A student is asked to draw the most stable chair conformation of cis-1-ethyl-2-methylcyclohexane. Which of the following statements accurately describes the arrangement of substituents in the most stable conformation?
- AThe ethyl group is axial, and the methyl group is equatorial.
- BBoth ethyl and methyl groups are in axial positions.
- CBoth ethyl and methyl groups are in equatorial positions.
- DThe ethyl group is equatorial, and the methyl group is axial.
Show answer & explanationAnswer & explanation
Correct answer: C. Both ethyl and methyl groups are in equatorial positions.
For cis-1,2-disubstituted cyclohexanes, one substituent must be axial and the other equatorial in the same chair conformation. However, the question asks for the *most stable* chair conformation. Larger groups prefer to be in the equatorial position to minimize steric strain (1,3-diaxial interactions). In cis-1-ethyl-2-methylcyclohexane, since the groups are cis, they must be on the same side of the ring. If one is axial, the other must be equatorial. To achieve the most stable conformation where both groups are equatorial, a ring flip would be required if they started axial/equatorial. However, due to the cis relationship on adjacent carbons, if one is equatorial, the other must be axial. The statement 'both ethyl and methyl groups are in equatorial positions' for cis-1,2 is incorrect as this would imply a trans relationship. Let's re-evaluate. Cis-1,2 means one up, one down. So, if C1 is 'up' (equatorial), then C2 must be 'up' (axial) to be cis. If C1 is 'up' (axial), then C2 must be 'up' (equatorial) to be cis. Therefore, for cis-1,2, one group must be axial and the other equatorial. The *more stable* conformation will have the *larger* group in the equatorial position. Ethyl is larger than methyl. So, the ethyl group should be equatorial and the methyl group axial. Upon re-reading the question and options, option B is incorrect for a cis-1,2 disubstituted cyclohexane. Let's re-examine the options considering the nature of cis-1,2. If we consider the example of cis-1,2-dimethylcyclohexane, one methyl is axial, and the other is equatorial. The more stable chair conformation will place the BULKIER group in the equatorial position. Ethyl is bulkier than methyl. So, the ethyl group should be equatorial, and the methyl group should be axial. This corresponds to option D. My initial reasoning for the explanation was flawed, let me correct it and the option selection.
Why the other options are wrong
- A. Incorrect. Placing the larger ethyl group in the axial position would result in more steric strain than placing the smaller methyl group there.
- B. Incorrect. Having both groups axial would lead to significant 1,3-diaxial interactions and thus be highly unstable.
- D. Correct. In cis-1,2-disubstituted cyclohexanes, one substituent is axial and the other is equatorial. To achieve the most stable conformation, the bulkier group (ethyl) should be in the equatorial position to minimize 1,3-diaxial interactions, while the smaller group (methyl) occupies the axial position.
Cyclohexane Chair Conformations
Cyclohexane exists predominantly in the chair conformation, which can interconvert via a ring flip. Substituents can be in axial or equatorial positions.
- Equatorial positions are generally more stable than axial due to less steric strain.
- Larger groups strongly prefer equatorial positions.
- 1,3-diaxial interactions destabilize axial substituents.
- Chair flip interconverts axial to equatorial and vice versa.
Memory trick: Equatorial is Easy, Axial is Awkward.