Draw the most stable conformation of:
(a) ethylcyclohexane
(b) 3-isopropyl-1,1-dimethylcyclohexane
(c) cis-1-tert-butyl-4-isopropylcyclohexane
The correct answer and explanation is :
Stable Conformations of Cyclohexane Derivatives
When drawing the most stable conformation of substituted cyclohexanes, the goal is to minimize steric hindrance and maximize the stability of the molecule. Cyclohexane adopts a chair conformation, where substituents can occupy axial or equatorial positions. The equatorial positions are more favorable because they are farther apart, resulting in less steric strain compared to axial positions. Here’s how to approach each molecule:
(a) Ethylcyclohexane
- Molecule: Ethylcyclohexane is a cyclohexane ring with an ethyl group (-CH₂CH₃) attached to one of the carbons.
Most Stable Conformation:
- To minimize steric strain, the ethyl group will preferentially occupy an equatorial position. This avoids the 1,3-diaxial interactions that would occur if it were in the axial position, which would create steric clashes with other axial hydrogens on the ring.
- The chair conformation has two types of positions for each substituent: axial and equatorial. The ethyl group in the equatorial position minimizes strain, making the conformation more stable.
(b) 3-Isopropyl-1,1-dimethylcyclohexane
- Molecule: This is a cyclohexane with an isopropyl group (-CH₂CH₃) on carbon 3, and two methyl groups (-CH₃) on carbon 1.
Most Stable Conformation:
- In this case, both the isopropyl and the two methyl groups must be arranged to minimize steric strain.
- The isopropyl group is large, and thus it will prefer to be in the equatorial position because it minimizes steric interactions with axial hydrogens.
- For the methyl groups, they are small and can also be placed in the equatorial positions. However, if the isopropyl group is in the axial position, one of the methyl groups may need to be axial, but this would increase steric strain. Therefore, placing all bulky groups (isopropyl and methyl groups) in equatorial positions provides the most stable conformation.
(c) cis-1-tert-butyl-4-isopropylcyclohexane
- Molecule: This is a cyclohexane with a tert-butyl group (-C₄H₉) at position 1 and an isopropyl group (-CH₂CH₃) at position 4. The “cis” designation means that both substituents are on the same side of the ring, either both axial or both equatorial.
Most Stable Conformation:
- The tert-butyl group is large and bulky, and thus it prefers to be in the equatorial position to minimize steric hindrance.
- The isopropyl group at position 4 will also prefer the equatorial position, but it may need to be placed in an axial position depending on how the ring flips.
- Since both groups must be in the same orientation (cis), the most stable conformation would place both bulky substituents in equatorial positions. However, if one of the groups needs to be axial due to ring flipping, the tert-butyl group would be placed axially to reduce strain, and the isopropyl group would be equatorial.
Summary of Stability:
- Ethylcyclohexane: Ethyl group in the equatorial position.
- 3-Isopropyl-1,1-dimethylcyclohexane: Isopropyl and methyl groups in equatorial positions for minimal steric strain.
- cis-1-tert-butyl-4-isopropylcyclohexane: Both bulky groups (tert-butyl and isopropyl) placed in the equatorial positions if possible for optimal stability.
In general, larger substituents prefer the equatorial position to minimize steric hindrance. The most stable conformations of these molecules feature bulky groups in the equatorial position, where they experience the least steric interference.