resonance vs induction

Question

In p-nitrophenol, nitro group reduces phenolic acidity via resonance or inductive effect?

RankGuru · Accepted Answer

1. **Acidity Concept**: Acidity of phenol depends on the stability of the phenoxide ion formed after losing a proton ($H^+$).
2. **Nitro Group ($-NO_2$)**: It is a powerful electron-withdrawing group via two mechanisms:
   - **Inductive Effect ($-I$)**: Withdraws electrons through $\sigma$ bonds.
   - **Resonance Effect ($-R$ or $-M$)**: Withdraws electrons through the $\pi$ system, especially from the para position.
3. **Effect on Phenoxide**: The $-NO_2$ group at the para position delocalizes the negative charge on oxygen more effectively, making the phenoxide ion more stable.
4. **Result**: Increased stability of conjugate base leads to **increased acidity**.
5. **Check**: $p$-nitrophenol is significantly more acidic than phenol ($pK_a$ 7.15 vs 10).

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resonance vs induction

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