The strongest conjugate acid will result from:
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Answer
(3) [figure]
Solution
Strongest conjugate acid ( ArNH3 ) ⇔ weakest base.
Basicity of aniline depends on the availability of the lone pair on N:
−OCH3 (+R) and −CH3 (+I, hyperconjugation) at para increase electron density on N ⇒ stronger bases than aniline.
−NO2 at para (−R, −I) pulls the lone pair into the ring and the nitro group ⇒ weakest base (pKb ≈ 13.0, aniline 9.38).
Basicity: p-anisidine > p-toluidine > aniline > p-nitroaniline
∴ p-nitroaniline gives the strongest conjugate acid.
Answer: (3)