Question

In spite of being an electron withdrawing group, why does chlorine act as ortho-para directing group in aromatic electrophilic substitution reactions?

Hint:

Remember: Halogens are exceptions – they are deactivating but ortho-para directing.

Answer 1

Step 1: Nature of chlorine as a substituent.
Chlorine is an electron withdrawing group due to its strong –I (inductive) effect because of high electronegativity. This tends to decrease electron density on the benzene ring.
Step 2: Resonance effect of chlorine.
Chlorine possesses lone pairs of electrons which it can donate to the benzene ring through resonance (+R effect). This resonance increases electron density particularly at the ortho and para positions of the benzene ring.
Step 3: Combined effect.
- The –I effect makes chlorine a deactivating group (reaction rate decreases).
- The +R effect directs the incoming electrophile to ortho and para positions.
Conclusion:
Although chlorine is overall deactivating due to its –I effect, it acts as an
ortho-para directing group in electrophilic substitution reactions because of its strong +R effect.