Explain the acidic nature of phenols and compare with that of alcohols.

Explanation

The reactions of phenol with metals (e.g. : `Na, Al`) and NaOH indicate its acidic nature. The hydroxyl group, in phenol is directly attached to the `sp^2` hybridised carbon of benzene ring which acts as an electron withdrawing group. The charge distribution in phenol molecule as depicted in its resonance structures, causes the oxygen of - OH group to be positive.

The reaction of phenol with NaOH solution indicates that phenols are stronger acids than alcohols and water. Let us compare the acidic nature of phenol with that of alcohol.
The ionisation of an alcohol and a phenol takes place as shown below.

Owing to the higher electronegativity of `sp^2` hybridised carbon of phenol to which - OH is attached, electron density decreases on oxygen. This increases the polarity of - OH bond and results in an increase in ionisation of phenols than that of alcohols.
The relative stabilities of alkoxide and phenoxide ions should be considered. In alkoxide ion, the negative charge is localised, on oxygen whereas in phenoxide ion, the charge is delocalised. The delocalisation of negative charge makes phenoxide ion more stable and favours the ionisation of phenol. There is delocalisation in unionised phenol also but the resonance structures have charge separation. Hence, phenol molecule is less stable than phenoxide ion.