In the most stable chair conformation of trans-1-isopropyl-3-methylcyclohexane,
A) Both the isopropyl and the methyl groups are axial
B) Both the isopropyl and the methyl groups are equatorial
C) The isopropyl group is equatorial; the methyl group is axial
D) The isopropyl group is axial; the methyl group is equatorial
The correct answer and explanation is :
The correct answer is D) The isopropyl group is axial; the methyl group is equatorial.
Explanation:
In cyclohexane rings, substituents can occupy two possible positions: axial and equatorial. The axial positions alternate up and down around the ring, whereas the equatorial positions point outward from the ring. The stability of a given conformation is influenced by the steric hindrance or 1,3-diaxial interactions that occur when bulky substituents occupy axial positions. To minimize these steric clashes, bulky substituents tend to prefer the equatorial positions because they are farther away from other axial substituents.
Now, let’s look at trans-1-isopropyl-3-methylcyclohexane:
- The molecule is trans, meaning that the two substituents (isopropyl and methyl groups) are on opposite sides of the ring (one is axial and the other is equatorial).
- The isopropyl group is a relatively bulky substituent. For the most stable conformation, bulky groups prefer the equatorial position. However, because of the trans configuration, the isopropyl group cannot be equatorial in this conformation because the methyl group must occupy the equatorial position.
- The methyl group, while smaller than the isopropyl group, also prefers the equatorial position to minimize steric strain. This preference results in the methyl group occupying the equatorial position in this case.
- The axial position of the isopropyl group creates steric interactions with other axial hydrogens or substituents on the cyclohexane ring, but this is less of a strain than placing the larger isopropyl group in the equatorial position, which would force it into unfavorable steric interactions.
Thus, in the most stable chair conformation of trans-1-isopropyl-3-methylcyclohexane, the isopropyl group is in the axial position, and the methyl group is in the equatorial position. This conformation minimizes steric strain and is therefore the most stable.