Identify the least stable conformation of a di-substituted cyclohexane with ethyl groups at the 1st and 2nd carbons.
trans-1,2-diethylcyclohexane, with both ethyl groups equatorial
trans-1,2-diethylcyclohexane, with both ethyl groups axial
cis-1,2-diethylcyclohexane, with one ethyl group axial and one ethyl group equatorial
They are the same compound, so they are all equal.
The correct answer and explanation is :
The least stable conformation of a di-substituted cyclohexane with ethyl groups at the 1st and 2nd carbons is:
▶️ trans-1,2-diethylcyclohexane, with both ethyl groups axial
Explanation (Approx. 300 words):
Cyclohexane rings can adopt chair conformations, where substituents occupy either axial (pointing up/down, perpendicular to the ring plane) or equatorial (pointing outward, roughly parallel to the ring) positions. Stability in substituted cyclohexanes depends on the positions of substituents: equatorial positions are more favorable due to reduced steric hindrance, especially for bulkier groups like ethyl.
In 1,2-disubstituted cyclohexanes, the relative positions of the two substituents (cis vs. trans) dictate their possible orientations in chair conformations:
- trans-1,2-diethylcyclohexane:
- In one chair flip, both ethyl groups can be equatorial – this is very stable, as both bulky groups avoid axial crowding.
- In the flipped chair, both ethyls become axial – this is very unstable, due to 1,3-diaxial interactions, where axial groups clash with axial hydrogens on the same side of the ring. Since ethyl is a bulky group, these clashes are significant, making this conformation least stable.
- cis-1,2-diethylcyclohexane:
- One group must be equatorial and the other axial, regardless of the chair flip. This results in a mixed interaction—intermediate stability—better than two axial groups but worse than two equatorial.
- The statement “They are the same compound” is incorrect because cis and trans isomers are distinct stereoisomers with different physical and chemical properties.
Conclusion:
Among the conformations listed, the trans-1,2-diethylcyclohexane with both ethyl groups in axial positions is the least stable due to the severe steric strain caused by the two bulky groups in axial positions.
✅ Correct answer: trans-1,2-diethylcyclohexane, with both ethyl groups axial.
