Identify the most acidic compound

The correct answer is Option (3) → (C)

All four compounds are para-substituted anilines. The acidity here refers to the ease of losing a proton from the –NH₂ group.

Key idea:

An electron-withdrawing group (EWG) increases acidity by stabilizing the conjugate base, while an electron-donating group (EDG) decreases acidity.

Substituent effects:

• –NO₂ → strong electron-withdrawing (−I, −R) → greatly increases acidity
• –Cl → weak −I but +R → slight increase
• –CH₃ → electron-donating (+I) → decreases acidity
• H (aniline) → baseline

Comparison:

• C (p-nitroaniline): strongest EWG → most acidic
• A (p-chloroaniline): moderately acidic
• B (aniline): reference
• D (p-toluidine): least acidic