When comparing the acidity of p-nitrobenzoic acid and p-methoxybenzoic acid, p-nitrobenzoic acid is more acidic. This difference in acidity can be explained using resonance structures and the electron-withdrawing or electron-donating effects of the substituents.
Resonance Structures and Acidity
In p-nitrobenzoic acid, the nitro group (-NO2) is an electron-withdrawing group. It withdraws electron density from the benzene ring through both inductive and resonance effects. This withdrawal of electron density stabilizes the conjugate base (the benzoate ion) by delocalizing the negative charge over the nitro group and the benzene ring. The resonance structures of the conjugate base of p-nitrobenzoic acid show that the negative charge can be delocalized onto the oxygen atoms of the nitro group, making the conjugate base more stable.
On the other hand, in p-methoxybenzoic acid, the methoxy group (-OCH3) is an electron-donating group. It donates electron density to the benzene ring through resonance, which destabilizes the conjugate base. The resonance structures of the conjugate base of p-methoxybenzoic acid show that the negative charge is less delocalized, making the conjugate base less stable.
Conclusion
Because the conjugate base of p-nitrobenzoic acid is more stable due to the electron-withdrawing effect of the nitro group, p-nitrobenzoic acid is more acidic than p-methoxybenzoic acid. The electron-donating effect of the methoxy group in p-methoxybenzoic acid makes its conjugate base less stable, resulting in lower acidity.