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 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.
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.
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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