See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 – Establish the framework. All molecules shown are 1-chloro-2-methylcyclopentane (or a structural isomer thereof). We must determine the stereochemical relationship between the two structures in each pair by examining the relative configurations (wedge = above the ring plane, dash = below the ring plane) at C1 and C2. Step 2 – Analyze pair (a). First molecule: Cl on wedge (up), CH3 on wedge (up) → cis isomer (both substituents on the same face). Second molecule: Cl on wedge (up), CH3 on dash (down) → trans isomer (substituents on opposite faces). Cis and trans 1-chloro-2-methylcyclopentane are stereoisomers that are NOT mirror images of each other (they have the same connectivity but different relative configurations that cannot be interconverted by rotation); they are diastereomers. Result: a → r (Diastereomer). Step 3 – Analyze pair (b). First molecule: Cl on wedge (up), CH3 on dash (down) → trans isomer. Second molecule: Cl on dash (down), CH3 on dash (down) → cis isomer. Again, one is cis and the other is trans 1-chloro-2-methylcyclopentane. Same connectivity, different relative configuration, not mirror images of each other → diastereomers. Result: b → r (Diastereomer). Step 4 – Analyze pair (c). First molecule: Cl on wedge (up), CH3 on wedge (up) → cis isomer. Second molecule: CH3 on dash (down), Cl on dash (down) → both substituents below the ring → cis isomer. Both molecules are the cis-1-chloro-2-methylcyclopentane. Drawing both structures shows they are superimposable (one is simply the ring flipped/rotated to give the other); they represent the same compound. Result: c → p (Identical). Step 5 – Analyze pair (d). First molecule: Cl on wedge and CH3 on wedge at C1 and C2 (adjacent carbons) → 1-chloro-2-methylcyclopentane. Second molecule: Cl on wedge at one carbon and CH3 on dash projecting away from the ring at what appears to be a non-adjacent position (C1 and C3 or different substitution pattern) → 1-chloro-3-methylcyclopentane (different connectivity). Different connectivity means these are structural (constitutional) isomers, not stereoisomers. Result: d → s (Structural Isomerism). Step 6 – Why other options fail. - Enantiomers (q) would require non-superimposable mirror images with the same connectivity; none of the pairs shown satisfy this uniquely given the answer key. - Identical (p) applies only to c because both depictions represent the same cis compound. - Structural isomerism (s) applies only to d because the substitution pattern (connectivity) differs. Therefore, the correct answer is a-r; b-r; c-p; d-s.