Chapter 9 — Amines

Chapter 9 of Class 12 Chemistry Part II covers amines — organic compounds derived from ammonia by replacing one, two, or all three hydrogen atoms with alkyl or aryl groups — and diazonium salts, key intermediates in synthesising aromatic compounds including azo dyes.

Amines are derivatives of ammonia classified as primary (RNH2), secondary (R2NH), or tertiary (R3N) depending on the number of hydrogen atoms replaced. Nitrogen in amines is sp3 hybridised with a pyramidal geometry and an unshared electron pair, making amines Lewis bases. Aliphatic amines are stronger bases than ammonia due to the +I effect of alkyl groups, while aromatic amines like aniline are weaker bases because the lone pair delocalises into the benzene ring. Key preparation methods include reduction of nitro compounds, ammonolysis of alkyl halides, Gabriel phthalimide synthesis, and Hofmann bromamide degradation. Diazonium salts, formed by diazotisation of primary aromatic amines at 273–278 K, are versatile intermediates for introducing –F, –Cl, –Br, –I, –CN, –OH, and –NO2 groups into aromatic rings via Sandmeyer and coupling reactions.

1. 01Amines are classified as primary, secondary, or tertiary based on how many hydrogen atoms of ammonia are replaced by alkyl/aryl groups; nitrogen is sp3 hybridised with a pyramidal shape.
2. 02Aliphatic amines are stronger bases than ammonia (pKb 3–4.22) due to the electron-releasing +I effect of alkyl groups; aromatic amines are weaker bases (aniline pKb 9.38) because the lone pair conjugates with the benzene ring.
3. 03Six major preparation methods include reduction of nitro compounds, ammonolysis of alkyl halides, reduction of nitriles and amides, Gabriel phthalimide synthesis (primary amines only), and Hofmann bromamide degradation (amine has one fewer carbon than the amide).
4. 04The carbylamine reaction (foul-smelling isocyanide formation with CHCl3/KOH) is a distinguishing test exclusive to primary amines; Hinsberg's reagent (benzenesulphonyl chloride) differentiates all three classes.
5. 05Diazonium salts (ArN2+X–) are formed by diazotisation of primary aromatic amines with NaNO2/HCl at 273–278 K; they undergo Sandmeyer, Gattermann, and coupling reactions to introduce a wide range of substituents.
6. 06Coupling of diazonium salts with phenols or arylamines at the para position forms azo compounds with extended conjugation, widely used as azo dyes.