Explain why the reactivity of the benzene ring increases due to the presence of the $-OH$ group in phenol.

(N/A) The $-OH$ group is an electron-donating group that activates the benzene ring towards electrophilic substitution reactions due to the following reasons:
$(a)$ Resonance Effect: The lone pair of electrons on the oxygen atom of the $-OH$ group is delocalized into the benzene ring through resonance. This increases the electron density at the ortho and para positions of the ring,making them more susceptible to attack by electrophiles.
$(b)$ Inductive Effect: The oxygen atom is more electronegative than carbon,which exerts a $-I$ effect,tending to withdraw electrons. However,the $+R$ (resonance) effect of the $-OH$ group is much stronger than its $-I$ effect.
Conclusion: Since the resonance effect dominates,the overall electron density in the benzene ring of phenol increases,especially at the ortho and para positions. Consequently,phenol is more reactive than benzene towards electrophilic aromatic substitution,and the $-OH$ group acts as an activating group.