Explain why ortho-nitrophenol is more acidic than ortho-methoxyphenol?

Ortho-nitrophenol is more acidic than ortho-methoxyphenol due to the influence of their respective substituents on the phenolic hydroxyl group.

Substituent Effects

The acidity of phenolic compounds is affected by the nature of the substituents attached to the aromatic ring. In this case, we have:

• Ortho-nitrophenol: Contains a nitro group (-NO2), which is a strong electron-withdrawing group.
• Ortho-methoxyphenol: Contains a methoxy group (-OCH3), which is an electron-donating group.

Impact on Acidity

The nitro group in ortho-nitrophenol stabilizes the negative charge that forms when the hydroxyl group donates a proton (H+). This stabilization occurs through resonance and inductive effects, making it easier for the compound to lose a proton and thus increasing its acidity.

In contrast, the methoxy group in ortho-methoxyphenol donates electron density to the ring, which destabilizes the negative charge that forms upon deprotonation. This makes it less likely for the compound to release a proton, resulting in lower acidity.

Conclusion

In summary, ortho-nitrophenol is more acidic than ortho-methoxyphenol because the nitro group effectively stabilizes the negative charge after deprotonation, while the methoxy group does not provide the same level of stabilization.