See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Solution: Effect of Substituents on pKa of Benzoic Acid Derivatives **Concept:** For benzoic acid derivatives ($C_6H_5$-COOH with X substituent), $pK_a$ decreases (acidity increases) when electron-withdrawing groups are attached to the benzene ring. **Analysis of each substituent:** **(A) $-NO_2$ (Nitro group):** - Strongly electron-withdrawing (via resonance and inductive effects) - Delocalizes negative charge on conjugate base - Stabilizes $COO^-$ anion → **increases acidity, decreases $pK_a$** ✓ **(B) $-NH_2$ (Amino group):** - Strongly electron-donating (via resonance) - Destabilizes $COO^-$ anion - Decreases acidity, **increases $pK_a$** ✗ **(C) $-OH$ (Hydroxyl group):** - Electron-donating (resonance dominates) - Destabilizes conjugate base - **Increases $pK_a$** ✗ **(D) $-OCH_3$ (Methoxy group):** - Electron-donating (resonance effect) - Destabilizes $COO^-$ anion - **Increases $pK_a$** ✗ **Answer: (A) $-NO_2$** is the only electron-withdrawing group that decreases $pK_a$ by stabilizing the carboxylate anion through resonance delocalization.