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Chemistry

Beckmann Rearrangement

Beckmann Rearrangement

Q: 4. Can you provide an example of a Beckmann rearrangement?

A classic example is the rearrangement of acetophenone oxime. When treated with a strong acid, the phenyl group, which is anti to the hydroxyl group, migrates to the nitrogen atom. After hydration and tautomerization, the final product formed is acetanilide, an important amide used in the synthesis of pharmaceuticals and dyes.

Reviewed by:

Ritika Singla

Beckmann rearrangement reaction is one of the important name reactions of organic chemistry. Ernst Otto Beckmann was a German Chemist who discovered the Beckmann rearrangement reaction. This is the reason the reaction is named after him and known as Beckmann rearrangement reaction.

In Beckmann rearrangement reaction oxime functional group (RR’C=NOH) is covered into amide (RC=ONR’R’’) in acid catalyzed conditions. The general form of the reaction is given below –

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The oxime is a chemical functional group belonging to imines. Oxime have a general formula RR’C=NOH where R is an alkyl group or organic side chain while R’ can be either hydrogen atom or another alkyl group or organic side chain. If R’ is a hydrogen atom in oxime, then it will form aldoxime and if it is another organic group (or alkyl group) then it will form ketoxime.

The amide is another organic compound of organic chemistry with the general formula RC(=O)NR’R’’ where R, R’, R’’ are either organic groups (or alkyl groups) or hydrogen atoms.

As the name suggests Beckmann rearrangement reaction is a type of rearrangement reaction. As you can observe in the general reaction above that R1 is attached to a carbon atom in the reactant while during the reaction rearrangement takes place and R1 gets attached to nitrogen atom.

Beckmann rearrangement has also been performed on haloimines and nitrones successfully. It results in lactams (a cyclic amide) when performed on haloimines and cyclic oximes.

When Beckmann rearrangement is performed on ketoximes then gives stereospecific product. Although Beckmann rearrangement reaction often takes place in presence of acidic – catalyst but some reagents also promote the rearrangement.

When Beckmann rearrangement is performed on aldoximes in gaseous phase then reaction occurs with stereospecificity while when it is performed on aldoximes in the solution phase then it occurs without stereospecificity.

Beckmann fragmentation is a different procedure or reaction than Beckmann rearrangement reaction. But in many processes Beckmann rearrangement and Beckmann fragmentation compete. Although specific promoting reagent and solvent can favor one over the other.

Caprolactam is used as raw material in the production of Nylon – 6. Caprolactam can be produced by Beckmann rearrangement reaction of cyclohexanone and oxime.

Acid catalytic solution which is used to catalyze Beckmann rearrangement is known as Beckmann solution and consists of acetic acid, hydrochloric acid and acetic anhydride. Apart from these the sulfuric acid is also commonly used.

Mechanism of Beckmann Rearrangement Reaction

Beckmann rearrangement reaction starts with the protonation of the alcoholic group of the oxime. Due to the protonation of the alcohol group a better leaving group is formed. The R group migrates to a nitrogen atom attached to the leaving group and a carbocation is formed with the release of a H2O molecule. Thus, formation of carbocation takes place by trans [1,2] – shift. Due to this regiochemistry of the reaction can be predicted. Now a water molecule attacks on the carbon atom of carbocation and through deprotonation and tautomerization, the final amide product is produced.

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Beckmann Rearrangement Reaction Assisted by Cyanuric Chloride

Beckmann rearrangement reaction can be carried out by using cyanuric chloride and zinc chloride as co-catalyst in the reaction. For example, the monomer unit of nylon 12 lactam can be produced by this type of Beckmann rearrangement using cyclododecanone as reactant. This reaction takes place by activation of hydroxyl group through aromatic nucleophilic substitution reaction by cyanuric chloride. Reaction is given below –

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Cyanuric chloride is a chemical compound with the formula (NCCl)3. Its structure is shown by the red color in the above reaction.

Beckmann Fragmentation

We have already discussed in the first section that Beckmann fragmentation is different from Beckmann rearrangement. So, let us discuss here what makes them two separate processes. Beckmann fragmentation takes place if a stable carbocation is formed. Various reaction conditions can also favor the Beckmann fragmentation pathway. For example, a quaternary carbon center promotes Beckmann fragmentation pathway as it stabilizes the carbocation formation through hyperconjugation. In the same way oxygen and nitrogen atoms also promote the fragmentation pathway. Sulfur and silicon are also capable of promoting the Beckmann fragmentation pathway.

