Preparing amines by reduction and ammonolysis

Reduction of nitro compounds: hydrogen over finely divided Ni, Pd or Pt, or a metal in acid, turns nitrobenzene into aniline; nitroalkanes give alkanamines the same way.

Iron scrap with hydrochloric acid is the preferred metal–acid pair. The FeCl₂ formed hydrolyses and gives back HCl, so only a little acid is needed to start the reaction.

Ammonolysis: an alkyl or benzyl halide heated with ethanolic ammonia in a sealed tube at 373 K undergoes nucleophilic substitution, –NH₂ replacing the halogen.

The primary amine formed is itself a nucleophile, so it reacts with more halide to give secondary and tertiary amines and finally a quaternary ammonium salt. A strong base frees the amine from its ammonium salt.

Ammonolysis therefore gives a mixture; using a large excess of ammonia makes the primary amine the main product. Halides react in the order RI > RBr > RCl.

Reduction of nitriles with LiAlH₄ or by catalytic hydrogenation gives primary amines. Since a nitrile comes from an alkyl halide and cyanide, this route adds one carbon (ascent of the amine series).

Reduction of amides with LiAlH₄ also gives amines.

Worked: CH₃CH₂Cl with ethanolic NaCN gives propanenitrile, which is reduced to propan-1-amine. Benzyl chloride gives phenylethanenitrile, and H₂/Ni turns it into 2-phenylethanamine.