To determine why p-nitrophenol (p-NP) is more acidic than phenol, let's analyze each statement:
I. -I effect of nitro group: The nitro group (-NO2) is an electron-withdrawing group due to its strong -I (inductive) effect. This withdraws electron density from the phenolic hydroxyl group in p-NP, stabilizing the conjugate base (phenoxide ion) formed after losing a proton, thus increasing acidity.
II. Greater resonance effect of p-nitrophenoxy group: After losing a proton, the phenoxide ion in p-NP can resonate with the nitro group, spreading the negative charge over the oxygen and nitrogen atoms of the nitro group. This resonance stabilization further enhances the acidity compared to phenol, where such resonance effects are absent.
III. Steric effect of bulky nitro group: The nitro group is not bulky enough to significantly affect the acidity compared to phenol. Steric hindrance typically affects the ease of approach of other molecules but doesn't directly influence acidity to the same extent as electronic effects.
Given these points, the correct explanation(s) for why p-nitrophenol is more acidic than phenol are:
C. I and II
