: 240
Amine basicity in aqueous solution depends on three competing factors: (1) Inductive effect — more alkyl groups increase electron density on N (3° > 2° > 1°). (2) Solvation — smaller, less substituted aminium ions are better hydrated, stabilising the conjugate acid (1° > 2° > 3°). (3) Steric effects — bulky groups hinder protonation and solvation.
The net result for aliphatic amines in water: 2° > 1° > 3° > NH3. In gas phase (no solvation): 3° > 2° > 1° > NH3.
Aromatic amines are much weaker bases than aliphatic. Aniline (pKb ~9.4) vs methylamine (pKb ~3.3). The nitrogen lone pair in aniline is delocalised into the ring, making it less available for protonation.
Substituent effects on aniline: EWGs (NO2, CN) decrease basicity further by pulling electron density from N. EDGs (CH3, OCH3) increase basicity by pushing electron density toward N. Key order: p-OCH3-aniline > p-CH3-aniline > aniline > p-Cl-aniline > p-NO2-aniline.
Special cases: Pyridine (pKb ~8.8) > aniline (9.4) because pyridine's lone pair is NOT in the pi system. Pyrrole is non-basic (lone pair IS in the aromatic sextet). Benzylamine (~4.7) behaves as aliphatic (CH2 insulates from ring).