1. Classification, structure and nomenclature
1° RNH₂, 2° R₂NH, 3° R₃N (count groups on N); R₄N⁺X⁻ quaternary salts; general formula CₙH₂ₙ₊₃N. N sp³, pyramidal (C–N–C ≈ 108°), lone pair → basic and nucleophilic. IUPAC: alkanamine with N-prefixes (N,N-diethylethanamine; hexane-1,6-diamine); aniline = benzenamine.
Key terms
- Primary amine
An amine with one carbon group on nitrogen, RNH₂.
- Quaternary ammonium salt
R₄N⁺X⁻, nitrogen bonded to four carbon groups.
2. Preparation
- ROH + NH₃ (Al₂O₃, 633 K) → mixture of amines.
- RX + alcoholic NH₃ (sealed tube) → mixture; excess NH₃ favours 1°.
- Nitriles: H₂/Ni, LiAlH₄ or Na/ethanol → RCH₂NH₂ (+1 C).
- Hofmann: (−1 C).
- Nitro compounds: Sn/HCl or Fe/HCl, then NaOH; H₂/Ni.
- Gabriel: potassium phthalimide + RX → hydrolysis → pure RNH₂ (not aryl amines).
| Method | Carbon change |
|---|---|
| Nitrile reduction | +1 |
| Hofmann | −1 |
| Gabriel | 0 |
Formulas
- Reduction of nitrobenzene
Key terms
- Gabriel synthesis
Preparation of pure primary amines from potassium phthalimide and an alkyl halide.
- Hofmann degradation
Conversion of an amide into a primary amine with one carbon fewer using Br₂ and KOH.
3. Physical properties
Lower amines: gases/volatile liquids, fishy smell; aniline darkens in air. 1° and 2° amines form N–H···N hydrogen bonds — weaker than O–H···O, so amines boil lower than alcohols of similar mass. Isomers: 1° > 2° > 3° (3° has no N–H). Lower amines water-soluble.
| C₃H₉N isomer | b.p. / K |
|---|---|
| (CH₃)₃N | 276 |
| CH₃NHC₂H₅ | 310 |
| CH₃CH₂CH₂NH₂ | 321 |
Key terms
- N–H···N hydrogen bond
Hydrogen bond between amine molecules; weaker than O–H···O.
4. Basicity
; smaller pKb → stronger base. Gas: 3° > 2° > 1° > NH₃ (+I). Water: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃; (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ — +I effect, solvation of the cation (more N–H better) and steric hindrance.
Aniline (pKb 9.38) ≪ NH₃ (4.75): lone pair delocalised into the ring. EDG (–CH₃, –OCH₃) increase, EWG (–NO₂, –Cl) decrease basicity. Benzylamine (N on CH₂) ≈ alkyl amines.
Key terms
- pKb
−log Kb; a smaller pKb means a stronger base.
5. Alkylation, acylation and tests
Alkylation (SN2 by N lone pair): 1° → 2° → 3° → R₄N⁺X⁻. Acylation (addition–elimination at C=O of RCOCl/(RCO)₂O; pyridine removes HCl): 1° and 2° → amides; 3° no reaction; benzoylation (Schotten–Baumann).
HNO₂: 1° aliphatic → ROH + N₂; 1° aromatic (273–278 K) → ArN₂⁺; 2° → yellow N-nitrosamine; 3° aliphatic → salt. Carbylamine (CHCl₃ + alc. KOH): 1° only → foul isocyanide. Hinsberg (C₆H₅SO₂Cl + KOH): 1° product soluble in KOH; 2° insoluble; 3° no reaction.
| Test | 1° | 2° | 3° |
|---|---|---|---|
| Carbylamine | − | − | |
| Hinsberg (KOH) | Soluble | Insoluble | No reaction |
| HNO₂ | N₂ (aliphatic) | Yellow oil | Salt |
Formulas
- Carbylamine reaction
Key terms
- Carbylamine test
Primary amines + CHCl₃ + alcoholic KOH → foul-smelling isocyanide.
- Hinsberg’s reagent
Benzenesulphonyl chloride, C₆H₅SO₂Cl.
6. Aniline
Preparation: C₆H₅NO₂ + Sn/HCl → C₆H₅NH₃Cl → (NaOH) C₆H₅NH₂; steam distil. –NH₂ reactions: salts with acids; acetylation (acetanilide), benzoylation (benzanilide), alkylation, carbylamine, diazotisation.
Ring: –NH₂ strongly activating (o/p): Br₂(aq) → 2,4,6-tribromoaniline; for monosubstitution acetylate first (protects from oxidation, moderates activation, avoids anilinium meta direction) → 4-nitro/4-bromoacetanilide → hydrolyse; sulphonation → sulphanilic acid (zwitter ion); no Friedel–Crafts.
Tests: azo dye (diazotise cold + alkaline 2-naphthol → orange-red); bromine water (white ppt); bleaching powder (violet). Uses: dyes, drugs, rubber chemicals, polyurethanes.
| Reagent | Product |
|---|---|
| Br₂(aq) | 2,4,6-Tribromoaniline |
| (CH₃CO)₂O | Acetanilide |
| NaNO₂/HCl, cold | C₆H₅N₂Cl |
| Conc. H₂SO₄, 453 K | Sulphanilic acid |
Formulas
- Bromination of aniline
Key terms
- Acetanilide
N-Phenylethanamide, C₆H₅NHCOCH₃.
- Sulphanilic acid
4-Aminobenzenesulphonic acid, a zwitter ion.
Exam tips
Common mistakes
- Classifying butan-2-amine as secondary (it is primary).
- Saying trimethylamine is the strongest base in water.
- Giving the carbylamine test for 2° amines.
- Nitrating aniline directly and expecting only the para product.
How marks are usually awarded
- Basicity (2–3 marks): order with +I, solvation, steric and resonance arguments.
- Tests (2–3 marks): reagent, observation for each class, reason.
- Aniline conversions (1 mark per step): protection, substitution, deprotection.