Nitro compounds can be reduced to primary amines under a variety of conditions. The various reduction stages of the nitro group are given below:
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.
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).
iii) Reduction in neutral medium
Zinc dust and ammonium chloride convert nitro benzene to the corresponding hydroxylamine.
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.
v) Reduction with LiAlH4
Aliphatic nitro compounds are reduced to p-amines with LiAlH4.
Aromatic nitro compounds on reduction with LiAlH4 give azo compounds.