See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Beta-keto acids (3-oxo acids) normally undergo facile decarboxylation via a cyclic six-membered transition state. In this transition state, the beta-carbonyl oxygen acts as an intramolecular base, abstracting the carboxyl OH proton simultaneously as CO2 is lost. This requires the molecule to adopt a planar, six-membered cyclic geometry involving: O=C-C-C(=O)-O-H. Step 2 - Requirement for decarboxylation: The beta-keto acid must be able to form a six-membered cyclic transition state. This demands that the two carbonyl groups (the keto C=O and the carboxyl C=O) and the intervening carbons can achieve the necessary planar, eclipsed-like cyclic arrangement. Step 3 - Analysis of option (b): In structure (b), the CO2H group and the C=O (ketone) are on adjacent carbons of the bicyclo[2.2.1]heptane (norbornane) skeleton. Critically, the CO2H is at a bridgehead-adjacent position and the ketone is also on the rigid bicyclic framework. Due to Bredt's rule considerations and the rigid geometry of the norbornane skeleton, the carboxyl group and the beta-keto group cannot achieve the coplanar, six-membered cyclic transition state required for decarboxylation. The rigid bicyclic ring system locks the relevant bonds in a geometry that prevents the necessary orbital overlap and ring formation for the concerted cyclic mechanism. Step 4 - Why other options undergo decarboxylation: - (a): Although also on a norbornane framework, the CO2H is on the carbon beta to the carbonyl with sufficient conformational freedom to form the cyclic TS. - (c): Ph-CO-CH2-CO2H is a standard beta-keto acid; the phenyl ketone provides the beta carbonyl, and the six-membered cyclic TS forms readily. - (d): CH3-CO-CH2-CO2H (acetoacetic acid) is the classic example of a beta-keto acid that decarboxylates easily via the six-membered cyclic TS. Step 5 - Conclusion: Structure (b) cannot achieve the required cyclic transition state geometry due to the rigid bicyclic framework constraining the spatial relationship between the CO2H and the beta-keto group, preventing decarboxylation. Therefore, the correct answer is B.