See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The reaction shown is cyanohydrin formation, where HCN adds across the carbonyl (C=O) of a ketone to give an alpha-hydroxy nitrile (cyanohydrin). The mechanism proceeds via nucleophilic addition of CN⁻ to the carbonyl carbon. A catalyst for this reaction must be able to generate CN⁻ from HCN, i.e., it must be a base strong enough to deprotonate HCN (pKa ≈ 9.2) but should not be so strong or nucleophilic as to cause side reactions. Step 1 - Identify what kind of catalyst is needed: The catalytic cycle requires a species that can abstract the proton from HCN to generate CN⁻, which then attacks the carbonyl. The catalyst must therefore be a base with appropriate pKa. Step 2 - Evaluate each option: (a) Cl⁻: Cl⁻ is an extremely weak base (conjugate acid HCl, pKa ≈ -7). It cannot deprotonate HCN (pKa ≈ 9.2). Cannot act as a base catalyst here. (b) CH3COO⁻ (acetate): Conjugate acid CH3COOH has pKa ≈ 4.75. Since pKa of CH3COOH (4.75) < pKa of HCN (9.2), acetate is not basic enough to deprotonate HCN effectively. It is a weaker base than CN⁻. (c) EtO⁻ (ethoxide): Conjugate acid EtOH has pKa ≈ 15.9. Since pKa of EtOH (15.9) > pKa of HCN (9.2), ethoxide IS basic enough to deprotonate HCN and generate CN⁻. The generated CN⁻ then attacks the carbonyl to form the cyanohydrin, and the proton is transferred back, regenerating the base catalyst. EtO⁻ thus acts as a true catalyst in this reaction. (d) HSO4⁻: This is an acid (pKa ≈ 1.99), not a base. It would protonate CN⁻ and suppress the reaction rather than catalyze it. Step 3 - Conclusion: Only EtO⁻ (option c) is a sufficiently strong base to deprotonate HCN and generate the active nucleophile CN⁻, acting as a base catalyst for cyanohydrin formation. Therefore, the correct answer is C.