Though chlorine shows strong $-I$ effect, why is it ortho/para directing?
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Solution and Explanation
The directing effect in electrophilic aromatic substitution is governed by resonance, not just the inductive effect.
Step 2: Analysis
Chlorine is strongly electronegative and withdraws electron density inductively ($-I$ effect), which would suggest meta direction. However, chlorine also has lone pairs on its atom. These lone pairs are in conjugation with the $\pi$-system of the ring and donate electron density through resonance ($+M$ effect) specifically at the ortho and para positions.
Step 3: Conclusion
The $+M$ (resonance donation) effect of chlorine dominates its $-I$ effect for directing purposes. The ortho and para positions are enriched with electron density through resonance structures, directing incoming electrophiles to these positions. However, the $-I$ effect deactivates the ring overall (Cl is a deactivating, ortho/para director).
Final Answer: Although $-$Cl has a $-I$ effect, its lone pairs donate electron density to ortho and para positions via resonance ($+M$ effect). The resonance effect dominates for directing, making Cl an ortho/para director (though a ring deactivator overall).
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