See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: The lowest-energy chair conformation of a substituted cyclohexane is the one that places the maximum number of substituents in equatorial positions, thereby minimizing 1,3-diaxial steric interactions. Step 2 - Assign the stereocenters: The compound is (1S, 2S, 4R)-1,2,4-trimethylcyclohexane. We need to determine which chair conformation places all three methyl groups (or the maximum number) in equatorial positions. Step 3 - Analyze each option: (a) Shows a chair with one methyl axial (up) at C1, making two methyls equatorial and one axial - not all equatorial. (b) Shows a chair with two methyl groups axial - high energy, not the lowest. (c) Shows a chair conformation where all three methyl groups appear in equatorial positions - this is the lowest energy arrangement. For the (1S,2S,4R) configuration, one chair conformer allows all three substituents to be equatorial simultaneously, and this is depicted in option (c). (d) Shows a chair with two axial methyl groups - higher energy than (c). Step 4 - Why (c) is correct: In the correct chair flip for (1S,2S,4R)-1,2,4-trimethylcyclohexane, the relative cis/trans relationships between the methyl groups allow all three to occupy equatorial positions in one chair conformer. Option (c) depicts this all-equatorial arrangement, which is definitively the lowest-energy conformation. Step 5 - Why others fail: Options (a), (b), and (d) all show at least one axial methyl group, introducing 1,3-diaxial strain and making them higher in energy than (c). Therefore, the correct answer is C.