Q.Arrange the following amines in the increasing order of their pKb values.
Aniline, Cyclohexylamine, 4-Nitroaniline

An amine's nitrogen lone pair makes it both basic and nucleophilic (reacting with acids to give a salt, e.g. aniline + HCl to anilinium chloride). In water, R-NH2 + H2O sits in equilibrium with R-NH3+ + OH-, an equilibrium lying far left, so amines are weak bases versus NaOH; the basicity constant Kb (larger Kb / smaller pKb = stronger base) measures this. STRUCTURE-BASICITY: an alkyl group's +I effect raises nitrogen's electron density, so alkylamines beat ammonia, and the expected GAS-PHASE order is R3N > R2NH > R-NH2 (more alkyl substitution, more donation). In AQUEOUS solution this order breaks, because a bigger, more alkyl-substituted ammonium cation is SOLVATED LESS well (more steric hindrance to the stabilising water shell) -- combining +I and solvation gives the real order 2 degree > 1 degree > 3 degree (e.g. (CH3)2NH > CH3NH2 > (CH3)3N > NH3). AROMATIC amines are different again: aniline's nitrogen lone pair delocalises into the ring by resonance, far less available for protonation, so aniline is markedly LESS basic than ammonia (pKb 9.38 vs 4.74). On a substituted aniline, electron-RELEASING ring substituents (-CH3, -OCH3, -NH2) raise basicity (lower pKb); electron-WITHDRAWING ones (-NO2, halogens, -COOH) lower it (raise pKb). The overall family order: Alkyl amines > Aralkyl amines (benzylamine) > Ammonia > N-Aralkyl amines (N-methylani …