Beckmann rearrangement

The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853-1923), is an acid-catalyzed rearrangement of an oxime to an amide. Cyclic oximes yield lactams.



This example reaction starting with cyclohexanone and forming caprolactam is one of the most important applications of the Beckmann rearrangement, as caprolactam is the feedstock in the production of Nylon 6.

The Beckmann solution consists of acetic acid, hydrochloric acid and acetic anhydride, and was widely used to catalyze the rearrangement. Other acids, such as sulfuric acid or polyphosphoric acid, can also be used.

Reaction mechanism
The reaction mechanism of the Beckmann rearrangement is generally believed to consist of an alkyl migration with expulsion of the hydroxyl group to form a nitrilium ion followed by hydrolysis. In one study, the mechanism is established in silico taking into account the presence of solvent molecules and substituents. The rearrangement of acetone oxime in the Beckmann solution involves three acetic acid molecules and one proton (present as an oxonium ion). In the transition state leading to the iminium ion (σ - complex), the methyl group migrates to the nitrogen atom in a concerted reaction and the hydroxyl group is expulsed. The oxygen atom in the hydroxyl group is stabilized by the three acetic acid molecules. In the next step the electrophilic carbon atom in the nitrilium ion is attacked by water and the proton is donated back to acetic acid. In the transition state leading to the N-methyl acetimidic acid, the water oxygen atom is coordinated to 4 other atoms. In the third step, an isomerization step protonates the nitrogen atom leading to the amide.



The same computation with a hydroxonium ion and 6 molecules of water has the same result but when the migrating substituent is phenyl in the reaction of acetophenone oxime with protonated acetic acid the mechanism favors the formation of an intermediate three-membered π - complex. This π - complex is again not found in the H3O+(H2O)6.



With the cyclohexanone-oxime, the relief of ring strain results in a third reaction mechanism leading directly to the protonated caprolactam in a single concerted step without the intermediate formation of a π - complex or σ - complex.

Cyanuric chloride assisted Beckmann reaction
The Beckmann reaction is known to be catalyzed by cyanuric chloride and zinc chloride co-catalyst. For example, cyclododecanone can be smoothly convered to the corresponding lactam, a monomer for the production of nylon 12.



The reaction mechanism for this reaction is based on a catalytic cycle with cyanuric chloride activating the hydroxyl group via a nucleophilic aromatic substitution. The reaction product is dislodged and replaced by new reactant via an intermediate Meisenheimer complex.