See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

These three structures represent tautomers of a bicyclic compound involving a six-membered ring fused or connected to a three-membered ring with varying positions of the double bond(s). Concept: Tautomer stability depends on the degree of conjugation, hyperconjugation, ring strain relief, and overall thermodynamic stability of the alkene system. Analysis of each structure: - Structure (I): Has a double bond within or adjacent to the cyclopropene/cyclopropane portion at one position on the six-membered ring. The cyclopropyl group conjugated with an adjacent double bond (homoconjugation or direct conjugation) provides some stabilization, but the geometry is less favorable. - Structure (II): Has two double bonds - one in the three-membered ring region and one in the six-membered ring. This structure benefits from conjugation between the two double bonds (cross-conjugated or directly conjugated system), making it the most stable tautomer. Conjugation lowers energy significantly. - Structure (III): The double bond is positioned such that it is least conjugated or is an isolated double bond within the system, and the cyclopropane ring (saturated three-membered ring) adds ring strain without compensating conjugation stabilization. This makes it the least stable tautomer. Stability reasoning: - Structure II is most stable due to maximum conjugation between the double bonds in the bicyclic framework. - Structure I is intermediate: some stabilization from cyclopropylcarbinyl-type conjugation (cyclopropane ring can conjugate with adjacent pi system through Walsh orbitals), but less effective than full conjugation in II. - Structure III is least stable: isolated double bond, high ring strain from cyclopropane without conjugative relief. Thus the order is: II > I > III Why other options fail: - (a) I > II > III: Incorrect, II is more stable than I due to better conjugation. - (b) III > II > I: Incorrect, III is the least stable. - (d) II > III > I: Incorrect, I is more stable than III because cyclopropylcarbinyl conjugation stabilizes I more than the isolated system in III. Therefore, the correct answer is C.