Passage
Preparation & Chemical reactions of amines
Preparation:
Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium, or platinum as catalyst. Nitriles on reduction with lithium aluminum hydride or catalytic hydrogenation produce primary amines. The Gabriel synthesis is used for the preparation of primary amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide.
Chemical reactions:
Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides, and esters by nucleophilic substitution reaction. This reaction is known as acylation. Both aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide give corresponding isocyanide. Benzenesulphonyl chloride also known as Hinsberg’s reagent reacts with primary and secondary amines to form sulphonamide. Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline. Aniline reacts with HNO3 in the presence of H2SO4 to give nitro derivatives. Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid or sulphanilic acid.
Which reducing agent is preferred for converting nitrobenzene into aniline?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Fe scrap and HCl
Nitro compounds are reduced to amines either by catalytic hydrogenation (\( \ce{H2} \) with finely divided Ni, Pd or Pt) or by a metal in acid (Sn + HCl or Fe + HCl):
\( \displaystyle \ce{C6H5NO2 + 6[H]} \xrightarrow{\ce{Fe}/\ce{HCl}} \ce{C6H5NH2 + 2H2O} \)
All of these methods convert nitrobenzene into aniline, but iron scrap and hydrochloric acid is the preferred method:
Iron scrap is cheap.
The \( \ce{FeCl2} \) formed is hydrolysed and releases HCl again, so only a small amount of acid is needed to start the reaction.
Why the other options are incorrect
Option (1) H2/Pd in ethanol — Incorrect. It does reduce nitrobenzene to aniline, but palladium is an expensive catalyst.
Option (3) Sn and HCl — Incorrect. It works too, but tin is costly and a large amount of acid is used up.
Option (4) H2/Ni — Incorrect. It also reduces the nitro group, but it needs hydrogen gas under pressure; it is not the method preferred here.
NCERT (2026-27 edition):
Preparation of Amines