Applications of Beckmann Rearrangement Reaction

It is used in various fields such as textile, pharmaceutical etc. Its few applications are listed below –

It is used in the production of the monomer unit of Nylon 12.
It is used in the production of raw material for Nylon 6. Caprolactam is used as raw material in the production of Nylon – 6. Caprolactam can be produced by Beckmann rearrangement reaction of cyclohexanone and oxime.
Drug paracetamol was developed by using Beckmann rearrangement at industrial level by Hoechst – Celanese. This process involves conversion of methyl ketone to acetanilide by Beckmann rearrangement reaction.
Androstenolone or DHEA can be synthesized by using Beckmann rearrangement.
It is also used for the production of benazepril, ceforanide, olanzapine, etazepine, enprazepine etc.
Schmidt reaction also involves Beckmann pathway.

This ends our coverage on the topic “Beckmann Rearrangement Reaction”. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.

FAQs on Beckmann Rearrangement

1. What is the Beckmann rearrangement?

The Beckmann rearrangement is an organic reaction that converts an oxime (a compound formed from a ketone or aldehyde) into an N-substituted amide. This transformation is typically catalysed by strong acids like sulfuric acid or phosphorus pentachloride. It is a fundamental name reaction in organic chemistry, known for its high efficiency and stereospecificity.

2. What is the general mechanism of the Beckmann rearrangement?

The mechanism involves several key steps that explain the conversion of an oxime to an amide:

Protonation: The acid catalyst protonates the hydroxyl (-OH) group of the oxime, turning it into a good leaving group (H₂O).
Rearrangement: The alkyl or aryl group that is anti-periplanar (on the opposite side) to the leaving group migrates to the nitrogen atom in a concerted step as the water molecule leaves. This forms a positively charged intermediate called a nitrilium ion.
Hydration: A water molecule attacks the electrophilic carbon of the nitrilium ion.
Tautomerization: A final deprotonation and rearrangement (keto-enol tautomerism) yields the stable N-substituted amide as the final product.

3. What reagents are commonly used to initiate a Beckmann rearrangement?

A variety of acidic reagents, known as Beckmann catalysts, can be used. The choice depends on the specific substrate and desired reaction conditions. Common examples include:

Protic acids: Concentrated sulfuric acid (H₂SO₄) and polyphosphoric acid (PPA).
Lewis acids: Thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), and phosphorus pentoxide (P₂O₅).
Other reagents: Acetic anhydride and hydrogen chloride gas.

4. Can you provide an example of a Beckmann rearrangement?

A classic example is the rearrangement of acetophenone oxime. When treated with a strong acid, the phenyl group, which is anti to the hydroxyl group, migrates to the nitrogen atom. After hydration and tautomerization, the final product formed is acetanilide, an important amide used in the synthesis of pharmaceuticals and dyes.

5. Why is the stereochemistry of the starting oxime so important in this reaction?

The stereochemistry is critical because the Beckmann rearrangement is stereospecific. Only the group that is in the anti-position (180° opposite) to the hydroxyl (-OH) group on the C=N bond is the one that migrates. The group in the syn-position (on the same side) does not migrate. This predictable behaviour allows chemists to control which group moves, leading to a specific amide product, which is a key reason for the reaction's synthetic utility.

6. What is the most significant industrial application of the Beckmann rearrangement?

The most important industrial application is the synthesis of caprolactam from cyclohexanone oxime. Caprolactam is the monomer required for the large-scale production of Nylon 6, a major synthetic polymer used globally to make textiles, carpets, industrial yarns, and engineering plastics. This single application highlights the reaction's immense economic and industrial importance.

7. How does the Beckmann rearrangement differ from the Schmidt rearrangement?

While both reactions can produce amides, they differ in their starting materials and mechanism. The Beckmann rearrangement starts with an oxime and an acid catalyst. In contrast, the Schmidt rearrangement starts with a ketone or carboxylic acid, which reacts directly with hydrazoic acid (HN₃) in the presence of an acid catalyst. The key intermediate and the specifics of the migrating group's origin are different, even though the final amide product can sometimes be the same.