Which of the following best describes the lowest energy conformation of $\mathrm{cis}$-$1$-ethyl-$3$-isopropylcyclohexane?

• A. Both the ethyl and isopropyl groups occupy axial positions on the cyclohexane ring.
• B. The ethyl group is axial and the isopropyl group is equatorial.
• C. The isopropyl group is axial and the ethyl group is equatorial.
• D. Both the ethyl and isopropyl groups occupy equatorial positions on the cyclohexane ring.

Answer: (D)

Verified step by step guidance

1. Identify the substituents and their positions on the cyclohexane ring: here, an ethyl group is at carbon 1 and an isopropyl group is at carbon 3, with a cis relationship between them.
2. Recall that in cyclohexane, substituents can occupy either axial (perpendicular to the ring plane) or equatorial (roughly in the plane of the ring) positions, and that equatorial positions are generally more stable for bulky groups due to less steric hindrance.
3. Draw or visualize the cyclohexane chair conformations with the substituents at carbons 1 and 3, considering both possible chair flips, and assign axial and equatorial positions to each substituent in each conformation.
4. Since the substituents are cis, they must be on the same face of the ring: if one is axial up, the other must also be axial up or equatorial up, depending on the carbon numbering and ring flip; analyze which combination allows both groups to be equatorial simultaneously.
5. Determine which conformation has the lowest energy by minimizing steric strain, which typically occurs when both bulky groups (ethyl and isopropyl) occupy equatorial positions, leading to the most stable conformation.