Amines and Nitrogen Compounds

Amines and Nitrogen Compounds

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Amines are organic derivatives of ammonia (NH₃) in which one or more N–H hydrogens are swapped for alkyl or aryl (R) groups; the nitrogen retains a lone pair, which gives amines their characteristic basicity and nucleophilicity. Classification rests on the number of carbon groups directly bonded to N: primary (1°) has one R group (R–NH₂), secondary (2°) has two (R₂NH), and tertiary (3°) has three (R₃N); a quaternary ammonium salt (R₄N⁺X⁻) forms when a 3° amine picks up a fourth alkyl group. For ECAT MCQs, lock in three high-yield facts: (1) basicity order in water for aliphatic amines is roughly 2° > 1° > 3° > NH₃, while aromatic amines (e.g., aniline) are far weaker because the lone pair delocalises into the benzene ring; (2) the carbylamine test with CHCl₃ + alcoholic KOH is positive only for 1° amines, producing a foul-smelling isocyanide R–NC; (3) diazotisation of aniline at 0–5 °C with NaNO₂/HCl gives the benzenediazonium chloride C₆H₅N₂⁺Cl⁻, which couples with phenols or aromatic amines to form azo dyes.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Structure and Nomenclature

The N atom in an amine is sp³-hybridised with a pyramidal geometry (bond angles ≈ 107°). The lone pair occupying the fourth sp³ orbital is what accepts a proton (Brønsted base behaviour) or attacks an electrophile (nucleophile). IUPAC names use the suffix -amine (e.g., ethanamine, benzenamine/aniline), with locants for the N position in diamines.

Basicity and the Kb Expression

For the equilibrium RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻, the base dissociation constant is

K_b = [RNH₃⁺][OH⁻] / [RNH₂], with pK_b = –log K_b.

Because pK_a + pK_b = 14 at 25 °C for any conjugate acid–base pair in water, a smaller pK_b means a stronger base. Aliphatic amines owe their strength to the +I (inductive) effect of alkyl groups pushing electron density onto N. In aqueous solution the simple gas-phase order 2° > 1° > 3° is moderated by solvation: 1° amines solvate the NH₃⁺ ion best (three N–H bonds available for H-bonding), so 2° > 1° ≈ 3° is commonly observed for small alkyl groups. Aniline (pK_b ≈ 9.4) is roughly a million times weaker than methylamine (pK_b ≈ 3.4) because the lone pair is conjugated with the aromatic π system.

Key Reactions

• Salt formation: R–NH₂ + HCl → R–NH₃⁺Cl⁻
• Acylation (1° and 2° only): R–NH₂ + R′COCl → R–NHCOR′ + HCl
• Carbylamine test (1° only): R–NH₂ + CHCl₃ + 3 KOH → R–NC + 3 KCl + 3 H₂O
• Hinsberg test: benzene-sulfonyl chloride distinguishes 1° (soluble base), 2° (insoluble sulfonamide), and 3° (no reaction) amines.
• Diazotisation (0–5 °C): C₆H₅NH₂ + NaNO₂ + 2 HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2 H₂O

Preparation Routes

Reduction of nitro compounds (RNO₂ → RNH₂ with Sn/HCl or catalytic H₂), Hofmann bromamide degradation (RCONH₂ → RNH₂ via Br₂/NaOH with loss of one carbon), ammonolysis of alkyl halides (excess NH₃), and LiAlH₄ reduction of nitriles (RCN → RCH₂NH₂).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Subtle Distinctions

ECAT traps frequently hinge on two confusions. First, amine classification counts groups on N, not on carbon — a molecule with the C bonded to two methyls but the N bonded to only one alkyl group is still 1° (e.g., isopropylamine, (CH₃)₂CHNH₂). Second, quaternary ammonium salts are not amines: with all four sp³ orbitals tied up in σ-bonds, nitrogen has no lone pair and behaves as a phase-transfer catalyst or surfactant, not as a base.

Aromatic Diazonium Chemistry in Depth

The diazonium cation Ar–N≡N⁺ (linear, sp-hybridised terminal N) is stable only below 5 °C. On warming with water it hydrolyses to a phenol (ArOH + N₂); with CuCl it gives the Sandmeyer aryl chloride (ArCl); with CuCN, an aryl nitrile; with H₃PO₂, the aryl group is deaminated back to ArH. Coupling reactions with electron-rich arenes (phenols, N,N-dialkylanilines) at the para position yield intensely coloured azo compounds (–N=N– bridge), the basis of dye chemistry. A common exam error is drawing the salt as Ar–N=N–OH; the correct Brønsted acid is Ar–N≡N⁺ with a free N₂ leaving group.

Worked Example

Rank, with reasoning, the basicity of: (a) methylamine, (b) dimethylamine, (c) aniline, (d) p-nitroaniline. Answer: (b) > (a) > (c) > (d). Dimethylamine benefits from two +I methyl groups and retains enough N–H bonds for solvation; aniline is weakened by resonance delocalisation of the lone pair; p-nitroaniline is the weakest because the –NO₂ group withdraws electron density through both induction and resonance, placing partial positive charge on N.

ECAT Strategy

This topic contributes roughly 1–2 MCQs in the 30-question Chemistry section (≈3% weight). High-yield items are nearly always on the carbylamine test (1° only), Hinsberg differentiation, the diazonium-coupling pathway to azo dyes, and basicity ordering. Allocate about 45 seconds per question; the long stem of “rank the basicity” prompts is where marks are lost, so always write the resonance/induction argument before bubbling.

Practice Prompts

1. Predict the products when aniline is treated with (i) acetyl chloride, (ii) NaNO₂/HCl at 0 °C followed by β-naphthol in NaOH.
2. Explain, using K_b values, why p-methoxyaniline is more basic than p-nitroaniline despite both being aromatic amines.

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