Organic Nitrogen Compounds — Amines and Diazonium Salts
Why This Chapter Matters
Amines and Diazonium salts give 5-7 marks in CBSE Class 12. Classification, preparation, reactions, and named reactions are all tested in both short and long answers.
Core Concepts
1. Classification of Amines
Based on number of alkyl/aryl groups attached to N:
Primary (1°): RNH₂ — one alkyl group. Example: CH₃NH₂ (methylamine).
Secondary (2°): R₂NH — two alkyl groups. Example: (CH₃)₂NH.
Tertiary (3°): R₃N — three alkyl groups. Example: (CH₃)₃N.
Aromatic amines: aniline (C₆H₅NH₂) — lone pair on N conjugated with ring.
2. Basic Nature of Amines
Amines are Lewis bases (donate lone pair of N).
Basicity order: Aliphatic > Ammonia > Aromatic amines.
Reason: Alkyl groups donate electrons to N (increase basicity).
Aryl groups withdraw electrons via resonance (decrease basicity of aromatic amines).
In water (aqueous solution): 2°>1°>3° for aliphatic (steric+solvation effects).
pKa of conjugate acid (pKb of amine): lower pKb = stronger base.
Aniline pKb = 9.38 (much weaker base than aliphatic amines, pKb ≈ 3-4).
3. Preparation of Amines
From nitro compounds: ArNO₂ + 3H₂ →(Sn/HCl or Fe/HCl)→ ArNH₂.
Aniline: C₆H₅NO₂ + 3[H] → C₆H₅NH₂ + H₂O.
From halides (Gabriel phthalimide synthesis):
Phthalimide + KOH + RX → N-alkylphthalimide → (hydrolysis) → RNH₂ + phthalic acid.
Gives ONLY primary amines.
Hoffmann bromamide degradation:
RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O.
Product has ONE less carbon than starting amide.
Reduction of nitriles: RCN + 2H₂ →(catalyst)→ RCH₂NH₂.
4. Chemical Reactions of Amines
With acids: RNH₂ + HCl → RNH₃⁺Cl⁻ (salt formation, water-soluble).
Acylation: RNH₂ + CH₃COCl → RNHCOCH₃ + HCl (N-acyl amine/amide).
Carbylamine reaction: 1° amine + CHCl₃ + 3KOH → isocyanide (RNC) + 3KCl + 3H₂O.
Isocyanide has extremely unpleasant smell. Test for 1° amines ONLY.
Hinsberg test: Distinguish 1°, 2°, 3° amines using benzene sulphonyl chloride (C₆H₅SO₂Cl):
1°: forms sulphonamide soluble in NaOH (N-H present → acidic H).
2°: forms sulphonamide INSOLUBLE in NaOH (no N-H).
3°: does NOT react.
5. Diazonium Salts
ArNH₂ + NaNO₂ + HCl →(0-5°C)→ ArN₂⁺Cl⁻ + NaCl + 2H₂O.
Diazotisation: must be done at 0-5°C (diazonium salt unstable at higher temp).
Reactions (Synthetic importance):
Sandmeyer: ArN₂⁺Cl⁻ + CuCl → ArCl + N₂ (+ CuBr for ArBr, + CuCN for ArCN).
Balz-Schiemann: ArN₂⁺BF₄⁻ →(heat)→ ArF + N₂ + BF₃.
Gattermann: ArN₂⁺X⁻ + HX →(Cu/Cu₂X₂)→ ArX + N₂.
With water: ArN₂⁺ + H₂O →(warm)→ ArOH + N₂ (synthesis of phenol from aniline).
With H₃PO₂: ArN₂⁺ + H₃PO₂ + H₂O → ArH + N₂ (replaces diazonium with H, deamination).
Azo coupling: ArN₂⁺ + ArOH → Ar-N=N-Ar'-OH (azo dye, orange/red colour).
PYQs (CBSE)
CBSE 2023: Explain Hinsberg test with equations.
1°: PhSO₂Cl + RNH₂ → PhSO₂NHR (soluble in NaOH due to acidic N-H).
2°: PhSO₂Cl + R₂NH → PhSO₂NR₂ (insoluble in NaOH, no acidic H).
3°: R₃N + PhSO₂Cl → no reaction.
CBSE 2022: Convert: Aniline → Benzene.
Aniline →(NaNO₂/HCl, 0-5°C)→ Benzenediazonium chloride →(H₃PO₂/H₂O)→ Benzene.