MCQ Detailed View

The acidity of phenolic compounds depends on the ability of the molecule to stabilize the negative charge on the oxygen atom after deprotonation. In phenol, the –OH group loses a proton to form the phenoxide ion. The negative charge on the oxygen is partially stabilized by resonance with the aromatic ring, but phenol itself is only weakly acidic.

When an electron-withdrawing group such as a nitro group (–NO₂) is introduced on the benzene ring, the acidity of phenol increases. The nitro group strongly attracts electrons due to both its –I effect (inductive electron withdrawal) and –R effect (resonance electron withdrawal). This stabilizes the negative charge on the phenoxide ion, making the release of a proton easier.

The position of the nitro group greatly affects the extent of this stabilization. In p-nitrophenol, the nitro group is located opposite to the –OH group. This allows maximum delocalization of negative charge through resonance and strong inductive withdrawal. As a result, p-nitrophenol is the most acidic among the given options.

In o-nitrophenol, the nitro group also withdraws electrons, but intramolecular hydrogen bonding occurs between the –OH group and the nitro oxygen. This decreases the ability of the molecule to donate a proton, making o-nitrophenol less acidic than p-nitrophenol.

In m-nitrophenol, the nitro group is positioned in such a way that it cannot participate in resonance with the phenoxide ion. Its effect is limited to inductive withdrawal only, so its acidity is lower than that of p-nitrophenol and o-nitrophenol.

Therefore, among phenol, o-nitrophenol, m-nitrophenol, and p-nitrophenol, the most acidic compound is p-nitrophenol, due to strong stabilization of the phenoxide ion through both inductive and resonance effects.