A student is asked to draw the most stable chair conformation of trans-1,4-dimethylcyclohexane. Which of the following descriptions accurately represents this conformation?
- ABoth methyl groups are in axial positions.
- BBoth methyl groups are in equatorial positions.
- CThe molecule rapidly interconverts between two equally stable conformations.
- DOne methyl group is axial and the other is equatorial.
Show answer & explanationAnswer & explanation
Correct answer: B. Both methyl groups are in equatorial positions.
For trans-1,4-dimethylcyclohexane, the two methyl groups are on opposite sides of the ring. In a chair conformation, to achieve the most stable state (minimizing 1,3-diaxial interactions), both bulky methyl groups prefer to be in equatorial positions. If one were axial, the other would have to be axial in the same direction to be trans, leading to significant steric strain. Therefore, the conformation with both groups equatorial is the most stable.
Why the other options are wrong
- A. If both were axial, they would be cis, not trans, and experience significant steric hindrance.
- C. While chair flips occur, the two conformations for trans-1,4-dimethylcyclohexane are not equally stable; the diequatorial is significantly more stable.
- D. This arrangement would be cis-1,4-dimethylcyclohexane, not trans.
Cyclohexane Chair Conformations
Cyclohexane exists predominantly in a chair conformation, which can interconvert through a 'chair flip', and substituents prefer equatorial positions to minimize steric strain.
- Equatorial positions are generally more stable for bulky substituents.
- Axial substituents experience 1,3-diaxial interactions.
- Chair flips interconvert axial and equatorial positions.
Memory trick: Equatorial Equals Easy Energy (low energy)