Reduction of nitroalkane

Nitro compounds can be reduced to primary amines under a variety of conditions. The various reduction stages of the nitro group are given below:

reducion stages of nitrogroup

The final product depends on the nature of the reducing agent as well as the pH of the medium.

i) Catalytic reduction

The nitro group is easily reduced by catalytic hydrogenation using Pd/C catalyst in ethanol.

catalytic hydrogenation of nitro group

formation of benzenamine from nitrobenzene

ii) Reduction by metal in acidic solutions

Metals (Fe, Sn and Zn) and HCl are sued for reducing a nitro group (-NO2) to an amino group (-NH2).

formation of ethanamine from nitro ethene

formation of aniline from nitrobenzene

formation of  amine from nitrobenzene

iii) Reduction in neutral medium

Zinc dust and ammonium chloride convert nitro benzene to the corresponding hydroxylamine.

Baker - Mullikens test

The hydroxyl amines on warning with ammoniacal silver nitrate solution (Tollen's reagent) get oxidised to nitroso compound and reduce the Tollens reagent to metallic silver. This reaction is used as a test for nitro compounds and is called Baker - Mullikens' test.

iv) Reduction in alkaline medium

Depending upon the nature of the reducing agent, nitrobenzene forms different products.

formation of azobenzene from nitrobenzene

formation of azobenzene from nitrobenzene

v) Reduction with LiAlH4

Aliphatic nitro compounds are reduced to p-amines with LiAlH4.

reduction of aliphatic nitro compounds

Aromatic nitro compounds on reduction with LiAlH4 give azo compounds.

aromatic nitro compounds on reduction

Acidic nature of alpha hydrogen atom
Alpha hydrogen atom in case of aliphatic nitro compounds becomes acidic due to electron withdrawing nature of nitro group.

aliphatic nitro compounds becomes acidic

This nucleophic anion performs reactions with electrophiles such as acetaldehyde to form aldol condensation product which on dehydration gives unsaturated nitro compound.

formation of unsaturated nitro compound

Hydrolysis of aliphatic nitro compounds
Primary aliphatic nitro compounds can be converted to aldehydes by treatment of their carbanion salts with sulphuric acid.

formation of aldehydes from aliphatic nitro compounds

When primary nitro compounds are treated with H2SO4 without prior conversion to salts, they give carboxylic acids.

formation of carboxylic acids from nitro compounds

Secondary nitro alkanes hydrolyze with boiling HCl to give ketones and nitrous oxide.

formation of ketones from nitro alkanes