Why is o-nitrophenol more acidic than o-methoxyphenol?
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Solution and Explanation
Acidity of substituted phenols depends on the stability of the phenoxide ion formed after losing the proton. More stable the phenoxide, more acidic the phenol.
Step 2: Analysis
The $-NO_2$ group at the ortho position has a strong electron-withdrawing effect by both resonance ($-M$) and induction ($-I$). This disperses the negative charge of the phenoxide ion over the ring and the nitro group, greatly stabilizing it. Ortho nitrophenoxide is further stabilized by intramolecular hydrogen bonding with the $-OH$ of the nitro group. The $-OCH_3$ group at the ortho position is electron donating by resonance ($+M$ effect), which destabilizes the phenoxide ion (makes it more negative) and reduces acidity.
Step 3: Conclusion
$-NO_2$ stabilizes the phenoxide (increases acidity); $-OCH_3$ destabilizes the phenoxide (decreases acidity). Therefore, o-nitrophenol is more acidic.
Final Answer: The electron-withdrawing $-NO_2$ group stabilizes the phenoxide anion (through $-I$ and $-M$ effects), increasing acidity. The electron-donating $-OCH_3$ group destabilizes the phenoxide (through $+M$ effect), decreasing acidity.
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