Which of the following is the most stable conformation of $\mathrm{cis}-1-\mathrm{isopropyl}-3-\mathrm{methylcyclohexane}$?

• A. Isopropyl group is axial and methyl group is equatorial
• B. Isopropyl group is equatorial and methyl group is axial
• C. Both isopropyl and methyl groups are in equatorial positions
• D. Both isopropyl and methyl groups are in axial positions

Answer: (B)

Verified step by step guidance

1. Step 1: Identify the substituents and their positions on the cyclohexane ring. Here, we have a cis-1-isopropyl-3-methylcyclohexane, meaning the isopropyl group is at carbon 1 and the methyl group is at carbon 3, and both substituents are on the same side of the ring (cis).
2. Step 2: Draw the cyclohexane chair conformations and place the substituents accordingly. Remember that in the chair form, substituents can be either axial (pointing straight up or down) or equatorial (pointing roughly outward around the ring).
3. Step 3: Consider the steric bulk of each substituent. The isopropyl group is bulkier than the methyl group, so it is generally more stable when placed in the equatorial position to minimize 1,3-diaxial interactions.
4. Step 4: Since the substituents are cis, if one substituent is axial, the other must be equatorial on the same side of the ring. Analyze both possible conformations: isopropyl axial with methyl equatorial, and isopropyl equatorial with methyl axial.
5. Step 5: Compare the steric strain and 1,3-diaxial interactions in both conformations. The most stable conformation will have the bulkier isopropyl group equatorial and the smaller methyl group axial, minimizing steric hindrance and strain.