Lecture on Cyclohexane Chair Conformation and Ring Flips

Introduction

• Discussed the chair conformation of cyclohexane.
• Explained how to draw a ring flip.

Cyclohexane Bond-Line Structure

• Basic bond-line structure of cyclohexane.
• Conversion to chair conformation involves drawing two sets of three parallel lines.

Types of Bonds

• Axial Bonds: Alternate between up and down.
• Equatorial Bonds: Slant to the side and alternate in direction.

Drawing Chair Conformation

• Start with parallel lines.
• Alternating axial and equatorial bonds to show hydrogen positions.

Example: Methylcyclohexane

• Draw bond-line structure.
• Two positions for the methyl group:
  • Axial Position: Prone to 1,3-diaxial strain.
  • Equatorial Position: More stable due to reduced strain.

Example: 1-tert-Butyl-4-methylcyclohexane

• Different combinations:
  1. Axial-axial
  2. Axial-equatorial
  3. Equatorial-axial
  4. Equatorial-equatorial (most stable)
• Stability order: Equatorial-equatorial > Equatorial-axial > Axial-equatorial > Axial-axial.

Identifying Cis and Trans Isomers

• