzamide is obtained as a product. Let’s see the reaction with the mechanism.
Beckmann rearrangement of acetophenone oxime
When acetophenone oxime undergoes a Beckmann rearrangement reaction, N-phenylacetamide is formed as a product.
Beckmann rearrangement of cyclohexanone oxime
Cyclohexanone oxime undergoes Beckmann rearrangement to give caprolactam as final products. It means when oximes of cyclic ketones undergo Beckmann rearrangement then lactams are formed. This can be explained in the following example.
The mechanism of this reaction can be given as:
Beckmann rearrangement applications
The application of Beckmann rearrangement reaction are listed below:
- It can be used for the preparation of isoquinoline.
- it can be used for the preparation of polymer such as perlon.
Beckmann rearrangement video
References:
- Morrison, R. T., & Boyd, R. N., Organic chemistry, Allyn and Bacon, Inc. 1987
- March, J., Advanced Organic Chemistry, Wiley Eastern Limited, 1986.
- Skyes, P., A Guide Book to Mechanism in Organic Chemistry, Second edition, Orient Longman Ltd., 1988
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