17.6 The Beckmann Rearrangement

  Aldehydes and Ketones

The acid catalysed transformation of a ketoxime to an N-substituted amide is known as the Beckmann rearrangement.

The rearrangement is catalysed by a variety of acidic reagents such as ${H_3}P{O_2},{\text{ }}{H_2}S{O_4},{\text{ }}SOC{l_2},{\text{ }}PC{l_5},$ etc. and a simplified mechanism can be written in the following steps.

The rearrangement is highly stereospecific in that the migrating group always approaches the nitrogen atom on the side opposite to the oxygen atom.

In the above example, there is migration of the R group to the electron deficient nitrogen, which is anti or trans to the hydroxyl group. The exclusive migration of the anti group has been confirmed in a number of cases, most notably in the conversion of 2-chloro-5-nitro-benzophenone oxime to a chlorobenzanilide. The configuration of this particular oxime has earlier been established by its ready conversion of a nitro-substituted phenylbenzisoxazole, showing that the nitrated benzene ring lies on the same side of the C = N linkage as the –OH group. When this oxime undergoes Beckmann rearrangement, it is found that the phenyl group, rather than the nitrated benzene ring, migrates to the nitrogen.

Since it is always the anti group that migrates, it appears that either the breakage of the N – O bond and the group migration are syunchronous or these two steps follow each other quickly. The migration of the anti group is so much certain in the Beckmann rearrangement that it is normally possible to establish the configuration of a particular oxime by identifying its rearrangement products.

Another mechanistic feature of the rearrangement has been worked out by Kuhara and later by Chapman who established that the rearrangement does not take place in the oxime itself, but in its acyl derivative which ionizes and subsequently rearranges. The ionization of the acyl derivative has been shown to be the rate-controlling step of the rearrangement. Typically, the benzene sulphonic ester of benzophenone oxime undergoes rearrangement without any acid catalyst.

It is also observed that rates of isomerisation of various oxime esters run parallel to the strength of the esterifying acids which is found to be in the following order.

Finally, the carbon atom of the migrating group retains its configuration in the Beckmann rearrangement. By the following series of reactions. Kenyon and Young not only proved retention of configuration but also correlated Beckmann and Hofmann rearrangements. Optically active 3-heptylcarboxylic acid was converted to its amide and subjected to Hofmann rearrangement. In another set of experiment the same acid was converted into 3-phenylmethyl ketone and then the oxime of this ketone was rearranged by the Beckmann method. Both these rearrangement yielded the same optical isomer of 3-heptylamine.

Since the retention of configuration at the migrating carbon has already been confirmed in the Hofmann rearrangement, it therefore follows that the Beckmann rearrangement also takes place with retention of configuration. Apparently the migrating group never becomes free from the remainder of the molecule and it also appears reasonable to believe that the breakage of C – C bond and the formation of the new C – N bond take place synchronously.

However, Beckmann rearrangement as a whole is not an intramolecular process since the carbonium ion, once formed, may combine with a hydroxyl ion present in solution. The rearrangement of benzophenone oxime into benzanilide was studied in presence of water containing heavy oxygen $\left( {^{18}O} \right)$. The product incorporated the heavy isotope.

It is therefore evident that the Beckmann rearrangement does not proceed by an intramoelcular exchange of the hydroxyl and the phenyl group; rather it passes through a carbonium ion which accepts hydroxyl ion or a water molecule from the solvent. Thus there is complete loss of the original oxime oxygen from the molecule during the reaction. A reasonable mechanism for the Beckmann rearrangement may be summed up as under:

In the presence of strong acids, the reaction is initiated by protonation of the oxime followed by loss of water to yield the same species as obtained by the decomposition of ester.

Beckamnn rearrangement is not only confined to ketoximes, but some aldoximes have also been shown to undergo this rearrangement. Thus $syn - $ and $anti - $benzaldoximes undergo rearrangement under the influence of PPA.

The formation of benzamide in the rearrangement of the syn-isomer has been explained by the ready conversion of some of the syn-form into the anti which then rearranges with a hydrogen migration.

Beckmann rearrangement has also been used for the enlargement of rings. An example of considerable industrial importance involves the rearrangement of cyclohexanone oxime to ?-caprolactam used for the manufacture of nylon-6.