Acidity of Phenol

Phenol is considerably more acidic than aliphatic alcohols: unlike an alcohol, phenol reacts with aqueous NaOH to form sodium phenoxide, C6H5-OH + NaOH gives C6H5-O-Na+ + H2O. The equilibrium C6H5-OH + H2O reversible-with C6H5-O- + H3O+ has Ka about 1x10^-10 at 25 degrees C -- many orders of magnitude larger than an alcohol's. This increased acidity is explained by the stability of the phenoxide ion: the negative charge on phenoxide's oxygen is delocalised into the aromatic ring by resonance, spreading the charge and lowering the ion's energy far more than is possible for a simple alkoxide. In SUBSTITUTED phenols, electron-withdrawing groups (-NO2, -Cl) at the ortho or para position further stabilise the phenoxide ion by extending this delocalisation, and so INCREASE acidity (Table 11.3: o- and p-nitrophenol, pKa about 7.2 and 7.1, are far more acidic than phenol itself, pKa 1 …