19.15:
Preparation of Amines: Reduction of Oximes and Nitro Compounds
Carbonyl compounds and hydroxylamines react to give oximes.
Oximes can be reduced to primary amines via hydrogenation, hydride reduction, or reduction using sodium metal.
Aromatic amines, like aniline, can be prepared by reducing nitroaromatics using catalytic hydrogenation or acidic reduction by active metal.
Catalytic hydrogenation is nonselective and, therefore, inefficient if the nitro compound contains other easily reducible functional groups.
In such cases, acidic reduction using SnCl2, followed by a basic workup, is employed to selectively reduce the –NO2 group to an –NH2 group in the presence of other groups.
A similar reduction of aliphatic nitro compounds gives aliphatic amines.
Since the reduction of the nitro group occurs under acidic conditions, a follow-up with a base is required to prevent protonation of the amino group.
A hydride reduction using LiAlH4 can convert oximes to amines, but nitroaromatics get reduced to azobenzenes.
The nitro group is, however, inert to NaBH4 reduction. To conclude, both metal hydride reducing agents are ineffective in reducing the –NO2 group to the –NH2 group.
19.15:
Preparation of Amines: Reduction of Oximes and Nitro Compounds
Oximes can be reduced to primary amines using catalytic hydrogenation, hydride reduction, or sodium metal reduction. The reduction of aliphatic and aromatic nitro compounds to primary amines takes place by either catalytic hydrogenation or by using active metals like Fe, Zn, and Sn in the presence of an acid.
Though catalytic hydrogenation can reduce nitrobenzenes, the reduction is nonselective in the presence of other functional groups. For instance, if nitrobenzene contains an aldehyde group, both nitro and aldehyde groups will get reduced.
Conversion of the nitro group to the amino group while retaining the aldehydic group can be achieved through acidic reduction using SnCl2, followed by treatment with base.
The reduction of nitrobenzene and its derivatives are useful in industries that manufacture various aniline dyes. This process also has medicinal importance—for example, the synthesis of topical anesthetic benzocaine.