Question

Some meta-directing substituents in aromatic substitution are given. Which one is most deactivating?

• A. $-NO_2$
• B. $-C = N$
• C. $-SO_3 H$
• D. $ -COOH$

Answer 1

The Correct Option is A

The question asks which meta-directing substituent is the most deactivating in aromatic substitution. Let's analyze the given substituents in terms of their electronic effects on the aromatic ring:

1. Meta-directing and Deactivating Nature:
   • Meta-directing groups are typically electron-withdrawing groups.
   • They decrease the electron density of the aromatic ring, making the ring less reactive to electrophilic aromatic substitution (EAS).
   • Such groups often exert a strong inductive or resonance effect that pulls electron density away from the aromatic ring.
2. Review of the Given Substituents:
   • -NO_2 (Nitro group): This is a strong electron-withdrawing group due to resonance and inductive effects. It withdraws electrons via resonance by forming a resonance structure that places a positive charge on the ring. It is one of the most deactivating substituents.
   • -C = N (Cyano group): This group also withdraws electrons through resonance as well as inductive effects but is generally not as deactivating as the nitro group.
   • -SO_3H (Sulfonic acid group): It is an electron-withdrawing group through inductive effects but is less deactivating than the nitro group.
   • -COOH (Carboxyl group): This group is electron-withdrawing through resonance effects, but not as strongly as the nitro group.
3. Conclusion: Among all the substituents provided, -NO_2 is the most deactivating due to its highly effective electron-withdrawing properties via both resonance and strong inductive effects.

Thus, the most deactivating meta-directing substituent in this group is -NO_2.