Amines

Amines

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your MDCAT.

Amines are organic derivatives of ammonia (NH₃) in which one, two, or three hydrogen atoms are replaced by alkyl or aryl groups. They are classified by how many carbon groups sit on nitrogen:

Class	Formula	Example
Primary (1°)	R–NH₂	Methylamine, CH₃NH₂
Secondary (2°)	R₂NH	Dimethylamine, (CH₃)₂NH
Tertiary (3°)	R₃N	Trimethylamine, (CH₃)₃N
Quaternary (4°)	R₄N⁺X⁻	Tetramethylammonium chloride

• The –NH₂ group is called the amino group; the lone pair on N makes amines both basic and nucleophilic.
• Aliphatic amines > NH₃ > aromatic amines in basicity because alkyl groups push electrons toward N, whereas the aniline lone pair is delocalised into the benzene ring (pKₐ of anilinium ≈ 4.6).
• The carbylamine (isocyanide) test detects primary amines only — a foul-smelling R–N≡C forms with CHCl₃ + alcoholic KOH.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nomenclature and Structure

Common names treat amines as alkyl-substituted ammonia (methylamine, dimethylamine, aniline). IUPAC names use the suffix -amine on the parent chain (methanamine, benzenamine). The C–N–H bond angle is roughly 107°, slightly less than the tetrahedral value because the lone pair compresses the bonds.

Basicity of Amines

Amines act as Brønsted–Lowry bases by accepting a proton on the lone pair:

$$\text{RNH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{RNH}_3^{+} + \text{OH}^{-}$$

$$K_b = \frac{[\text{RNH}_3^{+}][\text{OH}^{-}]}{[\text{RNH}2]}, \quad pK_b = -\log{10} K_b, \quad K_b \cdot K_a = K_w = 1.0 \times 10^{-14} \text{ at 25 °C}$$

Smaller pK_b means a stronger base; remember K_b × K_a = K_w when an MCQ mixes pK_b with pK_a of the conjugate acid.

Amine type	Aqueous basicity order	Reason
Aliphatic methylamines	(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃	Inductive donation raises N electron density; tertiary loses out to solvation and steric hindrance in water
Aromatic vs aliphatic	Aliphatic > NH₃ > aniline	Aniline lone pair is delocalised into the ring (resonance), reducing availability for protonation

Preparation Reactions

• Reduction of nitro compounds: R–NO₂ + 6[H] (Sn/HCl, Fe/HCl, or H₂/Pd) → R–NH₂ + 2H₂O.
• Hofmann bromamide degradation: R–CONH₂ + Br₂ + 4NaOH → R–NH₂ + Na₂CO₃ + 2NaBr + 2H₂O (the amide loses one carbon).
• Ammonolysis: R–X + NH₃ → R–NH₂, but over-alkylation gives a mixture.
• Gabriel synthesis: phthalimide + KOH → potassium phthalimide + R–X → N-alkyl phthalimide → R–NH₂ (clean primary-amine route).
• Reduction of nitriles: R–C≡N + 4[H] (LiAlH₄ or H₂/Ni) → R–CH₂–NH₂.

Identification Tests

Test	Reagent	Positive result
Carbylamine	CHCl₃ + alc. KOH, heat	1° amine gives foul-smelling isocyanide (R–N≡C)
Hinsberg	C₆H₅SO₂Cl + NaOH	1° → soluble salt; 2° → insoluble sulfonamide; 3° → no reaction
Diazotisation	NaNO₂ + HCl, 0–5 °C	1° aromatic amine → ArN₂⁺Cl⁻ (diazonium salt)

For Hinsberg, only the secondary-amine product is a neutral, water-insoluble sulfonamide — that single phrase is the usual MCQ discriminator.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Reactions of Diazonium Salts

The diazonium cation ArN₂⁺ is a versatile intermediate. Its three reaction families are heavily tested:

1. Substitution (Sandmeyer / Gattermann) — replacing N₂⁺ with CuCl/HCl → ArCl, CuBr/HBr → ArBr, CuCN/KCN → ArCN, H₃PO₂ → ArH.
2. Hydrolysis — warm water converts ArN₂⁺ to phenol (ArOH); used to make phenols that cannot be made by direct sulphonation routes.
3. Coupling — ArN₂⁺ attacks activated arenes (phenols, aromatic amines) at the para position to form brightly coloured azo dyes (e.g., p-hydroxyazobenzene, orange).

Trapping temperature matters: diazonium salts are stable only at 0–5 °C; warming decomposes them to phenols with N₂ loss.

Hofmann Elimination vs Hofmann Bromamide

These two reactions share the chemist’s name but differ entirely:

Reaction	Substrate	Reagent	Product
Hofmann bromamide	R–CONH₂	Br₂ / NaOH	R–NH₂ (one carbon shorter)
Hofmann elimination	R₄N⁺ OH⁻	heat	Least-substituted alkene (anti-Saytzeff)

In Hofmann elimination, the bulky trialkylammonium leaving group prefers to abstract the most accessible β-hydrogen, giving the terminal alkene as the major product — the opposite of Saytzeff’s rule.

Boiling Points, Solubility and a Worked Comparison

Primary and secondary amines exhibit intermolecular N–H hydrogen bonding, so their boiling points exceed those of alkanes of similar molar mass; tertiary amines (no N–H) boil lower than their 1° isomers. Lower amines dissolve in water through H-bonding; solubility drops sharply beyond C₅. Because they are basic, all amines dissolve in dilute HCl by forming water-soluble ammonium salts — a standard extraction trick.

Worked basicity comparison — rank ethylamine, diethylamine, triethylamine, and aniline in aqueous solution:

• Triethylamine: 3 ethyl groups donate electrons but steric crowding plus weak solvation of R₃NH⁺ make it weaker than the secondary amine in water.
• Diethylamine: two ethyl groups donate, and the conjugate acid is still well solvated → strongest of the four.
• Ethylamine: one ethyl group → moderate base, stronger than NH₃.
• Aniline: lone pair tied up in aromatic resonance → weakest, K_b ≈ 4 × 10⁻¹⁰.

Final aqueous order: (C₂H₅)₂NH > C₂H₅NH₂ > (C₂H₅)₃N > C₆H₅NH₂.

Common Exam Traps

• Tertiary amine vs quaternary salt: 3° amines are neutral (R₃N); only 4° salts carry a permanent positive charge (R₄N⁺).
• Gas-phase vs aqueous basicity: the methylamine order reverses between phases — water solvation makes 2° amines the strongest in solution.
• Carbylamine test: negative for 2° and 3° amines — they simply do not form isocyanides.
• Aniline K_b: do not assume –NH₂ means strong base; the ring steals the lone pair.

Practice Prompts

1. A compound with molecular formula C₇H₉N gives a foul-smelling product with CHCl₃/alc. KOH and forms a diazonium salt at 0 °C. Identify the compound, name the diazonium product, and write the coupling reaction with phenol.
2. Arrange methylamine, dimethylamine, trimethylamine, ammonia, and aniline in increasing order of pK_b. Justify why the aqueous order differs from the gas-phase order for the three methylamines.

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