Chapter 13 — Amines — introduces nitrogen-containing organic compounds that are derivatives of ammonia. From the adrenaline in your body to the aniline used in dyes, amines are biologically and industrially vital. This chapter carries 5-7 marks in Board exams. Focus on basicity comparison, diazonium salt reactions, and distinction tests.
Key Concepts
Classification
| Type | Structure | Example |
|---|---|---|
| Primary (1°) | R−NH₂ | CH₃NH₂ (methylamine), C₆H₅NH₂ (aniline) |
| Secondary (2°) | R₂NH | (CH₃)₂NH (dimethylamine) |
| Tertiary (3°) | R₃N | (CH₃)₃N (trimethylamine) |
Preparation
(Excess NH₃ → mainly 1° amine)
2. Gabriel Phthalimide Synthesis (1° amine only):
Phthalimide + KOH → potassium phthalimide + RX → N-alkylphthalimide →(N₂H₄) RNH₂ + phthalhydrazide
3. Reduction of Nitro Compounds:
RNO₂ + 3H₂/Ni → RNH₂ (catalytic reduction)
ArNO₂ + Sn/HCl → ArNH₂ (aniline — standard lab method)
4. Hoffmann Bromamide Degradation:
RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O
(Amide → amine with ONE LESS carbon — very useful!)
Basicity of Amines
(C₂H₅)₂NH > C₂H₅NH₂ > (C₂H₅)₃N > NH₃ > C₆H₅NH₂
In gas phase (no solvation):
3° > 2° > 1° > NH₃ (purely inductive effect)
Why aniline is a weak base: The lone pair on N is delocalised into the benzene ring (resonance), making it less available for protonation. Hence aniline is much weaker than aliphatic amines.
Effect of Substituents on Basicity
- Electron-donating groups (−CH₃, −OCH₃) on ring: Increase basicity of aromatic amines
- Electron-withdrawing groups (−NO₂, −Cl) on ring: Decrease basicity
- p-Toluidine (p-CH₃C₆H₄NH₂) > aniline > p-nitroanilline
Reactions of Amines
Distinction Tests (Hinsberg Test)
1° amine → sulphonamide (dissolves in NaOH — acidic H on N)
2° amine → sulphonamide (insoluble in NaOH — no acidic H)
3° amine → no reaction with Hinsberg reagent
Carbylamine Test (for 1° amines only)
Only 1° amines (aliphatic and aromatic) give this test.
Reaction with Nitrous Acid (NaNO₂ + HCl)
| Amine Type | Reaction | Product |
|---|---|---|
| 1° aliphatic | RNH₂ + HNO₂ | ROH + N₂↑ + H₂O (unstable diazonium → alcohol) |
| 1° aromatic | ArNH₂ + HNO₂ (0-5°C) | ArN₂⁺Cl⁻ (diazonium salt — stable at 0-5°C!) |
| 2° (both) | R₂NH + HNO₂ | R₂N−N=O (nitrosoamine — yellow oily liquid) |
| 3° aliphatic | R₃N + HNO₂ | R₃N⁺H·NO₂⁻ (salt) |
| 3° aromatic | Ar₃N + HNO₂ | p-nitroso compound (electrophilic substitution on ring) |
Diazonium Salts — The Most Versatile Intermediate!
Reactions replacing N₂:
ArN₂⁺Cl⁻ + CuCl/HCl → ArCl + N₂ (Sandmeyer — chloride)
ArN₂⁺Cl⁻ + CuBr/HBr → ArBr + N₂ (Sandmeyer — bromide)
ArN₂⁺Cl⁻ + CuCN/KCN → ArCN + N₂ (Sandmeyer — cyanide)
ArN₂⁺Cl⁻ + KI → ArI + N₂ + KCl (no Cu catalyst needed!)
ArN₂⁺Cl⁻ + H₃PO₂ + H₂O → ArH + N₂ (replace NH₂ with H)
ArN₂⁺Cl⁻ + HBF₄ → ArF + N₂ + BF₃ (Balz-Schiemann — fluoride)
ArN₂⁺Cl⁻ + H₂O →(warm) ArOH + N₂ + HCl (phenol)
Coupling reactions (N₂ retained):
ArN₂⁺ + C₆H₅OH → Ar−N=N−C₆H₄OH (azo dye — coloured!)
