Beckmann Rearrangement: Mechanism and application

Table of Contents

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

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.

beckmann rearrangement

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.

beckmann rearrangement

Step 3: Hydrolysis of nitrilium cation

The nitrilium cation is hydrolyzed in a basic medium to give amide as the final product.

beckmann rearrangement

Note: Beckmann rearrangement also takes place in presence of PCl5. Let’s see how the reaction takes place in presence of PCl5.

beckmann rearrangement
beckmann rearrangement reaction mechanism

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:

abnormal beckmann rearrangement

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.

benzophenone oxime beckmann rearrangement

Beckmann rearrangement of acetophenone oxime

When acetophenone oxime undergoes a Beckmann rearrangement reaction, N-phenylacetamide is formed as a product.

acetonphenone
The mechanism can be expressed in a similar manner as described above, which is shown as:
beckmann rearrangement of acetophenone oxime

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.

Caprolactam

The mechanism of this reaction can be given as:

beckmann rearrangement of benzophenone oxime

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

If you have any confusion related to this topic, please comment down. If you want to learn about chromic acid oxidation reaction then click here.

Beckmann rearrangement

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