See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: Decarboxylation (loss of CO2 on heating) occurs readily for specific classes of carboxylic acids. The two classical cases are: (1) beta-keto acids, where the carbonyl group is beta (two carbons away, i.e., at the beta position) to the carboxyl group, allowing a six-membered cyclic transition state; and (2) malonic acid derivatives (1,3-diacids), which also decarboxylate via a similar cyclic mechanism. Step-by-step analysis of each compound: Compound 1 (cyclohexane-1,4-dione): This is a diketone with no carboxylic acid group. No COOH means no decarboxylation possible. Does NOT undergo decarboxylation. Compound 2 (methyl 1-oxocyclohexane-2-carboxylate, a beta-keto ester): This compound has an ester group (CO2CH3), not a free carboxylic acid. Beta-keto esters can undergo hydrolysis followed by decarboxylation, but on simple heating alone (without hydrolysis), a free acid is required for the classic decarboxylation mechanism. The ester cannot form the required cyclic transition state for direct decarboxylation. Does NOT readily undergo direct decarboxylation on heating. Compound 3 (2-oxocyclohexane-1-carboxylic acid): This is a beta-keto acid. The carboxylic acid (CO2H) is directly attached to the ring carbon that is alpha to the ring ketone (C=O). This means the carboxyl group is beta to the carbonyl (or equivalently, the carbonyl is beta to the COOH), fulfilling the beta-keto acid criterion. Beta-keto acids readily undergo decarboxylation on heating via a six-membered cyclic transition state involving the beta-carbonyl oxygen, losing CO2 to give cyclohexanone. UNDERGOES decarboxylation. Compound 4 (cyclohexane-1,2-dicarboxylic acid): This is an ortho (1,2) diacid (phthalic acid analog on cyclohexane). Malonic acid (1,3-diacid) decarboxylates, but a 1,2-diacid (succinic acid analog) does not readily decarboxylate; instead it tends to form a cyclic anhydride on heating. This is NOT a beta-keto acid or a malonic-type diacid. Does NOT undergo decarboxylation. Why other options fail: - Option (a) '2 and 3': Compound 2 is a beta-keto ester, not a beta-keto acid, so it does not directly decarboxylate under simple heating. - Option (b) '3 and 4': Compound 4 is a 1,2-diacid, not a 1,3-diacid (malonic type), so it does not decarboxylate. - Option (d) '1 and 4': Neither compound 1 nor compound 4 has the structural requirement for decarboxylation. Only Compound 3 (a beta-keto acid) meets the structural requirement for facile thermal decarboxylation. Therefore, the correct answer is C.