Question

Explain why hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group is attached to an alkyl group?

Answer 1

Hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group is attached to an alkyl group. Phenols are a million times more acidic than alcohols.
In alcohols,$-OH$ group is attached to $C$ atom which is $s{p}^3$ hybridised. In phenols, $-OH$ group is attached to $C$ atom which is $s{p}^2$ hybridised. $s{p}^2$ hybridised $C$ atom is more electronegative than $s{p}^3$ hybridised. Thus electron density on $O$ atom in phenol is lower than the electron density on $O$ atom in alcohol. Hence, the polarity of $-OH$ bond and ionization in phenol is more than that in alcohol.
Also when alcohols lose a proton, the resulting alkoxide ion has negative charge localised on $O$ atom. However when phenols lose a proton, the resulting phenoxide ion has negative charge of $O$ atom delocalised over ortho and para positions of aromatic ring. Hence, phenoxide ion is more stable than alkoxide ion.