Which of the following compound is more acidic than phenol?

o-nitrophenol

The correct answer is option 4. o-nitrophenol.

Let us analyze each compound in detail to understand why o-nitrophenol is more acidic than phenol, while the others are not.

Acidity and Substituent Effects on Phenol

The acidity of a compound is often described by its ability to donate a proton \((H^+)\). For phenol \((C_6H_5OH)\), the phenoxide ion \((C_6H_5O^-)\) formed after deprotonation is stabilized by resonance, making phenol more acidic than simple alcohols like ethanol.

The presence of substituents on the aromatic ring can either increase or decrease the acidity of phenol based on their electron-donating or electron-withdrawing properties.

1. Ethanol \((C_2H_5OH)\)

Ethanol is less acidic than phenol. In ethanol, the ethoxide ion \((CH_3CH_2O^-)\) formed after deprotonation is not stabilized by resonance. The lack of resonance stabilization makes the ethoxide ion less stable than the phenoxide ion \((C_6H_5O^-)\), resulting in lower acidity.

2. o-Methoxyphenol (2-methoxyphenol)

o-Methoxyphenol is less acidic than phenol. The methoxy group \((-OCH_3)\) is an electron-donating group due to its \(+M\) (resonance) effect and inductive effect. It donates electron density through resonance to the aromatic ring, particularly to the ortho and para positions. This electron-donating effect destabilizes the phenoxide ion by increasing the electron density on the oxygen, making it less willing to donate a proton \((H^+)\).

3. o-Methylphenol (2-methylphenol or o-cresol)

o-Methylphenol is less acidic than phenol. The methyl group \((-CH_3)\) is an electron-donating group through its \(+I\) (inductive) effect. It pushes electron density towards the aromatic ring, particularly at the ortho and para positions, increasing the electron density on the phenoxide ion, making it less stable, and thus, reducing the acidity of the phenol.

4. o-Nitrophenol (2-nitrophenol)

o-Nitrophenol is more acidic than phenol. The nitro group \((-NO_2)\) is a strong electron-withdrawing group due to its \(-M\) (resonance) and \(-I\) (inductive) effects. It withdraws electron density from the aromatic ring through resonance and inductive effects. This electron-withdrawing effect stabilizes the negative charge on the phenoxide ion by delocalizing the negative charge, making it easier for the phenol to lose a proton \((H^+)\). The stabilization of the phenoxide ion significantly increases the acidity of o-nitrophenol compared to phenol.

Conclusion: Among the given compounds, o-nitrophenol is more acidic than phenol due to the electron-withdrawing effect of the nitro group, which stabilizes the phenoxide ion formed after deprotonation. The other substituents (methoxy and methyl) are electron-donating and decrease the acidity of phenol.

Therefore, the correct answer is: o-Nitrophenol.