Explain, why is chlorobenzene less reactive than benzene in electrophilic substitution reaction?

Chlorobenzene is less reactive than benzene in electrophilic aromatic substitution (EAS) due to the electron-withdrawing inductive effect of the chlorine atom and the deactivation by resonance.

Benzene (π-electron cloud)Benzene (π-electron cloud)Chlorobenzene (Cl withdraws electrons)Chlorobenzene (Cl withdraws electrons)

1. Inductive Effect: Chlorine is more electronegative than carbon, pulling electron density away from the benzene ring through the σ-bond (–I effect). This reduces the overall electron density of the ring, making it less attractive to electrophiles.

2. Resonance Effect: While chlorine has lone pairs that can donate electrons into the ring via resonance (+R effect), this donation is weaker than the –I effect. The net result is a deactivated ring (lower electron density) compared to benzene, which has no such competing effects.

3. Directing Effect: Chlorine is an ortho/para director but a deactivator because its –I effect dominates. The partial positive charge on the ring (due to –I) slows down the rate of EAS.

Thus, chlorobenzene undergoes EAS slower than benzene, and the substitution occurs preferentially at the ortho and para positions relative to the chlorine atom.