In a cyclohexane chair conformation, if two substituents are both on axial positions on adjacent carbons, what is the relationship between these substituents?

• A. They are constitutional isomers.
• B. They are $\mathrm{trans}$ to each other.
• C. They are $\mathrm{cis}$ to each other.
• D. They are enantiomers.

Answer: (B)

Verified step by step guidance

1. Recall that in a cyclohexane chair conformation, each carbon has one axial and one equatorial position, with axial positions alternating up and down around the ring.
2. Understand that substituents on adjacent carbons can be either both axial, both equatorial, or one axial and one equatorial, and their relative stereochemistry (cis or trans) depends on their orientation (up or down).
3. Note that axial substituents on adjacent carbons point in opposite directions: if one axial substituent is pointing up, the adjacent axial substituent points down.
4. Since cis substituents are on the same side of the ring and trans substituents are on opposite sides, two axial substituents on adjacent carbons must be trans to each other because they point in opposite directions.
5. Therefore, the relationship between two substituents both in axial positions on adjacent carbons in a cyclohexane chair is that they are trans to each other.