ACIDITY OF PHENOLS:
Phenols are considerably more acidic than alcohols however less so than carboxylic acids or even carbonic acid. This is indicated in the order of acidity constants:
Phenols are acidic due to the formation of stable phenoxide ion in aqueous solution. For example, phenols itself gives phenoxide ion on dissociation.
The phenoxide ion is stable due to resonance:
Notice that the negative charge is spread throughout benzene ring, and thereby effectively dispersed. This charge delocalisation is a stabilizing factor in the phenoxide ion. One the other hand, no resonance is possible in alkoxide ions (NO–) derived from alcohols. The negative charge is concentrated (localized) on a single oxygen atom. Consequently, alcohols are much weaker acids than phenols.
EFFECT OF SUBSTITUENTS ON ACIDITY:
Effect Of Electron-Withdrawing Substituents. An electron-withdrawing group (e.g.,-NO2.-Cl, -CN, _CHO, –COOH) on the aromatic ring is acid-strengthening. It enables the ring to withdraw more electrons from the phenoxy oxygen. This stabilises the phenoxide ion still further and results in a stronger acid. For example, p-nitrophenol is more acidic than phenol.
Effect of Electron-Releasing Substituents. An electron-releasing group (e.g., -CH3, -OCH3, -NH2) on the aromatic ring is acid-weakening. It strengthens the negative charge on phenoxy oxygen and inhibits the charge delocalization due to resonance. This destabilises the phenoxide ion and result in a weaker acid. For example, p-cresol is less acidic than phenol.
The relative acid-strengths of some phenols are as follows: