Lesson 12 of 12 · 17 min
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Must-know facts
23 facts
- 01Amine N: sp³, pyramidal, one lone pair; C–N–C in trimethylamine 108°.
- 021° RNH₂, 2° R₂NH, 3° R₃N. IUPAC: alkanamine; N-locants for groups on nitrogen; aniline = benzenamine.
- 03Nitro → amine: H₂ with Ni/Pd/Pt or metal + acid; Fe scrap + HCl preferred (FeCl₂ regenerates HCl).
- 04Ammonolysis: ethanolic NH₃, sealed tube, 373 K; gives a mixture up to R₄N⁺X⁻; large excess NH₃ favours 1°; RI > RBr > RCl.
- 05Nitrile + LiAlH₄ or H₂/Ni → 1° amine, one carbon more than the halide.
- 06Gabriel: 1° aliphatic amines only; aryl amines cannot be made (aryl halides resist substitution).
- 07Hoffmann bromamide: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O; one carbon fewer.
- 08b.p. (K), mass 72–74: n-butylamine 350.8 > diethylamine 329.3 > ethyldimethylamine 310.5 > 2-methylbutane 300.8; butan-1-ol 390.3.
- 09Isomeric amine b.p.: 1° > 2° > 3° (N–H hydrogen bonding).
- 10pKb NH₃ 4.75; aliphatic amines 3–4.22 (stronger bases); aniline 9.38 (weaker).
- 11Gas phase: 3° > 2° > 1° > NH₃. Water: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃ and (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃.
- 12Aniline: 5 resonance structures vs anilinium's 2 → weaker base than NH₃. EDG (–OCH₃, –CH₃) raise, EWG (–NO₂, –COOH, –X) lower basicity.
- 13Acylation needs pyridine; aniline + (CH₃CO)₂O → acetanilide; benzoylation uses C₆H₅COCl.
- 14Carbylamine (CHCl₃ + ethanolic KOH, heat): foul isocyanide, 1° amines only.
- 15HNO₂: 1° aliphatic → alcohol + N₂ (quantitative); 1° aromatic at 273–278 K → diazonium salt.
- 16Hinsberg (C₆H₅SO₂Cl): 1° product soluble in alkali; 2° insoluble; 3° no reaction. p-Toluenesulphonyl chloride now used.
- 17Aniline + Br₂ water → 2,4,6-tribromoaniline (white ppt). Protect as acetanilide for monosubstitution.
- 18Aniline nitration (288 K): 51% p, 47% m, 2% o, because anilinium ion is meta-directing.
- 19Sulphonation at 453–473 K → sulphanilic acid; aniline gives no Friedel-Crafts reaction.
- 20Diazotisation: ArNH₂ + NaNO₂ + 2HCl at 273–278 K; use at once; fluoroborate stable at room temperature.
- 21Sandmeyer: Cu₂Cl₂/HCl, Cu₂Br₂/HBr, CuCN/KCN. Gatterman: Cu powder + HX. Sandmeyer yield is better.
- 22KI → ArI; HBF₄ then heat → ArF; H₃PO₂ or ethanol → ArH; warm to 283 K → phenol; NaNO₂/Cu on fluoroborate → ArNO₂.
- 23Coupling: with phenol (OH⁻) → p-hydroxyazobenzene (orange); with aniline (H⁺) → p-aminoazobenzene (yellow).
Common traps
Where marks are lost
Formulas
5 to know
Base strength
pKb = −log Kb; Kb ratio = 10^(ΔpKb)
Ammonia (4.75) vs aniline (9.38): 10^4.63 ≈ 4.3 × 10⁴.
Hoffmann bromamide degradation
RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O
The amine has one carbon fewer than the amide.
Carbylamine reaction
R–NH₂ + CHCl₃ + 3KOH → R–NC + 3KCl + 3H₂O
Heat; primary amines only; foul-smelling isocyanide.
Diazotisation
C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O
At 273–278 K; use the salt at once.
Nitrous acid on a 1° aliphatic amine
R–NH₂ + HNO₂ → [R–N₂⁺Cl⁻] → ROH + N₂ + HCl
NaNO₂ + HCl; N₂ released quantitatively.
Key terms
13 terms
- Amine
- A derivative of ammonia with one or more H replaced by alkyl or aryl groups.
- Ammonolysis
- Cleavage of a C–X bond by ammonia, giving amines and finally a quaternary salt.
- Quaternary ammonium salt
- R₄N⁺X⁻, nitrogen carrying four carbon groups.
- Gabriel phthalimide synthesis
- Primary aliphatic amines from potassium phthalimide, an alkyl halide and alkaline hydrolysis.
- Hoffmann bromamide degradation
- Amide + Br₂ + NaOH → primary amine with one carbon fewer.
- pKb
- −log Kb; the smaller it is, the stronger the base.
- Acylation
- Replacing an H on nitrogen by an acyl group to give an amide.
- Isocyanide (carbylamine)
- R–NC, the foul-smelling product of the carbylamine test.
- Hinsberg's reagent
- Benzenesulphonyl chloride, used to tell 1°, 2° and 3° amines apart.
- Acetanilide
- N-Phenylethanamide, aniline protected by acetylation.
- Diazotisation
- Converting a primary aromatic amine to its diazonium salt with HNO₂ at 273–278 K.
- Sandmeyer reaction
- Replacing –N₂⁺ by Cl, Br or CN using Cu(I) salts.
- Coupling reaction
- A diazonium salt joining a phenol or arylamine through –N=N– to form an azo dye.