Why is ortho-nitrophenol more acidic than ortho-methoxyphenol?
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Solution and Explanation
• Acidity depends on the stability of the phenoxide ion formed after losing a proton.
• Electron-withdrawing groups (EWG) stabilize the negative charge and increase acidity.
• Electron-donating groups (EDG) destabilize the negative charge and decrease acidity.
Step 1: Analyze the nitro group (\( -NO_2 \))
The nitro group is a strong EWG via both \( -I \) and \( -R \) effects. It pulls electron density away from the ring and the phenoxide oxygen, significantly stabilizing the anion.
Step 2: Analyze the methoxy group (\( -OCH_3 \))
The methoxy group is an EDG via the \( +R \) effect (resonance). It pushes electron density into the ring, increasing the negative charge density on the phenoxide oxygen and destabilizing the anion.
Step 3: Conclusion
Since nitrophenoxide is much more stable than methoxyphenoxide, ortho-nitrophenol releases its proton much more easily. The final answer is due to the electron-withdrawing nature of the nitro group.
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