Question

Though chlorine shows strong $-I$ effect, why is it ortho/para directing?

Hint:

Cl = ortho/para director (due to $+M$) but deactivator (due to $-I$ dominant for overall ring activity). $-M \gt -I$ wins for directing.

Answer 1

Step 1: Two effects of Cl on the ring.
Chlorine has two opposing effects: a strong electron-withdrawing inductive effect ($-I$, due to high electronegativity) AND an electron-donating resonance effect ($+M$, because Cl lone pairs can conjugate with the ring $\pi$ system).
Step 2: Where resonance donation enriches electron density.
When Cl donates a lone pair into the ring through resonance, the electron density is directed specifically to the ortho and para positions (resonance structures place negative charge at these positions). Meta positions are NOT enriched.
Step 3: Which effect controls directing?
For directing purposes in EAS, the resonance ($+M$) effect dominates over the inductive ($-I$) effect. Electrophiles attack the electron-rich ortho and para positions.
Step 4: Conclusion.
Cl is an ortho/para director because resonance lone-pair donation activates ortho/para positions, overriding its $-I$ effect for directing. Overall, Cl is a deactivating ortho/para director (ring is less reactive than benzene, but attack occurs at ortho/para positions).