19.15 : Preparação de Aminas: Redução de Oximas e Compostos Nitro

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.