Which of the following chair conformations represents $\mathrm{trans}-1,2-\mathrm{dimethylcyclohexane}$?

• A. One methyl group is axial and the other is equatorial on adjacent carbons
• B. Both methyl groups are equatorial on adjacent carbons
• C. Both methyl groups are axial on adjacent carbons
• D. Both methyl groups are on the same side of the ring

Answer: (A)

Verified step by step guidance

1. Identify the positions of the substituents on the cyclohexane ring—in this case, the 1 and 2 positions on the ring are substituted with methyl groups.
2. Recall that in cyclohexane chair conformations, substituents can be either axial (pointing straight up or down) or equatorial (pointing roughly along the equator of the ring).
3. Understand that 'trans' means the two substituents are on opposite sides of the ring plane: if one is 'up', the other must be 'down'.
4. Analyze the possible chair conformations: if one methyl group is axial 'up' at carbon 1, then for trans-1,2-dimethylcyclohexane, the methyl at carbon 2 must be equatorial 'down', or vice versa.
5. Conclude that the correct trans conformation has one methyl group axial and the other equatorial on adjacent carbons, reflecting their opposite sides on the ring.