For each of the following structures, draw the most stable chair conformer.
a.
b.

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Step 1: Identify the substituents on the cyclohexane ring for each structure. In structure (i), the substituents are CH2CH3, H, CH3, and (CH3)2CH. In structure (ii), the substituents are CH2CH3, H, H, and CH3COOC.

Step 2: Recall that in a chair conformation, substituents can occupy either axial (vertical) or equatorial (angled) positions. The equatorial position is generally more stable for bulky groups due to reduced steric hindrance.

Step 3: For structure (i), place the largest substituent, (CH3)2CH, in the equatorial position to minimize steric hindrance. Then, arrange the other substituents (CH2CH3, CH3, and H) based on their positions in the original structure while maintaining the chair conformation.

Step 4: For structure (ii), place the bulky group CH3COOC in the equatorial position for stability. Arrange the other substituents (CH2CH3 and H) accordingly, ensuring that the chair conformation is maintained.

Step 5: Verify that the most stable chair conformer for each structure minimizes steric hindrance and unfavorable interactions between substituents. Double-check the orientation (up or down) of each substituent based on the original structure.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Chair Conformation

Chair conformation is a three-dimensional arrangement of cyclohexane that minimizes steric strain and torsional strain. In this conformation, carbon atoms are staggered, allowing for more stable interactions between substituents. The chair form can exist in two orientations: axial (up or down) and equatorial (outward), with equatorial positions generally being more stable for larger substituents.

Axial vs. Equatorial Positions

In cyclohexane chair conformers, substituents can occupy axial or equatorial positions. Axial substituents are aligned parallel to the axis of the ring, which can lead to 1,3-diaxial interactions and increased steric strain. Equatorial substituents, on the other hand, extend outward from the ring, reducing steric hindrance and making the molecule more stable, especially for larger groups.

Stability of Conformers

The stability of chair conformers is influenced by the size and orientation of substituents. Larger substituents prefer equatorial positions to minimize steric interactions, while smaller groups can occupy either position with less impact on stability. When analyzing conformers, the most stable arrangement is typically the one that maximizes equatorial substituents and minimizes steric strain.

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Textbook Question

Using the wedge-and-dash notation, draw the nine stereoisomers of 1,2,3,4,5,6-hexachlorocyclohexane.

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Textbook Question

Explain why the enantiomers of 1,2-dimethylaziridine can be separated even though one of the “groups” attached to nitrogen is a lone pair.

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