Electron-withdrawing groups enhance the acidic strength of phenol, with a more pronounced effect at ortho and para positions. Effective delocalization of negative charge in the phenoxide ion occurs when the substituent is at these positions.
How do electron-withdrawing groups, like nitro groups, impact the acidic strength of substituted phenols, and why is the effect more pronounced at ortho and para positions?
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Electron-withdrawing groups, like nitro groups, enhance the acidic strength of substituted phenols by withdrawing electron density from the phenoxide ion. This stabilizes the negative charge on the oxygen, making it easier for the phenol to donate a proton. The effect is more pronounced at ortho and para positions because these positions are closer to the site of electron withdrawal. The presence of the nitro group at these positions intensifies the resonance effect, increasing charge delocalization and further stabilizing the resulting phenoxide ion, making ortho- and para-substituted phenols more acidic than their meta-substituted counterparts.