Question

Why is phenol more reactive in electrophilic aromatic substitutions?

Answer 1

The phenyl ring is activated by delocalization of the lone pairs of oxygen around the ring........

Explanation:

Benzene normally requires Lewis acid activation for electrophilic aromatic substitution; for instance, bromination of the `C_6H_6` ring requires the bromine (obviously), and some Lewis acid catalysis. (Typically aluminum or iron metal which is oxidized up to `MBr_3`, and this acts as a bromine transfer reagent).

By constrast, when phenol is treated with bromine, presumably because of the donation of `pi` electrons to the ring by oxygen, bromination is facile. And 2,4,5-substitution of bromine (with respect to the `OH` group) may be observed, sometimes even in the absence of Lewis acid catalysis.

At any rate, there is nothing I can say here that you won't learn better than from your organic chemistry text. Read the chapters on electrophilic aromatic substitution, and digest their account.