Draw the two chair conformations of each of the following substituted cyclohexanes. In each case, label the more stable conformation.
(a) cis-1-ethyl-2-methylcyclohexane
(b) trans-1,2-diethylcyclohexane
(c) cis-1-ethyl-4-isopropylcyclohexane
(d) trans-1-ethyl-4-methylcyclohexane
The Correct Answer and Explanation is :
To draw the two chair conformations for each substituted cyclohexane, we first need to understand how chair conformations work. Cyclohexane adopts a chair form that minimizes steric strain by placing bulky substituents in equatorial positions, which are more favorable than axial positions because axial substituents cause 1,3-diaxial interactions. The most stable conformation for a substituted cyclohexane places the larger substituents in the equatorial positions.
(a) cis-1-ethyl-2-methylcyclohexane
- Structure Explanation:
- In the cis conformation, both substituents (ethyl and methyl) are on the same side of the ring.
- For the most stable chair form, the ethyl group (larger) should occupy the equatorial position, and the methyl group (smaller) should be axial.
- The less stable chair form would have the ethyl group axial and the methyl group equatorial.
- Most stable conformation: The chair where the ethyl group is in the equatorial position and the methyl group is axial.
(b) trans-1,2-diethylcyclohexane
- Structure Explanation:
- In the trans conformation, the two ethyl groups are on opposite sides of the ring.
- For the most stable conformation, each ethyl group should be placed in the equatorial position to avoid steric hindrance.
- The less stable chair would have the ethyl groups in axial positions.
- Most stable conformation: The chair where both ethyl groups are equatorial.
(c) cis-1-ethyl-4-isopropylcyclohexane
- Structure Explanation:
- In the cis conformation, both the ethyl and isopropyl groups are on the same side of the ring.
- For the most stable conformation, the larger isopropyl group should occupy the equatorial position to reduce steric strain, while the ethyl group would be placed in the axial position.
- The less stable chair would have the ethyl group axial and the isopropyl group axial.
- Most stable conformation: The chair where the isopropyl group is equatorial, and the ethyl group is axial.
(d) trans-1-ethyl-4-methylcyclohexane
- Structure Explanation:
- In the trans conformation, the ethyl and methyl groups are on opposite sides of the ring.
- The more stable conformation places the larger ethyl group in the equatorial position, and the methyl group will be placed in the axial position.
- The less stable chair would have the ethyl group in the axial position and the methyl group in the equatorial position.
- Most stable conformation: The chair where the ethyl group is equatorial, and the methyl group is axial.
Conclusion:
In all cases, the more stable conformation is the one where bulky substituents are placed in the equatorial position. This reduces steric hindrance from 1,3-diaxial interactions, leading to a lower energy state for the molecule.