Amines
'Amines are like AMMONIA — but with organic "decorations" that change their properties and reactivity in fascinating ways.'
1. Chapter Overview
Amines are BASIC nitrogen-containing organic compounds. Topics include: CLASSIFICATION (1°, 2°, 3° amines), NOMENCLATURE, METHODS OF PREPARATION (from haloalkanes, nitro compounds, nitriles, amides, and the Hoffmann bromamide degradation), PHYSICAL PROPERTIES (boiling points, solubility, odour), BASICITY of amines (comparing aliphatic and aromatic amines, effect of substituents), and CHEMICAL REACTIONS (acylation, alkylation, carbylamine reaction, reaction with nitrous acid, and electrophilic substitution in aromatic amines). The chapter also covers DIAZONIUM SALTS — their preparation and synthetic applications.
2. Classification and Nomenclature
Classification
- 1° (Primary): R−NH₂. 2° (Secondary): R₂NH. 3° (Tertiary): R₃N.
- Aliphatic amines: N attached to aliphatic carbon. Aromatic amines: N attached directly to aromatic ring (e.g., C₆H₅NH₂ = aniline).
IUPAC Naming
- 1° amines: Replace −e of alkane with −amine. Example: CH₃NH₂ (methanamine), (CH₃)₂CHNH₂ (propan-2-amine).
- 2°/3° amines: Treat the LARGEST alkyl group as the parent. Example: CH₃NHCH₂CH₃ (N-methylethanamine).
- Aromatic amines: C₆H₅NH₂ (aniline), C₆H₅NHCH₃ (N-methylaniline).
3. Preparation of Amines
- Reduction of nitro compounds: RNO₂ + 6[H] → RNH₂ + 2H₂O. Using Sn/HCl, Fe/HCl, or catalytic hydrogenation.
- Reduction of nitriles: RCN + 4[H] → RCH₂NH₂ (LiAlH₄ or H₂/Ni).
- Reduction of amides: RCONH₂ + 4[H] → RCH₂NH₂ + H₂O (LiAlH₄).
- Hoffmann bromamide degradation: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O. 'A PRIMARY amine with ONE LESS carbon than the starting amide.'
- From haloalkanes (Gabriel phthalimide synthesis) : Primary alkyl halide + potassium phthalimide → N-alkylphthalimide → RNH₂ (hydrolysis). 'ONLY for PRIMARY amines — NO over-alkylation.'
- Reductive amination: RCHO + NH₃ + H₂ (Ni) → RCH₂NH₂.
4. Physical Properties
| Property | Observation | Reason |
|---|---|---|
| Boiling points | 1° > 2° > 3° (for isomeric amines) | H-bonding capability decreases |
| Boiling points vs alcohols | LOWER than alcohols | N−H bonds are weaker than O−H bonds |
| Solubility | Lower amines are WATER SOLUBLE | H-bonding with water |
| Odour | Fishy smell (e.g., trimethylamine in rotting fish) | Volatile amines |
| Aniline | OILY liquid, slightly soluble in water | Larger hydrophobic ring |
5. Basicity of Amines
In Aqueous Solution
- 'Amines are BASES — they ACCEPT a proton (H⁺) from water, forming alkylammonium ions and OH⁻ ions.'
- RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻.
- K_b and pK_b: Larger K_b = STRONGER base. Smaller pK_b = STRONGER base.
Aliphatic Amines
- Basicity order (gas phase): 3° > 2° > 1° > NH₃ (alkyl groups are electron-releasing — stabilise the positive charge on nitrogen).
- Basicity order (aqueous): 2° > 1° > 3° > NH₃ (in water, solvation of the ammonium ion matters. 3° amines are LESS solvated due to steric hindrance).
Aromatic Amines
- 'Aniline is MUCH LESS BASIC (pK_b ≈ 9.4) than aliphatic amines (pK_b ≈ 3-4).'
- Reason: The lone pair on nitrogen is DELOCALISED into the aromatic ring by resonance — LESS available for protonation.
- Effect of substituents on aniline: Electron-wITHDRAWING groups (NO₂, CN) DECREASE basicity. Electron-RELEASING groups (CH₃, OCH₃) INCREASE basicity.
6. Chemical Reactions of Amines
6.1 Reactions Involving the Nitrogen Lone Pair
| Reaction | Reagent | Product |
|---|---|---|
| Alkylation | R−X | R₂NH, R₃N, R₄N⁺X⁻ (quaternary ammonium salt) |
| Acylation | RCOCl / (RCO)₂O | RCONHR (N-substituted amide) |
| Carbylamine reaction | CHCl₃ + KOH (alcoholic) | RNC (isocyanide — FOUL SMELL). ONLY for 1° amines. |
| Hinsberg test | C₆H₅SO₂Cl (benzenesulphonyl chloride) + KOH | 1°: SULPHONAMIDE SOLUBLE in KOH. 2°: SULPHONAMIDE INSOLUBLE in KOH. 3°: NO REACTION. |
| Reaction with HNO₂ | NaNO₂ + HCl (cold, 0-5°C) | 1° aliphatic → N₂ gas. 1° aromatic → DIAZONIUM SALT (stable at low T). 2° → N-NITROSAMINE (yellow oil). 3° → NITROSAMINE SALT. |
6.2 Carbylamine Reaction
- RNH₂ + CHCl₃ + 3KOH (alc.) → RNC + 3KCl + 3H₂O.
