Resonance stabilization plays a key role in nucleophilic aromatic substitution (NAS) by stabilizing the intermediate, making the reaction much easier. Here’s a clear explanation:
1. Mechanism recap
Most NAS reactions follow the addition–elimination (SnAr) mechanism:
- Nucleophile attacks the electron-deficient carbon → forms a negatively charged intermediate called a Meisenheimer complex.
- Leaving group departs → aromaticity is restored.
2. Role of resonance stabilization
- In the Meisenheimer complex, the negative charge on the carbon can delocalize over the ring if there are electron-withdrawing groups (EWGs) attached.
- For example, a nitro group (–NO₂) at the ortho or para position can pull electron density toward itself, spreading the negative charge over several atoms.
Effect:
- The intermediate is more stable.
- Lower activation energy → faster reaction.
3. Why this matters
- Without resonance stabilization, the intermediate would be highly unstable, and NAS would be very slow or impossible.
- That’s why NAS is rare in simple benzene rings without EWGs—they can’t stabilize the negative charge.
4. Summary
- Resonance stabilization = spreading the negative charge of the intermediate over the ring and substituents.
- Result: Makes the intermediate more stable → makes nucleophilic attack feasible → increases reaction rate.