Class 12 Chemistry Chapter 9 Amines – Revision Notes

Quick revision notes for Class 12 Chemistry Chapter 9 – Amines, covering classification, basicity trends, key name reactions, and diazonium salt chemistry. Ideal for last-minute board exam revision.

Last Updated: September 23, 2026

Classification and Structure

Amines (R–NH₂, R₂NH, R₃N) are classified as primary, secondary or tertiary based on the number of carbon groups attached to nitrogen — unlike alcohols/halides, this classification is based on substitution at N, not at a carbon. Nitrogen is sp³ hybridised with a pyramidal shape and one lone pair, which is responsible for both its basicity and its nucleophilicity.

Basicity Trends

In aqueous solution, basicity of simple alkyl amines follows: secondary>primary>tertiary>NH₃ — a balance between the electron-donating (+I) effect of alkyl groups (favouring more substitution) and steric hindrance to solvation of the ammonium ion (penalising the bulky tertiary amine). In the gas phase (no solvation effects), basicity increases steadily with alkyl substitution: tertiary>secondary>primary>NH₃. Aromatic amines are weaker bases than both aliphatic amines and ammonia, since the nitrogen lone pair is delocalised into the ring by resonance; electron-withdrawing ring substituents (–NO₂) weaken this basicity further, while electron-donating groups (–CH₃) strengthen it.

Key Name Reactions

Hofmann’s bromamide degradation: RCONH₂+Br₂+4NaOH→RNH₂+2NaBr+Na₂CO₃+2H₂O — the product amine has one carbon fewer than the starting amide. Gabriel phthalimide synthesis: potassium phthalimide+1° alkyl halide (S_N2)→N-alkylphthalimide→(hydrolysis)→pure primary amine; fails for aromatic amines since aryl halides cannot undergo S_N2. Carbylamine reaction: a primary amine+CHCl₃+alc. KOH, heat→a foul-smelling isocyanide — positive only for 1° amines. Diazotisation: ArNH₂+NaNO₂+2HCl at 273–278K→ArN₂Cl+NaCl+2H₂O; stable for aromatic amines, but aliphatic diazonium salts decompose instantly even in the cold.

Diazonium Salt Reactions

Displacement of N₂ (ring retained): Sandmeyer reaction (Cu₂Cl₂/Cu₂Br₂/CuCN→Ar–Cl/Ar–Br/Ar–CN), Gattermann reaction (Cu powder+HX, similar products without a cuprous salt), KI (direct iodination, no catalyst), Balz–Schiemann (HBF₄, then heat→Ar–F), H₃PO₂/H₂O (reduction to Ar–H), hydrolysis (H₂O/Δ→Ar–OH). Retention of N₂ (coupling reactions): with phenols (mildly alkaline medium) or aromatic amines (mildly acidic medium), giving brightly coloured azo compounds — the basis of azo dyes; coupling occurs mainly para to the activating group (ortho if para is blocked).

Identification and Distinguishing Tests

Hinsberg’s test (benzenesulfonyl chloride): 1° amine→sulfonamide soluble in KOH (has an acidic N–H); 2° amine→sulfonamide insoluble in KOH (no acidic N–H); 3° amine→no reaction, separates unreacted. Carbylamine test: positive only for 1° amines. Bromine water test: aniline gives an instant white precipitate of 2,4,6-tribromoaniline; benzylamine (whose –NH₂ is not conjugated to a ring) does not. Reaction with HNO₂: aliphatic 1° amines give brisk N₂ effervescence; aromatic 1° amines give a stable diazonium salt; 2° amines give a yellow oily N-nitrosamine; 3° amines simply form a salt.

Written by Satish

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