Lecture on Cyclohexane Chair Conformation and Ring Flips

Introduction to Cyclohexane

• Discussed the bond line structure of cyclohexane.
• Conversion to chair conformation involves drawing parallel lines to represent the molecule's shape.

Axial and Equatorial Bonds

• Axial Bonds: Alternate up and down.
• Equatorial Bonds: Slanted up or down, alternate direction.

Stability in Chair Conformations

• Methylcyclohexane:
  • Axial vs. Equatorial positions.
  • Axial position causes 1,3-diaxial strain with nearby hydrogens.
  • Equatorial position more stable due to reduced strain.

Substituted Cyclohexanes

• 1-tert-Butyl-4-methylcyclohexane:
  • Different combinations: Both in axial, both in equatorial, or mixed positions.
  • Stability ranking: Most stable when both groups in equatorial; least stable when both axial.
  • Bulkiest group should be in equatorial position for stability.

Cis and Trans Isomers in Chair Conformations

• Identification:
  • Cis: Both substituents in the same direction (both up or both down).
  • Trans: Substituents in opposite directions (one up, one down).

Ring Flips

• Procedure:
  • Draw mirror image for the flip.
  • Positions switch between axial and equatorial, but up/down direction remains.
  • Numbering shifts in clockwise or counterclockwise directions depending on the form.
• Equilibrium Arrows:
  • Unequal arrows when one conformation is more stable than the other.

Practice Examples

• Performing Ring Flips:
  • Move numbering clockwise for A to B, counterclockwise for B to A.
  • Convert axial to equatorial and vice versa, while maintaining up/down direction.
  • Compare stability: Bulkiest group in the equatorial position is more stable.

Chair Conformation Conversion

• Converting bond line structures to chair conformations.
• Matching conformations to cis or trans isomers based on wedge/dash notation.

Key Points

• Equatorial Positions: More stable for bulky groups.
• 1,3-Diaxial Strain: Avoided by placing groups in equatorial positions.
• Conformation Interconversion: Equilibrium favors more stable conformation.

These notes summarize the essential details of drawing and understanding cyclohexane chair conformations, including the effects of substituents on stability and how to perform ring flips.