Answer: p-nitroaniline < aniline < p-toluidine.
- A p-nitroaniline < aniline < p-toluidine
- B p-toluidine < aniline < p-nitroaniline
- C aniline < p-nitroaniline < p-toluidine
- D p-nitroaniline < p-toluidine < aniline
Correct answer: A. p-nitroaniline < aniline < p-toluidine
Explanation: The electron-withdrawing nitro group lowers basicity while the electron-releasing methyl group raises it: p-nitroaniline < aniline < p-toluidine.
In aniline, the nitrogen's lone pair is pulled into the benzene ring through resonance, leaving less electron density available to accept a proton - which is why aniline is a much weaker base than methylamine, where the lone pair is fully available.
Concept context
Nitrogen-containing organic compounds. Covers classification (primary, secondary, tertiary), basicity comparison, preparation methods, and reactions including diazotization and coupling, key for understanding dyes and pharmaceuticals.