Chapter 9 — Amines

Chapter 9 of the Class 12 Chemistry NCERT textbook, "Amines", covers 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.

• Structure and classification of amines — The chapter treats amines as ammonia derivatives — primary, secondary or tertiary by how many hydrogens are replaced — with sp3 nitrogen carrying a lone pair, a pyramidal shape, and Lewis-base character that underlies their reactivity.
• What controls basic strength — It explains why aliphatic amines are stronger bases than ammonia through the electron-releasing +I effect, while aromatic amines like aniline are weaker because the nitrogen lone pair is drawn into the benzene ring.
• Preparing amines — Several preparation routes are compared — reduction of nitro compounds, ammonolysis, Gabriel synthesis and Hofmann bromamide degradation — alongside distinguishing tests such as the carbylamine reaction that identify amine class.
• Diazonium salts as synthetic hubs — Formed by diazotising primary aromatic amines at low temperature, diazonium salts act as versatile intermediates that introduce many substituents into aromatic rings and couple with phenols or arylamines to make widely used azo dyes.

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.