1. Structure, classification and nomenclature
Amines are derivatives of ammonia: primary RNH₂, secondary R₂NH, tertiary R₃N (class = number of carbon groups on N). Nitrogen is sp³ and pyramidal with a lone pair (C–N–C about 108°).
IUPAC: alkanamine (ethanamine, propan-2-amine); groups on N as N-prefixes (N-methylethanamine, N,N-dimethylmethanamine); diamines (ethane-1,2-diamine); aryl amines as benzenamines (aniline = benzenamine, N,N-dimethylbenzenamine).
Key terms
- Primary amine
An amine with one carbon group on nitrogen, RNH₂.
- Aryl amine
An amine with nitrogen bonded directly to an aromatic ring, e.g. aniline.
2. Preparation of amines
- Reduction of nitro compounds: H₂/Ni, Pt, Pd; Fe/HCl (preferred — FeCl₂ hydrolyses and regenerates HCl) or Sn/HCl: .
- Ammonolysis of alkyl halides: — gives a mixture of 1°, 2°, 3° amines and quaternary salts.
- Reduction of nitriles (LiAlH₄ or H₂/Ni): — one carbon more than the alkyl halide.
- Reduction of amides (LiAlH₄).
- Gabriel phthalimide synthesis: phthalimide → K salt → N-alkylphthalimide → NaOH(aq) → pure 1° aliphatic amine (not aryl amines).
- Hoffmann bromamide degradation: — one carbon fewer.
Key terms
- Ammonolysis
Reaction of an alkyl halide with ammonia, giving a mixture of amines.
- Gabriel synthesis
Preparation of pure primary amines from potassium phthalimide and alkyl halides.
- Hoffmann bromamide degradation
Conversion of an amide into a primary amine with one carbon fewer, using Br₂ and NaOH.
3. Physical properties and basicity
Physical: 1° and 2° amines form N–H···N hydrogen bonds (weaker than O–H···O): boiling points between alkanes and alcohols; for isomers 1° > 2° > 3° (propan-1-amine 321 K, N-methylethanamine 310 K, trimethylamine 276 K). Lower amines are water-soluble; aryl amines are colourless when pure but darken in air (oxidation).
Basicity: ; smaller pKb = stronger base. pKb: (C₂H₅)₂NH 3.00, (C₂H₅)₃N 3.25, (CH₃)₂NH 3.27, C₂H₅NH₂ 3.29, CH₃NH₂ 3.38, (CH₃)₃N 4.22, NH₃ 4.75, C₆H₅NH₂ 9.38.
- Alkylamines > NH₃: +I effect of alkyl groups.
- In water the order also depends on solvation of the ammonium ion (more N–H, better H-bonding) and steric hindrance: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃; (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃. In the gas phase: 3° > 2° > 1° > NH₃.
- Aryl amines ≪ NH₃: the lone pair is delocalised into the ring (resonance), lost on protonation. –CH₃, –OCH₃ on the ring increase basicity; –NO₂ decreases it.
| Amine | pKb |
|---|---|
| (C₂H₅)₂NH | 3.00 |
| CH₃NH₂ | 3.38 |
| (CH₃)₃N | 4.22 |
| NH₃ | 4.75 |
| C₆H₅NH₂ | 9.38 |
Key terms
- pKb
Negative logarithm of the base dissociation constant; smaller pKb means a stronger base.
- Solvation effect
Stabilisation of the protonated amine by hydrogen bonding with water.
4. Chemical reactions of amines
- Salts with acids; amine released by NaOH.
- Alkylation to 2°, 3° amines and quaternary ammonium salts.
- Acylation (1° and 2° only): (pyridine removes HCl); benzoylation with C₆H₅COCl.
- Carbylamine test (1° only, aliphatic and aromatic): — foul-smelling isocyanide.
- Nitrous acid: 1° aliphatic → alcohol + N₂ (); aniline at 273–278 K → benzenediazonium chloride.
- Hinsberg test (C₆H₅SO₂Cl): 1° → sulfonamide soluble in alkali; 2° → sulfonamide insoluble in alkali; 3° → no reaction.
- Aniline ring: –NH₂ strongly activating, o/p-directing. Bromine water → 2,4,6-tribromoaniline (). Protect by acetylation () for monosubstitution. Direct nitration gives oxidation and about 47% meta product (anilinium ion is meta-directing). Sulphonation → sulphanilic acid (zwitterion). No Friedel–Crafts reaction (salt with AlCl₃).
| Test | 1° | 2° | 3° |
|---|---|---|---|
| Carbylamine | Foul smell | No | No |
| Hinsberg product + KOH | Dissolves | Insoluble | No reaction |
Key terms
- Carbylamine test
Test for primary amines: warming with CHCl₃ and alcoholic KOH gives a foul-smelling isocyanide.
- Hinsberg’s reagent
Benzenesulfonyl chloride, used to distinguish 1°, 2° and 3° amines.
- Acetanilide
N-Phenylethanamide; the acetylated, less activated form of aniline.
5. Diazonium salts
Preparation: at 273–278 K. Arenediazonium salts are stabilised by resonance; they are used in cold solution.
Replacement of N₂: Sandmeyer (CuCl/HCl, CuBr/HBr, CuCN/KCN) → ArCl, ArBr, ArCN; Gattermann (Cu + HX) → ArCl, ArBr; KI → ArI; HBF₄ then heat → ArF (Balz–Schiemann); warm water → phenol; H₃PO₂ or ethanol → arene ().
Coupling (N retained): with phenol (alkaline) → 4-hydroxyazobenzene (orange); with aniline (mildly acidic) → 4-aminoazobenzene (yellow). Azo compounds are dyes.
Importance: introduce F, I, CN, OH and H into rings where direct substitution fails or is unselective; make azo dyes.
Key terms
- Diazotisation
Conversion of a primary aromatic amine into a diazonium salt with NaNO₂ and HCl at 273–278 K.
- Coupling reaction
Reaction of a diazonium ion with an activated arene to form an azo compound.
- Sandmeyer reaction
Replacement of –N₂⁺ by Cl, Br or CN using copper(I) salts.
Exam tips
Common mistakes
- Classifying amines by the carbon atom (as for alcohols) instead of by the number of groups on nitrogen.
- Using Gabriel synthesis to make aniline.
- Saying tertiary amines are always the strongest bases (not in water).
- Claiming 2° or 3° amines give the carbylamine test.
- Forgetting to protect –NH₂ before mono-bromination or nitration of aniline.
- Warming diazonium salts when another product is intended (they give phenol).
How marks are usually awarded
- 1-mark items: product, reagent, class or order.
- Basicity explanations (2–3 marks): +I effect, solvation, steric effect and/or resonance, each worth a mark.
- Tests (2 marks): reagent and result for each compound.
- Conversions (2–3 marks): correct reagents and conditions (temperature for diazotisation).