Phenols are more acidic than alcohols. This can be explained by considering various properties such as resonance, electronic structure, inductive effect, dissociation constant and bond properties.
Acidic Nature of Phenols
In phenols, the negative charge on the oxygen atom of the phenoxide ion can undergo delocalization of electrons throughout the ring, which results in the stabilization of the phenoxide ion (C6H5O–). On the other hand, alcohols does not have any delocalization of electrons. While both phenol and the phenolate ion have delocalized electrons, the resonance energy of the phenolate ion is greater than that of phenol because three of phenol’s resonance contributors have separated charges. The loss of a proton from phenol, therefore, is accompanied by an increase in resonance energy.
The acidity of phenols is due to the presence of the hydroxyl (OH) group. Phenols can dissociate to form a phenoxide ion (C6H5O–) and a hydrogen ion (H+), thereby increasing the dissociation constant (Ka) of the hydroxyl group of the phenols.
The aromatic ring in phenols acts as an electron-withdrawing group and shows an withdrawing effect, called as inductive effect (- I effect), which stabilizes the negative charge on the oxygen atom of the phenoxide ion. Greater inductive electron withdrawal by the sp2 carbon stabilizes the conjugate base by decreasing the electron density of its negatively charged oxygen. This makes the phenoxide ion (C6H5O–) more stable. This results in phenols being stronger acids than alcohols.
Acidic Nature of Alcohols
On the other hand, alcohols lack an electron-withdrawing group and therefore shows lesser inductive effect. The OH group in alcohols is attached directly to a sp3 carbon atom, which makes the dissociation of the hydroxyl group less favourable. This results in a lower dissociation constant (Ka) and makes alcohols weaker acids than phenols. Neither alcohols nor its conjugate base has delocalized electrons, so loss of a proton is not accompanied by an increase in resonance energy.
In conclusion, the presence of the aromatic ring in phenols makes the hydroxyl group more acidic than in alcohols. The loss of a proton from phenol to form phenolate ion, therefore, is accompanied by an increase in resonance energy. The aromatic ring acts as an electron-withdrawing group, making the phenoxide ion more stable and increasing the dissociation constant (Ka) of the hydroxyl group. Hence, phenols are more acidic than alcohols.