Question

Explain the following briefly.

The electrophilic substitution reactions in haloarenes occur slowly as compared to those in benzene.

Answer 1

1. Nature of C–X Bond: In haloarenes, the carbon to halogen bond (C–X) is partly covalent with the halogen having lone pairs. The halogen exerts both an electron-withdrawing inductive effect (-I) and an electron-donating resonance effect (+R) on the aromatic ring.
2. Inductive Effect Dominates: The halogen is more electronegative, so the -I effect (electron withdrawal) is stronger, decreasing the electron density on the benzene ring. Lower electron density means the ring is less reactive towards electrophiles, slowing down electrophilic substitution.
3. Resonance Effect and Halogen’s Position: The halogen donates electrons through resonance (+R effect), but this delocalization tends to stabilize only certain positions (ortho/para). Overall, the electron-withdrawing effect predominates, making the net reactivity of haloarenes lower than benzene.
4. Deactivation of the Ring: Due to electron density decrease, haloarenes are deactivated towards electrophilic substitution relative to benzene.
   Thus, the reaction proceeds more slowly.
5. Reaction Equation (example – chlorination of chlorobenzene):
   \[\mathrm{C_6H_5Cl} + \mathrm{Cl_2} \xrightarrow{FeCl_3}\mathrm{C_6H_4Cl_2} + \mathrm{HCl}\]
   The reaction requires a catalyst (FeCl₃) but proceeds slower than chlorination of benzene due to deactivation.