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

• A. Both the $\mathrm{ethyl}$ and $\mathrm{methyl}$ groups occupy equatorial positions on the cyclohexane ring.
• B. The $\mathrm{ethyl}$ group is axial and the $\mathrm{methyl}$ group is equatorial.
• C. Both the $\mathrm{ethyl}$ and $\mathrm{methyl}$ groups occupy axial positions on the cyclohexane ring.
• D. The $\mathrm{methyl}$ group is axial and the $\mathrm{ethyl}$ group is equatorial.

Answer: (A)

Verified step by step guidance

1. Identify the substituents on the cyclohexane ring and their positions: an ethyl group at carbon 1 and a methyl group at carbon 3, with a trans relationship between them.
2. Recall that in cyclohexane chairs, 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 due to less steric hindrance.
3. Determine the possible chair conformations for trans-1,3-disubstituted cyclohexane: one where the substituent at C1 is axial and the substituent at C3 is equatorial, and the other where the substituent at C1 is equatorial and the substituent at C3 is axial.
4. Evaluate the steric bulk of the substituents: the ethyl group is larger than the methyl group, so placing the larger group in the equatorial position reduces 1,3-diaxial interactions and increases stability.
5. Conclude that the most stable conformation is the one where both the ethyl and methyl groups occupy equatorial positions, which can be achieved by ring flipping to the chair conformation that places both substituents equatorial.