See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: β-Keto carboxylic acids undergo decarboxylation via a six-membered cyclic transition state (pericyclic, concerted mechanism). The COOH proton transfers to the β-carbonyl oxygen while CO2 is lost, requiring a specific geometrical arrangement: the carboxylic acid C=O, the O-H, and the β-keto C=O must be able to adopt a six-membered cyclic conformation. Step 1 - Mechanism requirement: For decarboxylation via the cyclic transition state, the carboxyl group and the β-ketone must be able to achieve a planar or near-planar six-membered ring geometry. This requires free rotation or appropriate spatial alignment between the -CO2H and the β-C=O. Step 2 - Analysis of option (a): In the norbornane (bicyclo[2.2.1]heptane) skeleton shown in (a), the CO2H and the ketone C=O are both attached to the same carbon at a bridgehead-adjacent position. The rigid bicyclic framework locks the geometry such that the carboxylic acid group and the β-ketone CANNOT adopt the required six-membered cyclic transition state geometry. The rigidity of the norbornane skeleton prevents the necessary eclipsed/syn conformation needed for the cyclic proton-transfer transition state. The CO2H and C=O are held in a fixed geometry by the bicyclic ring that is geometrically incompatible with the six-membered cyclic TS. Step 3 - Why (b) and (c) can decarboxylate: In option (b), despite being on a norbornane skeleton, the relative orientation of CO2H and C=O allows formation of the cyclic TS. In option (c), while the quaternary carbon has additional substituents, the CO2H and β-C=O can still achieve the required geometry for the cyclic transition state. Step 4 - Key distinction for (a): The specific stereochemical and geometric constraint in structure (a) places the CO2H and the β-ketone in a syn-periplanar locked arrangement that is actually geometrically unable to form the required six-membered cyclic transition state due to the bicyclic ring constraints, making decarboxylation impossible under normal heating conditions. Therefore, the correct answer is A.