Chapter 3 (Structure: Isomers) 2B. Which best describes the most stable conformation of cis-1-chloro-4-isopropylcyclohexane? (A) The isopropyl group is equatorial, and the chloro group is axial. (B) The isopropyl group is axial, and the chloro group is equatorial. (C) Both the chloro and isopropyl group are equatorial. (D) Both the chloro and isopropyl group are axial.
The Correct Answer and Explanation is:
Correct Answer: (A) The isopropyl group is equatorial, and the chloro group is axial.
Explanation:
Cis-1-chloro-4-isopropylcyclohexane is a substituted cyclohexane in which the two substituents (a chlorine atom and an isopropyl group) are on carbons 1 and 4 of the cyclohexane ring and are on the same side of the ring (cis configuration). To determine the most stable conformation, we must evaluate the possible chair conformations of the molecule and apply steric strain principles.
Cyclohexane rings prefer to adopt a chair conformation to minimize angle strain and torsional strain. In chair conformations, substituents can occupy either axial (perpendicular to the ring) or equatorial (roughly parallel to the ring plane) positions. Equatorial positions are more stable, especially for bulky groups, because they minimize 1,3-diaxial interactions (steric hindrance between axial substituents on carbon atoms separated by one carbon).
The isopropyl group is bulkier than the chloro group, so for the most stable conformation, it should be placed in the equatorial position. Placing the isopropyl group in the axial position would result in significant 1,3-diaxial interactions, which are energetically unfavorable.
Given the cis relationship between the two groups at positions 1 and 4, when the isopropyl group is equatorial at C4, the chloro group must be axial at C1 to maintain the cis stereochemistry (i.e., both substituents point in the same direction, either both up or both down). This conformation places the bulkier group in the more favorable equatorial position, making it the most stable conformation overall.
Therefore, (A) is the correct answer: The isopropyl group is equatorial, and the chloro group is axial. This arrangement minimizes steric strain while maintaining correct stereochemistry.
