Basicity of Amines

The lone pair on the amine nitrogen makes amines both basic and nucleophilic -- they react with acids to form salts (e.g. aniline + HCl gives anilinium chloride, C6H5-NH3+Cl-). In water, R-NH2 + H2O sits in equilibrium with R-NH3+ + OH-, an equilibrium that lies far to the left, so amines are weak bases compared with NaOH. The BASICITY CONSTANT, Kb = [R-NH3+][OH-]/[R-NH2], measures how readily the amine accepts a proton from water: the larger the Kb (equivalently, the smaller the pKb), the stronger the base. STRUCTURE-BASICITY RELATIONSHIP: an electron-DONATING (+I) alkyl group on nitrogen raises electron density there, making the lone pair more available for protonation -- so alkylamines are stronger bases than ammonia itself, and the expected gas-phase order is R3N > R2NH > R-NH2 (3 degree > 2 degree > 1 degree), since each added alkyl group donates further. In AQUEOUS solution this simple order breaks down (as the measured pKb table shows), because SOLVATION also matters: the protonated ammonium cation is stabilised by hydrogen-bonding water molecules, and a BIGGER cation (more alkyl bulk) is solvated LESS well and creates more steric hindrance to that solvation -- so combining the +I (basicity-raising) and solvation (basicity-lowering with size) effects gives the real aqueous order 2 degree > 1 degree > 3 degree, e.g. (CH3)2NH > CH3NH2 > (CH3)3N > NH3. AROMATIC AMINES: in aniline, the nitrogen lone pair delocalises into the benzene ring by resonance, making it far less available for protonation -- so aniline is markedly LESS basic than ammonia or any alkylamine (pKb 9.38 vs 4.74 for NH3). On a substituted aniline ring, electron-RELEASING groups (-CH3, -OCH3, -NH2) raise basic stren …