Question

Explain the following briefly.

In chlorobenzene, the electrophilic substitution takes place at o- and p-positions.

Answer 1

In chlorobenzene, electrophilic substitution takes place primarily at the ortho (o-) and para (p-) positions due to the following reasons:

1. Electron-donating effect of -Cl substituent: Chlorine attached to the benzene ring is an electron-withdrawing group by induction but also has lone pairs that can participate in resonance, donating electron density into the ring, increasing electron density at the o- and p- positions.
2. Resonance stabilization of the sigma complex (arenium ion): During electrophilic substitution, the intermediate carbocation formed is more stabilized when the electrophile attacks the o- or p- positions because resonance structures involve the lone pair of chlorine, delocalizing the positive charge.
3. Activation of o- and p-positions: Due to the above effects, these positions are more reactive sites for electrophilic substitution compared to the meta position.
4. Electrophilic aromatic substitution reaction example (chlorination):
   \[\text{C}_6\text{H}_5\text{Cl} + \text{Cl}_2 \xrightarrow{\text{FeCl}_3} \text{o- or p-dichlorobenzene} + \text{HCl}\]

Thus, in chlorobenzene, electrophilic substitution preferentially occurs at ortho and para positions due to resonance donation by the chlorine substituent stabilizing intermediates in these positions.