Nitrogen Compounds for JEE Main: Amines & Diazonium

Nitrogen compounds contribute three to five marks to JEE Main Chemistry every session, testing basicity comparisons, preparation methods for amines, and diazonium chemistry. This chapter has a high conceptual density: the basicity of amines depends simultaneously on inductive effects, resonance, and solvation — three competing factors that students often apply incorrectly. This guide resolves each factor clearly and gives you a reliable framework for every basicity question.

Classification and Basicity of Amines

Amines are classified by the number of carbon groups on nitrogen: primary (1°, one C attached), secondary (2°), tertiary (3°). The lone pair on nitrogen makes amines basic (Lewis base and Brønsted base). Basicity comparisons: in the gas phase, the order is 3° > 2° > 1° > NH₃ (more alkyl groups, more electron-donating induction). In aqueous solution, the order for aliphatic amines becomes 2° > 1° ≈ 3° > NH₃, because solvation stabilises the conjugate acid (ammonium ion) — tertiary amines have less solvation due to steric hindrance. This aqueous vs gas-phase difference is a classic JEE question.

Aromatic amines (aniline): the lone pair delocalises into the benzene ring, making aniline far less basic than aliphatic amines. Electron-withdrawing groups (NO₂) on the ring further reduce basicity; electron-donating groups (CH₃) slightly increase it. Ortho, para positions matter for the resonance effect; meta positions are influenced only inductively. The basicity comparison "aliphatic amines >> aniline" is directly tested. Take a free nitrogen compounds mock to test your basicity ordering speed.

Preparation of Amines

Key preparation methods JEE tests: (1) Reduction of nitro compounds (with Fe/HCl or H₂/Ni): ArNO₂ → ArNH₂. This is the main industrial route to aniline. (2) Gabriel phthalimide synthesis: phthalimide + KOH → potassium phthalimide, then reaction with primary alkyl halide → N-substituted phthalimide, then hydrolysis with NaOH → primary amine. Limitation: gives only primary amines, and aryl halides do not work (no SN2 on arenes). (3) Hoffmann bromamide degradation: primary amide RCONH₂ + Br₂ + NaOH → primary amine with one fewer carbon (RNH₂). This reaction degrades the chain by one carbon — a structural change that JEE tests directly. (4) Reductive amination: aldehyde or ketone + NH₃ + H₂/Ni → amine.

Diazonium Salts and Coupling Reactions

Diazonium salts (ArN₂⁺Cl⁻) are prepared by diazotisation: ArNH₂ + NaNO₂ + HCl at 0–5°C → ArN₂⁺Cl⁻. The low temperature is essential — above 5°C, the diazonium salt decomposes. Diazonium salts are highly versatile synthetic intermediates. Replacement reactions (Sandmeyer reactions): diazonium + CuCl/HCl → ArCl; + CuBr/HBr → ArBr; + KCN/CuCN → ArCN; + Cu₂O/HNO₂ → ArNO₂. Direct replacement: diazonium + HBF₄ + heat → ArF (Balz-Schiemann); + H₂O/heat → ArOH; + H₃PO₂ → ArH (reduction, removes NH₂).

Coupling reactions: diazonium salt reacts with aniline (weakly alkaline medium) or phenol (alkaline medium) at the para position to give azo dyes (–N=N– linkage). This coupling only works at the para position of activated rings (aniline, phenol). The resulting azo compounds are coloured because the extended conjugated system absorbs visible light. JEE tests which conditions (pH) favour coupling with phenol vs aniline. For complete organic coverage, pair this guide with our carbonyl compounds guide and organic reactions mechanism guide.