Explain how does the –OH group attached to a carbon of benzene ring activate it towards electrophilic substitution?

Explanation

Phenol may be regarded as a resonance hybrid of structures, I-V as shown in Section `12.10` (b). As a result of `+R` effect of the `(OH)` group, the electron density in the benzene ring increases, thereby facilitating the attack by an electophile. In other words, the presence of `(OH)` group activates the benzene ring towards electrophilic substitution reactions. Further, since the electron density is relatively higher at the two o- and p-position, therefore, electrophilic substitution occurs mainly at o-and p-position.