Phenol is more acidic than alcohols (like ethanol) and water mainly because the conjugate base formed after losing a proton, the phenoxide ion, is resonance stabilized. In phenol, the negative charge on the oxygen after losing H+H^+H+ is delocalized over the aromatic benzene ring via resonance, spreading out the charge and making the ion more stable. This resonance stabilization reduces the tendency for the phenoxide ion to reabsorb the proton, thus increasing phenol's acidity. In contrast, when water or ethanol loses a proton, the resulting ions (hydroxide or ethoxide) do not have resonance stabilization. The negative charge remains localized on the oxygen atom, making these conjugate bases less stable and less likely to remain ionized. Additionally, electron-donating groups like the ethyl group in ethanol increase electron density on oxygen, reducing acidity further. Thus, the key factors making phenol more acidic are:
- Resonance stabilization of the phenoxide ion by delocalization of negative charge over the benzene ring.
- Lack of such resonance stabilization in water and alcohols.
- Electron withdrawing effect of the aromatic ring enhances acidity, unlike electron donating alkyl groups in alcohols.
This explains why phenol is more acidic than both water and ethanol.
