Lecture on Cyclohexane Conformations

Overview

• Focus on mono- and di-substituted cyclohexane conformations.
• Cyclohexane rings adopt a chair conformation for ideal bond angles (109.5°).
• Cyclohexane rings can flip, changing equatorial substituents to axial and vice versa.

Monosubstituted Cyclohexane

Example: Methylcyclohexane

• Methyl group can be placed in axial or equatorial positions.
• Ring Flipping: Marks the shift of positions
  • Axial becomes equatorial and vice versa.
  • Important to number carbons to track changes during a flip.

Stability of Conformers

• Use Newman Projections to analyze interactions:
  • Choose bonds that include the carbon with the methyl group.
  • Most stable conformation is anti; least stable is totally eclipsed.

Newman Projection for Axial Position

• Methyl group and carbon 5 are gauche.

Newman Projection for Equatorial Position

• Methyl group is anti to large group on Carbon 2.
• Conclusion: Equatorial position is more stable due to reduced steric hindrance.

Disubstituted Cyclohexane

1,3-Dimethylcyclohexane

• Cis Isomer: Both methyls can be equatorial or axial.
  • Equatorial position is more stable.
• Trans Isomer: One methyl is axial, one equatorial due to arrangement.
• Cis isomer more stable due to accessibility of di-equatorial form.

1,2-Dimethylcyclohexane

• Cis Isomer: Always one methyl axial, one equatorial.
• Trans Isomer: Can achieve both methyls in equatorial positions in one form.
  • Trans isomer more stable due to possibility of both equatorial positions.

Influence of Substituent Size

• Larger groups have higher preference for equatorial position.
• In mixed substituents (e.g., one ethyl, one methyl):
  • Larger group (ethyl) prefers equatorial position to minimize 1,3-diaxial interactions.

Summary

• Equatorial positions are generally more stable due to minimized steric interactions.
• Larger substituents exacerbate axial interactions, increasing preference for equatorial positions.
• Understanding these conformations is critical for predicting the stability of cyclohexane derivatives.