Which is the most stable chair conformation of ${\mathrm{trans}}_{1,2}$-dimethylcyclohexane?

• A. Both methyl groups in the same position (both axial or both equatorial)
• B. One methyl group axial and one methyl group equatorial
• C. Both methyl groups in axial positions
• D. Both methyl groups in equatorial positions

Answer: (B)

Verified step by step guidance

1. Identify the positions of the two methyl groups on the cyclohexane ring. Since the compound is trans-1,2-dimethylcyclohexane, the two methyl groups are on carbons 1 and 2, and they are on opposite faces of the ring (one up and one down).
2. Recall that in cyclohexane chair conformations, substituents can be either axial (pointing straight up or down, perpendicular to the ring plane) or equatorial (pointing roughly outward, around the ring's equator).
3. For 1,2-disubstituted cyclohexanes, the trans relationship means that one substituent must be axial and the other equatorial in the most stable chair conformation. This is because both substituents cannot be equatorial or both axial simultaneously while maintaining the trans stereochemistry.
4. Analyze the steric interactions: axial substituents experience 1,3-diaxial interactions, which are sterically unfavorable. Equatorial substituents generally have less steric hindrance and are more stable.
5. Therefore, the most stable chair conformation for trans-1,2-dimethylcyclohexane is the one where one methyl group is axial and the other is equatorial, minimizing steric strain while maintaining the trans configuration.