See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Stability of cyclohexane (or bicyclic) conformations/isomers depends on minimizing steric strain and 1,3-diaxial interactions. For substituents on a cyclohexane ring, equatorial positions are preferred over axial. For bicyclic systems (norbornane-type), exo vs endo and the nature of substituents at bridgeheads determine stability. Looking at the four options, they all appear to be bicyclo[2.2.1]heptane (norbornane) frameworks with OH and OMe substituents at different positions and stereochemistries. Step 1: Identify the framework. Options (b), (c), and (d) clearly show bicyclic (norbornane-type) structures. Option (a) appears to be a cyclohexane chair with axial HO and equatorial OCH3. Step 2: For option (a) - cyclohexane chair with HO axial and OCH3 equatorial: having a large group (OH) axial introduces 1,3-diaxial strain, making it less stable. Step 3: For options (b) and (c) - norbornane systems: In (b), OH is at the bridgehead (top) and OMe is equatorial/exo; in (c), OMe is at the bridgehead and OH is equatorial. Bridgehead substituents in norbornane experience significant steric compression from the syn-axial hydrogens on the bridge. Step 4: For option (d) - norbornane framework with OH and OMe both positioned to minimize steric interactions. In option (d), the OH is at the bridgehead position with exo orientation and OMe is at an adjacent carbon also in exo orientation. The specific stereochemical arrangement in (d) places both substituents in positions that minimize syn-periplanar and steric interactions within the rigid bicyclic framework. Step 5: In norbornane systems, exo substituents are generally more stable than endo due to less steric interaction with the C7 bridge. Option (d) has both OH and OMe in exo-like orientations on the norbornane skeleton, minimizing steric strain. Additionally, the possibility of intramolecular hydrogen bonding between OH and OMe (or the oxygen lone pairs) in option (d) can provide extra stabilization. Step 6: Why other options fail: - (a): Axial OH on cyclohexane introduces 1,3-diaxial strain. - (b): OH at bridgehead of norbornane with certain endo/exo arrangement leads to more strain. - (c): OMe at bridgehead introduces more steric strain than OH at bridgehead; also the arrangement is less favorable. - (d): Both substituents in the most favorable positions (exo) with possible intramolecular H-bonding stabilization makes it the most stable. Therefore, the correct answer is D.