See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Analysis of pKₐ Values in Phenolic Compounds **Key Principle:** Higher pKₐ means weaker acid (less acidic). For phenols, electron-withdrawing groups decrease pKₐ (increase acidity) by stabilizing the conjugate base anion. **Examining each compound:** **(A) Phenol:** - Baseline phenolic acid with no substituents - pKₐ ≈ 10 **(B) o-Nitrophenol:** - Single $NO_2$ group in ortho position - $NO_2$ is electron-withdrawing but only one substituent - pKₐ ≈ 7.2 **(C) 2,4-Dinitrophenol:** - Two $NO_2$ groups (ortho and para positions) - **Two electron-withdrawing groups** stabilize the conjugate base more effectively - pKₐ ≈ 4 **(D) 4-Nitrophenol:** - Single $NO_2$ in para position - Para substitution is slightly less effective than ortho at stabilizing anion - pKₐ ≈ 7.1 **Conclusion:** Compound **(B)** has the **highest pKₐ** because it has: - Only **one** electron-withdrawing $NO_2$ group (compared to C's two) - The least stabilization of the conjugate base anion - Therefore, the weakest acidity **Answer: (B)** — o-Nitrophenol is the weakest acid among these compounds.