: 220
Phenol (pKa ~10) is far more acidic than alcohols (pKa ~16-18) because the phenoxide ion is resonance-stabilised — the negative charge delocalises into the benzene ring across 5 resonance structures. Phenol reacts with NaOH but not NaHCO3. This NaHCO3 test distinguishes phenols from carboxylic acids (which react with both).
Substituent effects on phenol acidity follow predictable patterns. Electron-withdrawing groups (EWGs) increase acidity by stabilising the phenoxide: -NO2 (-I, -M) > -Cl (-I) > -H. Electron-donating groups (EDGs) decrease acidity: -NH2 (+M) < -OCH3 (+M) < -CH3 (+I) < -H. Position matters: para-EWG is more effective than meta (direct resonance with O-) while meta-EWG operates only through -I effect.
Important acidity order: 2,4,6-trinitrophenol (pKa ~0.4) > p-nitrophenol (7.15) > p-chlorophenol (9.38) > phenol (9.98) > p-cresol (10.26). The intramolecular H-bonding in o-nitrophenol makes it steam-volatile (separable from p-nitrophenol by steam distillation).