Amines

Concept overview

Amines are the nitrogen analogues of ammonia — one, two or three of the N–H bonds of ammonia replaced by carbon chains give primary, secondary or tertiary amines respectively. Aromatic amines (aniline and its derivatives) behave quite differently from their aliphatic cousins because of resonance donation of the lone pair into the aromatic ring.

Basicity is the hallmark of amines. In the gas phase, tertiary > secondary > primary > ammonia (alkyl groups donate electrons inductively, stabilising the ammonium cation). In aqueous solution, the trend gets modulated by hydration — secondary amines typically end up most basic (2° > 1° > 3° > NH₃ in water for many alkyl amines). Aromatic amines are weaker bases than aliphatic ones because the aniline lone pair is delocalised into the ring and is less available for protonation.

Preparation of amines is a JEE staple. Gabriel synthesis (using phthalimide + alkyl halide + hydrolysis) gives pure primary amines and avoids the over-alkylation problem of direct SN2 on ammonia. Hoffmann bromamide rearrangement converts primary amides to primary amines with one fewer carbon. Reduction of nitriles (LiAlH₄) gives primary amines; reduction of amides (LiAlH₄) gives the corresponding amine; reduction of nitro compounds (Sn/HCl) gives primary amines from nitroarenes.

Aromatic amines undergo diazotisation with NaNO₂/HCl at 0–5°C to give aryl diazonium salts, which are among the most useful synthetic intermediates in organic chemistry. They couple with phenols to give azo dyes, replace –N₂⁺ with –Cl, –Br, –OH, –CN, –H (Sandmeyer, Gattermann, Balz-Schiemann reactions), and generally serve as anchors for building complex aromatic systems.

Key reactions & principles

• Basicity in aqueous: 2° > 1° > 3° > NH₃ (for typical aliphatic amines; solvation matters)
• Aromatic vs aliphatic: Aliphatic amines >> aromatic amines (resonance depletes aniline lone pair)
• Gabriel synthesis: Phthalimide + R–X + hydrazine → pure 1° amine
• Hoffmann rearrangement: R–CONH₂ + Br₂ / NaOH → R–NH₂ + CO₂ (one carbon shorter)
• Diazotisation: Ar–NH₂ + NaNO₂ / HCl / 0–5°C → Ar–N₂⁺ Cl⁻
• Sandmeyer reactions: Ar–N₂⁺ + CuCl → Ar–Cl; CuBr → Ar–Br; CuCN → Ar–CN
• Azo coupling: Ar–N₂⁺ + Ar'–OH/NH₂ → azo dye Ar–N=N–Ar'

Common mistakes to avoid

• Predicting basicity purely from inductive effects — solvation flips the gas-phase trend in aqueous solution.
• Assuming aniline is a strong base — it's much weaker than aliphatic amines because the lone pair delocalises into the ring.
• Using diazotisation at room temperature — aryl diazonium salts are unstable above 5°C and decompose.
• Attempting Gabriel synthesis with an aryl halide — SN2 on aryl halide fails; Gabriel works with alkyl halides only.

JEE Main relevance

JEE Main asks 1–2 amine questions every paper — usually one basicity-ordering and one preparation or diazonium-reaction problem. Sandmeyer and Gabriel are the two most-tested named reactions.

JEE Advanced relevance

Advanced problems combine amine chemistry with EAS — for instance, using diazonium salts as a masking group to control regioselectivity in a poly-substituted aromatic synthesis. Retrosynthesis problems involving amines as the disconnection point are also standard Advanced patterns.

Previous-year questions (PYQ)

Filter the PYQ bank by 'Amines' and target Sandmeyer + Hoffmann + Gabriel in the first pass — these three reactions cover the majority of the year-to-year archetypes.