The –OH group attached to a benzene ring activates it towards electrophilic substitution through the resonance effect. The lone pair of electrons on the oxygen atom of the –OH group gets delocalized into the benzene ring through resonance.

This resonance effect makes the ortho and para positions of the benzene ring electron-rich. As a result, these positions become more susceptible to attack by electrophiles. The activation is so strong that reactions like halogenation of phenol can occur even in the absence of a Lewis acid catalyst (such as FeBr₃), unlike benzene which requires such catalysts for electrophilic substitution.