Question

Which of the following compound has most stable conjugate acid? 

• A. \(p\)-Methoxyaniline
• B. \(p\)-Nitroaniline
• C. \(p\)-Methylaniline
• D. Aniline

Answer 1

The Correct Option is A

To determine which compound has the most stable conjugate acid, we need to analyze the effect of substituents on aniline's nitrogen atom when it is protonated. The stability of the conjugate acid is influenced by the electron donating or withdrawing nature of the substituents on the benzene ring.

Explanation:

1. \(p\)-Methoxyaniline: The methoxy group (\(OCH_3\)) is an electron-donating group (+R effect). It donates electron density through resonance, stabilizing the positive charge on the nitrogen when protonated. This increases the stability of the conjugate acid.
2. \(p\)-Nitroaniline: The nitro group (\(NO_2\)) is an electron-withdrawing group (-R effect). It withdraws electron density, destabilizing the positive charge on the nitrogen when protonated, making the conjugate acid less stable.
3. \(p\)-Methylaniline: The methyl group (\(CH_3\)) is a weak electron-donating group (+I effect) through induction. It slightly donates electron density, but not as effectively as the methoxy group, resulting in less stabilization of the conjugate acid.
4. Aniline: It has no additional groups to donate or withdraw electron density, so its conjugate acid is less stable compared to \(p\)-methoxyaniline.

Conclusion: The conjugate acid of \(p\)-methoxyaniline is the most stable due to the strong electron-donating resonance effect of the methoxy group.