See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

**Thermodynamic Acidity Analysis** Thermodynamic acidity is determined by the stability of the conjugate base ($pK_a$ — lower $pK_a$ = stronger acid). **Step 1: Identify the acidic groups** - (a) Benzenesulfonic acid: $-SO_3H$ group - (b) p-Nitrophenol: phenolic $-OH$ - (c) p-Sulfobenzoic acid: both $-SO_3H$ and $-COOH$ groups - (d) 2,4-Dinitrophenol: phenolic $-OH$ with electron-withdrawing nitro groups **Step 2: Compare acidic strengths by conjugate base stability** For phenols (b,d): Electron-withdrawing groups (like $NO_2$) stabilize the phenoxide anion by resonance. However, phenols are inherently weak acids. For carboxylic acids (c): The $-COOH$ group is a much stronger acid than phenolic $-OH$ or sulfonic acid alone because the carboxylate anion is stabilized by: - Resonance delocalization across $C=O$ and $C-O^-$ - Inherent acidity: $pK_a \approx 3-5$ For sulfonic acid (a): Sulfonic acids are extremely strong ($pK_a < 0$), but benzenesulfonic acid alone lacks additional stabilization. **Step 3: p-Sulfobenzoic acid advantage** Option (c) contains **both** $-SO_3H$ and $-COOH$ in close proximity. The $-COOH$ is enhanced by the electron-withdrawing $-SO_3H$ group, maximizing conjugate base stabilization through inductive effects and potential resonance effects. **Answer: (c) p-Sulfobenzoic acid** has the highest thermodynamic acidity due to dual electron-withdrawing acidic groups that synergistically stabilize the conjugate base.