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Must-know facts

23 facts

1Amine N: sp³, pyramidal, one lone pair; C–N–C in trimethylamine 108°.
21° RNH₂, 2° R₂NH, 3° R₃N. IUPAC: alkanamine; N-locants for groups on nitrogen; aniline = benzenamine.
3Nitro → amine: H₂ with Ni/Pd/Pt or metal + acid; Fe scrap + HCl preferred (FeCl₂ regenerates HCl).
4Ammonolysis: ethanolic NH₃, sealed tube, 373 K; gives a mixture up to R₄N⁺X⁻; large excess NH₃ favours 1°; RI > RBr > RCl.
5Nitrile + LiAlH₄ or H₂/Ni → 1° amine, one carbon more than the halide.
6Gabriel: 1° aliphatic amines only; aryl amines cannot be made (aryl halides resist substitution).
7Hoffmann bromamide: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O; one carbon fewer.
8b.p. (K), mass 72–74: n-butylamine 350.8 > diethylamine 329.3 > ethyldimethylamine 310.5 > 2-methylbutane 300.8; butan-1-ol 390.3.
9Isomeric 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

Classifying an amine as primary, secondary or tertiary by the carbon that holds nitrogen, as for alcohols.

Amines are classified by how many groups are on nitrogen: (CH₃)₃CNH₂ is a primary amine even though its carbon is tertiary.

Making aniline by the Gabriel phthalimide synthesis.

Aryl halides do not undergo nucleophilic substitution with the phthalimide anion, so Gabriel gives only aliphatic primary amines.

Keeping the carbon count in the Hoffmann bromamide reaction.

The amine has one carbon fewer than the amide: butanamide gives propanamine, benzamide gives aniline.

Ranking aqueous basicity purely by the +I effect (3° > 2° > 1°).

That order holds only in the gas phase. In water solvation and steric effects give (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ and (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃.

Reading a larger pKb as a stronger base.

Smaller pKb means larger Kb and a stronger base: diethylamine (3.00) is stronger than ammonia (4.75), aniline (9.38) far weaker.

Expecting benzylamine to be as weak a base as aniline because both have a ring.

In phenylmethanamine the –NH₂ is on a CH₂, not the ring, so its lone pair is not delocalised: pKb 4.70 against aniline's 9.38.

Using the carbylamine test to pick out a secondary amine.

Only primary amines, aliphatic or aromatic, give the foul isocyanide. Use Hinsberg's reagent to separate 2° from 3°.

Saying the Hinsberg product of a secondary amine dissolves in alkali.

It has no H on nitrogen, so it is not acidic and stays insoluble. The primary amine's sulphonamide, with an acidic N–H, dissolves.

Expecting direct nitration of aniline to give only ortho and para products.

In strong acid aniline is protonated to the meta-directing anilinium ion: 51% para, 47% meta, 2% ortho, plus tarry oxidation products.

Running Friedel-Crafts alkylation or acylation on aniline.

Aniline forms a salt with AlCl₃; the positive nitrogen deactivates the ring and no reaction occurs.

Storing a diazonium salt or letting it warm up.

Arenediazonium chlorides last only a short time at 273–278 K; above about 283 K the solution hydrolyses to phenol. They are used at once.

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.

Test yourself: 10 questionsExam-style questions on Amines, with full solutions.Start