Amines
Amines questions concentrate on basicity trends, preparative routes that control the degree of substitution, and diazonium chemistry.
Basicity
- in the gas phase, more alkyl groups mean higher basicity (electron-releasing +I effect)
- in water, solvation and steric hindrance compete; for ethylamines the order is Et2NH > EtNH2 > Et3N > NH3
- aromatic amines are much weaker: the lone pair is delocalised into the ring; aniline is weaker than ammonia and methylamine
- electron-withdrawing groups reduce basicity: p-nitroaniline < aniline
Preparation
- Hofmann bromamide: amide + Br2/KOH gives a primary amine with one carbon fewer
- Gabriel phthalimide synthesis gives pure primary amines but fails for aryl amines
- reduction of nitriles gives primary amines; reduction of amides or nitro compounds also works
- reductive amination of aldehydes/ketones gives 2° amines
Identification and diazonium salts
- carbylamine test: primary amines with CHCl3/KOH give foul-smelling isocyanides
- Hinsberg test with benzenesulfonyl chloride separates 1°, 2° and 3° amines
- diazotisation: aniline with NaNO2 + HCl at 273–278 K
- coupling with phenol gives p-hydroxyazobenzene dye; Sandmeyer gives aryl halides
Worked example
Why is aniline a weaker base than ethylamine? In aniline the nitrogen lone pair participates in resonance with the benzene ring, so it is less available for protonation; in ethylamine the alkyl group instead pushes electron density onto nitrogen and stabilises the ammonium ion. Hence ethylamine protonates far more readily.
Traps that cost marks
- Quoting the gas-phase order for aqueous basicity of alkylamines.
- Using Gabriel synthesis for aniline.
- Forgetting that diazotisation must be kept near 0–5 °C.