Amines
Amines is examined through basicity comparisons, tests distinguishing 1°, 2° and 3° amines, and diazonium chemistry.
Core rules
- Basicity in water: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃. The inductive effect favours more alkyl groups, solvation favours fewer, and steric hindrance opposes the 3° amine.
- Aniline is weaker than ammonia because the lone pair is delocalised into the ring. Electron-withdrawing nitro groups weaken it further, while CH₃ or OCH₃ strengthen it.
- Hinsberg test with benzenesulphonyl chloride: 1° amine gives a product soluble in alkali, 2° an insoluble sulphonamide, 3° no reaction.
- Carbylamine test (CHCl₃ + KOH) is positive only for 1° amines, giving a foul-smelling isocyanide.
- Gabriel phthalimide synthesis gives pure 1° amines only; it fails for 2° and 3° amines.
- Hofmann bromamide: RCONH₂ + Br₂/KOH → RNH₂, with one carbon fewer.
- Diazonium salts: aniline + NaNO₂/HCl at 273–278 K gives benzenediazonium chloride; Sandmeyer with CuCl/HCl gives chlorobenzene; coupling with phenol in mild alkali gives an azo dye.
Worked example
Arrange p-nitroaniline, aniline and ethylamine in increasing basicity. The nitro group withdraws electron density and destabilises the anilinium ion, making p-nitroaniline weakest. Ethylamine has an electron-releasing alkyl group on an sp³ nitrogen with no ring delocalisation, so it is strongest. Order: p-nitroaniline < aniline < ethylamine.
Traps that cost marks
- Confusing aqueous-phase basicity with gas-phase basicity, where 3° amines are strongest.
- Saying 3° amines give a Hinsberg derivative.
- Nitrating aniline directly on a large scale - protect it by acetylation first.
- Forgetting to keep the diazonium salt near 0–5 °C, or it decomposes to phenol.
- Mixing up Hofmann bromamide (loses a carbon) with Gabriel synthesis (chain retained).