Beckmann Rearrangement

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 (RRC=NOH) is covered into amide (RC=ONRR’’) 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 RRC=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)NRR’’ 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. 

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