ArN₂⁺ + C₆H₅NH₂ → Ar−N=N−C₆H₄NH₂ (azo dye)
Important Definitions
| Term | Definition |
|---|---|
| Amine | Organic derivative of ammonia where H atoms are replaced by alkyl/aryl groups |
| Diazotisation | Conversion of 1° aromatic amine to diazonium salt using NaNO₂ + HCl at 0-5°C |
| Coupling Reaction | Reaction of diazonium salt with activated aromatic compound to form azo dye |
| Carbylamine Reaction | Test for 1° amine using CHCl₃ + KOH → foul-smelling isocyanide |
| Hofmann Degradation | Conversion of amide to amine with one less carbon using Br₂/NaOH |
Solved Examples — NCERT Based
Example 1: Basicity Comparison
Q: Arrange in increasing order of basicity: C₆H₅NH₂, C₂H₅NH₂, (C₂H₅)₂NH, NH₃
Solution:
C₆H₅NH₂ is weakest (lone pair delocalised into ring). NH₃ is next. Among aliphatic: 2° > 1° in aqueous solution (balance of inductive and solvation effects).
C₆H₅NH₂ < NH₃ < C₂H₅NH₂ < (C₂H₅)₂NH
Example 2: Conversion via Diazonium Salt
Q: Convert aniline to chlorobenzene.
Solution:
Step 1: C₆H₅NH₂ + NaNO₂ + 2HCl →(0-5°C) C₆H₅N₂⁺Cl⁻ (diazonium salt)
Step 2: C₆H₅N₂⁺Cl⁻ + CuCl/HCl → C₆H₅Cl + N₂ (Sandmeyer reaction)
Example 3: Hinsberg Test
Q: How can you distinguish between ethylamine, diethylamine, and triethylamine using Hinsberg reagent?
Solution:
Treat each with C₆H₅SO₂Cl (Hinsberg reagent):
Ethylamine (1°): Forms sulphonamide → dissolves in NaOH (due to acidic −NH)
Diethylamine (2°): Forms sulphonamide → does NOT dissolve in NaOH (no acidic H on N)
Triethylamine (3°): No reaction with Hinsberg reagent
Example 4: Gabriel Synthesis
Q: Prepare propan-1-amine using Gabriel synthesis.
Solution:
Phthalimide + KOH → potassium phthalimide
Potassium phthalimide + CH₃CH₂CH₂Br → N-propylphthalimide
N-propylphthalimide + N₂H₄ (hydrazine) → CH₃CH₂CH₂NH₂ (propan-1-amine) + phthalhydrazide
Important Questions for Board Exams
1 Mark Questions
- Why is aniline a weaker base than methylamine?
- What is the carbylamine reaction?
- What is diazotisation?
- Name the reagent used in Hinsberg test.
2 Mark Questions
- How do primary, secondary and tertiary amines react with nitrous acid?
- Explain why (CH₃)₃N is a weaker base than (CH₃)₂NH in aqueous solution.
- Write the Gabriel phthalimide synthesis reaction for methylamine.
- Give the Hofmann bromamide degradation reaction.
3 Mark Questions
- What are diazonium salts? How are they prepared? Write any 4 reactions.
- How will you distinguish between 1°, 2° and 3° amines using Hinsberg test?
- Compare the basicity of methylamine, dimethylamine, aniline and ammonia with reasons.
5 Mark Questions
- What are amines? Discuss their classification and preparation (any 3 methods). Compare their basic character.
- Describe the chemistry of diazonium salts: preparation, Sandmeyer reaction, coupling reactions, and importance in synthesis.
Quick Revision Points
- Amines: 1° (RNH₂), 2° (R₂NH), 3° (R₃N)
- Gabriel synthesis: only 1° amines, not aromatic amines
- Hofmann degradation: amide → amine (1 less C)
- Basicity (aq): 2° > 1° > 3° > NH₃ >> aniline
- Aniline weak base: lone pair delocalised into ring
- Carbylamine test: 1° amines only → foul smell of isocyanide
- Hinsberg: 1° dissolves in NaOH, 2° doesn’t, 3° no reaction
- Diazonium salt: ArNH₂ + NaNO₂ + HCl at 0-5°C
- Sandmeyer: ArN₂⁺ + CuX/HX → ArX (Cl, Br, CN)
- Coupling: diazonium + phenol/amine → azo dye
Chapter Navigation
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- Biomolecules Class 12 Notes
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