Phenol is less acidic than which of the following compounds?

o-nitrophenol

The correct answer is option 2. o-nitrophenol.

Let us break down the acidity of phenol and its comparison to other compounds:

Phenol is more acidic than alcohols like ethanol due to the resonance stabilization of the phenoxide ion (\( C_6H_5O^- \)). The negative charge on the oxygen in the phenoxide ion can be delocalized into the aromatic ring, stabilizing the ion and making phenol more acidic than alcohols.

o-Nitrophenol

In o-nitrophenol, the nitro group (\( -NO_2 \)) is positioned ortho to the hydroxyl group. The nitro group is a strong electron-withdrawing group through both inductive and resonance effects. When the nitro group withdraws electron density from the phenol ring, it stabilizes the negative charge on the phenoxide ion even more effectively. The nitro group can stabilize the phenoxide ion by delocalizing the negative charge through resonance. The nitro group pulls electron density away from the phenoxide ion through inductive effects, further stabilizing it.

These effects make o-nitrophenol significantly more acidic than phenol.

o-Methylphenol

In o-methylphenol (o-cresol), the methyl group (\( -CH_3 \)) is positioned ortho to the hydroxyl group. The methyl group is an electron-donating group through its inductive effects. The electron-donating methyl group increases the electron density on the phenoxide ion, destabilizing it. The methyl group does not stabilize the negative charge on the phenoxide ion. Therefore, o-methylphenol is less acidic than phenol because the methyl group destabilizes the phenoxide ion.

o-Methoxyphenol

In o-methoxyphenol (o-vanillin), the methoxy group (\( -OCH_3 \)) is positioned ortho to the hydroxyl group. The methoxy group is a strong electron-donating group through both inductive and resonance effects. The methoxy group donates electron density into the aromatic ring, increasing the electron density on the phenoxide ion. The electron-donating effect makes the phenoxide ion less stable. Consequently, o-methoxyphenol is less acidic than phenol because the methoxy group destabilizes the phenoxide ion.

Summary

Most Acidic: o-Nitrophenol (due to strong electron-withdrawing effects of the nitro group)

Least Acidic: o-Methoxyphenol (due to strong electron-donating effects of the methoxy group)

So, phenol is less acidic than o-nitrophenol, making option 2 the correct answer.