See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: Oxaloacetic acid contains a beta-keto acid moiety with a ketone carbonyl (the central C=O, i.e., the alpha-keto group between the two carboxylic acids). This ketone carbonyl can undergo nucleophilic addition of water to form a gem-diol (hydrate). The extent of hydration of a carbonyl group depends on the electrophilicity of the carbonyl carbon. Step 1: Identify the hydrating carbonyl. In oxaloacetic acid (3-oxopropane-1,1,3-tricarboxylic acid framework: HOOC-CO-CH2-COOH), the ketone carbonyl (C=O adjacent to both carboxyl groups) is the site of hydration. Studies show oxaloacetate exists predominantly as the hydrate in aqueous solution. Step 2: Understand the pH dependence of hydration. The degree of hydration of the keto group is influenced by the ionization state of the neighboring carboxyl groups. When the carboxyl groups are protonated (low pH), their electron-withdrawing inductive effect is somewhat different than when they are ionized. Research on oxaloacetic acid shows that the hydrate form is maximized at low pH (acidic conditions, around pH 0), because at low pH the molecule is fully protonated and the two flanking carboxylic acid groups exert maximum electron-withdrawing inductive effects on the keto carbonyl through the sigma framework, making it more electrophilic and thus more susceptible to nucleophilic addition of water. Step 3: At higher pH values, the carboxylate anions (after deprotonation) actually push electron density toward the carbonyl through resonance/inductive effects differently, and/or the equilibrium shifts. Experimentally, the hydrate content of oxaloacetic acid decreases as pH increases from 0 upward. Step 4: Evaluate options. pH = 0 corresponds to maximally acidic conditions where the hydration equilibrium is most favorable. pH = 4 is near the pKa values (2.2 and 3.98) where partial ionization occurs. pH = 6 and pH = 12 correspond to increasingly basic conditions where the carboxylates are fully deprotonated and hydration is reduced. Step 5: Why other options fail. At pH = 4, the molecule is partially ionized; at pH = 6, it is largely fully ionized as dianion; at pH = 12, it is completely deprotonated. In all these cases, the hydrate content is lower than at pH = 0. Conclusion: Maximum hydrate of oxaloacetic acid is present at the lowest pH (most acidic condition), which is pH = 0. Therefore, the correct answer is A.