Although chlorine shows strong \( -I \) effect, yet it is ortho- and para-directing in electrophilic aromatic substitution reactions. Why?
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Solution and Explanation
• Substituents on a benzene ring affect reactivity and orientation via two competing effects: Inductive (\( I \)) effect and Resonance (\( R \) or \( M \)) effect.
• Inductive effect works through sigma bonds; Resonance works through pi systems.
Step 1: Explain the deactivating nature (\( -I \))
Chlorine is highly electronegative. It withdraws electron density from the benzene ring through the sigma bond (\( -I \) effect). This reduces the overall electron density on the ring, making it less reactive (deactivated) compared to benzene.
Step 2: Explain the directing nature (\( +R \))
Chlorine has lone pairs of electrons. These lone pairs can be delocalized into the benzene ring through resonance (\( +R \) effect).
Resonance structures show that the electron density increases specifically at the ortho and para positions.
Step 3: Reconcile the effects
While the \( -I \) effect is stronger and controls the reactivity (deactivation), the \( +R \) effect stabilizes the intermediate carbocation formed during attack at ortho/para positions.
Therefore, the incoming electrophile is directed to the ortho and para positions despite the ring being deactivated. The final answer is due to the \( +R \) effect which stabilizes ortho and para attack intermediates.
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