See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the compound type: Compound A is a trimethylcyclohexanone. Looking at the structure, it is 2,2,6-trimethylcyclohexan-1-one (or equivalently described as having a ketone at C1, a gem-dimethyl group at one adjacent carbon C2, and a methyl at the other adjacent carbon C6), with defined stereochemistry at the stereocenters. Step 2 - Identify stereocenters in A: The gem-dimethyl carbon (C2) is not a stereocenter (two identical methyl groups). The carbons bearing single methyl groups adjacent to and further from the ketone are stereocenters. In compound A, the wedge and dash bonds define the absolute configuration at these centers. Step 3 - Definition of diastereomers: Diastereomers are stereoisomers that are not mirror images of each other. They have the same connectivity (same constitutional structure) but differ in configuration at one or more stereocenters, and are not enantiomers. Step 4 - Eliminate non-isomers: Structure (2) lacks a stereocenter arrangement consistent with being a stereoisomer of A (it appears to be a different arrangement or possibly a constitutional isomer based on wedge/dash pattern). Structure (3) appears to have the same connectivity with all configurations inverted (could be enantiomer or diastereomer). Structure (5) has same connectivity but configuration differences. Step 5 - Analyze structures 1 and 4: Structure (1) has the same molecular formula and connectivity as A but with different stereochemistry at one or more centers (not a mirror image of A), making it a diastereomer. Structure (4) has the same connectivity as A with the configurations at the stereocenters changed such that it is not the mirror image of A, making it also a diastereomer. Step 6 - Confirm why other options fail: Structure (3) alone (option a) is insufficient. Structures (2) and (3) (option c) - structure (2) does not match as a proper stereoisomer with same connectivity. Structure (5) alone (option d) may be the enantiomer of A rather than a diastereomer. Structures (1) and (4) both share the same constitutional structure as A but differ in configuration at only some stereocenters (not all), satisfying the diastereomer criterion. Therefore, the correct answer is B.