ChemistryCHEM 13
Amines
Basicity order, carbylamine test, diazonium salts, coupling reactions, Hofmann degradation
Concept Core
Essential theory — everything NCERT tests on Amines
CLASSIFICATION & BASICITY
1°: RNH₂. 2°: R₂NH. 3°: R₃N. 4°: R₄N⁺ (quat. ammonium salt, not a base).
Basicity in gas phase: 3° > 2° > 1° > NH₃ (more alkyl groups = more e-donation). In aqueous solution (water solvation effect): 2° > 1° > 3° > NH₃.
Aromatic amines WEAKER base than aliphatic (lone pair delocalised into ring). Aniline pKb >> NH₃ pKb.
REACTIONS OF AMINES
Acylation: R-NH₂ + RCOCl → RNHCOR (amide). Carbylamine test: 1° amine + CHCl₃ + KOH → isocyanide (foul smell). Only 1° amines give this. Distinguishes 1° from 2° and 3°.
Reaction with HNO₂: 1° aliphatic amine → unstable diazonium → N₂ + alcohol. 2° amine → yellow N-nitrosoamine. 3° amine → N-nitroso (rearranges).
DIAZONIUM SALTS
ArN₂⁺Cl⁻ formed from Ar-NH₂ + NaNO₂ + HCl at 0–5°C. Stable at low temperature.
Coupling reaction: diazonium + phenol/aniline (at high pH) → azo dye (−N=N− chromophore). Orange-red dyes.
Sandmeyer reaction: ArN₂⁺ + CuCl → ArCl; +CuBr → ArBr; +KI → ArI; +KCN → ArCN; +H₃PO₂ → ArH (deamination).
IMPORTANT AMINES & PROPERTIES
Aniline (aminobenzene): weaker base. In EAS: activates ring at o/p. Acetylation used to protect -NH₂ before nitration.
Hofmann degradation: amide + Br₂ + NaOH → 1° amine (less C). RCONH₂ → RNH₂. Carbon lost as CO₂.
Fact & Formula Vault
High-yield facts, numbers, and formulas
Basicity Order
Aqueous: 2°>1°>3°>NH₃>ArNH₂
Gas phase: 3°>2°>1°>NH₃
Aniline: very weak base
Quat. ammonium: not basic
Diazonium Reactions
→ ArCl: Sandmeyer (CuCl)
→ ArBr: Sandmeyer (CuBr)
→ ArI: KI directly
→ ArF: Balz-Schiemann (HBF₄)
→ ArH: H₃PO₂ (deamination)
Carbylamine Test
Only 1° amines give CHI₃ in iodoform...
Wait - carbylamine: CHCl₃ + KOH
Product: isocyanide (very foul smell)
3° amines: negative
Worked Examples
NEET-style questions solved step-by-step
EASYCarbylamine test is used to identify:▾
Carbylamine test is used to identify:
Primary amines (1°) only. RNH₂ + CHCl₃ + KOH → R-N≡C (isocyanide, foul smell). Secondary and tertiary amines do NOT give this reaction.
MEDIUMBasicity order of methylamine (CH₃NH₂), dimethylamine ((CH₃)₂NH), trimethylamine ((CH₃)₃N) in water?▾
Basicity order of methylamine (CH₃NH₂), dimethylamine ((CH₃)₂NH), trimethylamine ((CH₃)₃N) in water?
In aqueous solution: dimethylamine > methylamine > trimethylamine > NH₃. (2° > 1° > 3°). Due to water solvation of the conjugate acid: 2° ammonium ion is optimally solvated.
HARDAniline is treated with NaNO₂ + HCl at 0°C, then coupled with phenol at pH 9. Product?▾
Aniline is treated with NaNO₂ + HCl at 0°C, then coupled with phenol at pH 9. Product?
Diazotisation of aniline → benzene diazonium chloride (C₆H₅N₂⁺Cl⁻). Coupling with phenol (alkaline medium) → para-hydroxyazobenzene (azo dye, orange-red). The -N=N- azo group acts as chromophore.
Mistake DNA
Common NEET traps for this chapter
⚠ Basicity: aqueous vs gas phase
Gas phase: 3° > 2° > 1° (more alkyl = more donation). Aqueous: 2° > 1° > 3° (solvation of conjugate acid). NEET asks aqueous basicity.
✓ Fix: Aqueous solution = NEET context. 2° is most basic in water.
⚠ Carbylamine vs iodoform
Carbylamine test: 1° amines + CHCl₃/KOH → isocyanide. Iodoform test: CH₃CO- group + I₂/NaOH → CHI₃. Different tests.
✓ Fix: Carbylamine = isocyanide (foul). Iodoform = CHI₃ (yellow ppt).
⚠ Diazonium stability
Aromatic diazonium salts (ArN₂⁺) are stable at 0–5°C. Aliphatic diazonium salts are unstable → decompose immediately → N₂ + carbocation → alcohol. NEET only tests aromatic.
✓ Fix: Aromatic diazonium = stable at low T. Aliphatic = instantly decomposes.
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
1–2 Q/year. Carbylamine test (1° amines only), aqueous basicity order (2°>1°>3°), Sandmeyer reactions, azo dye coupling reaction.
High-Yield
Aqueous basicity: 2° > 1° > 3°. Aniline: weakest (lone pair in ring). Carbylamine: 1° only. Diazonium: 0–5°C, Sandmeyer (CuX), coupling with phenol/aniline.
Strategy
Memorise Sandmeyer reactions as a table. Carbylamine test: 1° amine signature. Basicity order: gas vs aqueous are opposite for 1° vs 3°.
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