JEE Chemistry Amines Notes Set 07

Preparation of Aniline

1. Reduction of nitrobenzene with Sn/HCl or Fe/HCl, or by catalytic hydrogenation (H₂ with Ni or Pd). (NH₄)₂S selectively reduces one nitro group of a dinitro compound.
2. Hofmann bromamide degradation: a primary amide RCONH₂ with Br₂/NaOH gives R–NH₂ with one carbon fewer (alkyl or aryl R).

Diazonium Salts

1. Diazotisation: aromatic primary amines with NaNO₂/HCl at 0-5 °C give arenediazonium salts, stable only at low temperature. Primary aliphatic amines give unstable diazonium ions that lose N₂ at once.
2. Replacement of N₂⁺: boiling water gives phenol; CuCl/HCl or CuBr/HBr (Sandmeyer) and Cu/HCl or Cu/HBr (Gattermann) give aryl halides; H₃PO₂ or ethanol replaces N₂⁺ by H; HBF₄ then heat gives the aryl fluoride.
3. Coupling: with phenols in mildly alkaline medium (pH 8-10) or with aromatic amines (weakly acidic medium) the diazonium ion forms coloured azo dyes.

Tests for Aniline

1. Carbylamine reaction: primary amines (aliphatic or aromatic) with CHCl₃ and alcoholic KOH on heating give foul-smelling isocyanides. Secondary and tertiary amines do not.
2. Azo dye test: diazotise, then couple with 2-naphthol in alkali to get an orange-red dye.
3. Bromine water gives a white precipitate of 2,4,6-tribromoaniline.