- 'The CARBYLAMINE TEST is SPECIFIC for PRIMARY amines — the isocyanide product has a CHARACTERISTIC FOUL SMELL.'
7. Diazonium Salts
Preparation
- C₆H₅NH₂ + NaNO₂ + 2HCl (0-5°C) → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O.
- 'Diazotisation requires LOW TEMPERATURE (0-5°C). Above 5°C, the diazonium salt decomposes to give PHENOL.'
Synthetic Applications (Replacement Reactions)
| Reagent | Product | Type |
|---|---|---|
| H₂O (warm) | C₆H₅OH | Phenol |
| KI | C₆H₅I | Iodobenzene |
| CuCl/HCl | C₆H₅Cl | Chlorobenzene (Sandmeyer reaction) |
| CuBr/HBr | C₆H₅Br | Bromobenzene (Sandmeyer) |
| CuCN/KCN | C₆H₅CN | Benzonitrile (Sandmeyer) |
| HBF₄ | C₆H₅F | Fluorobenzene |
| H₃PO₂ | C₆H₆ | Benzene (replacement by H) |
Coupling Reactions
- C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅−N=N−C₆H₄OH(p) (AZO DYE — orange colour).
- C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ → C₆H₅−N=N−C₆H₄NH₂(p) (azo dye — yellow).
- 'Azo coupling produces VIVIDLY COLOURED azo compounds — the basis of MANY synthetic dyes.'
8. Common Mistakes
- Hoffmann bromamide degradation: The product has ONE LESS carbon than the starting amide. RCONH₂ → RNH₂ (not RCH₂NH₂ — that's amide reduction with LiAlH₄).
- Gabriel synthesis: ONLY gives PRIMARY amines. DOES NOT work with alkyl halides that undergo elimination.
- Carbylamine test: Positive ONLY for PRIMARY amines. Does NOT distinguish between 2° and 3° amines.
- Hinsberg test: 3° amines do NOT react with benzenesulphonyl chloride — they are SEPARATED from the reaction mixture as INSOLUBLE in both acid and base.
- Diazonium salt stability: MUST be kept at 0-5°C. At room temperature, it decomposes to phenol.
9. CBSE Exam Focus
- Classification and nomenclature of amines
- Preparation — reduction of nitro compounds, nitriles, amides; Hoffmann bromamide degradation; Gabriel phthalimide synthesis
- Basicity — aliphatic vs aromatic, effect of substituents
- Chemical reactions — acylation, alkylation, carbylamine test, Hinsberg test, reaction with HNO₂
- Diazonium salts — preparation, Sandmeyer reaction, coupling reactions
10. Self-Test
Q1: Arrange in increasing order of basicity: Aniline, NH₃, Methylamine, Dimethylamine. A1: Aniline < NH₃ < Methylamine < Dimethylamine. (In aqueous solution, 2° aliphatic > 1° > NH₃ > aromatic.)
Q2: How will you convert benzene to aniline? A2: Benzene → Nitrobenzene (conc. HNO₃/H₂SO₄) → Aniline (Sn/HCl or Fe/HCl reduction).
Q3: Write the product: C₆H₅N₂⁺Cl⁻ + H₂O (warm) → ? A3: C₆H₅OH (phenol) + N₂ + HCl.
Q4: Distinguish between aniline and N-methylaniline using chemical tests. A4: Hinsberg test: Aniline (1°) → sulphonamide soluble in KOH. N-methylaniline (2°) → sulphonamide INSOLUBLE in KOH. Carbylamine test: Aniline → positive (foul smell). N-methylaniline → negative.
Q5: Complete: C₆H₅NH₂ + CHCl₃ + alc. KOH → ? A5: Carbylamine reaction. Product: C₆H₅NC (phenyl isocyanide) + 3KCl + 3H₂O.
11. Conclusion
Amines are the NITROGEN-BEARING workhorses of organic chemistry:
- BASICITY: 'Amines are organic BASES — their lone pair on nitrogen accepts protons.'
- DIAZONIUM SALTS: 'Aromatic amines can be converted to diazonium salts — the MOST VERSATILE intermediates in organic synthesis for introducing diverse functional groups.'
- AZO DYES: 'Diazonium coupling produces brilliantly coloured compounds — the foundation of the synthetic dye industry.'
- 'From the aniline in synthetic dyes to the trimethylamine in rotting fish — amines are EVERYWHERE in chemistry and biology.'
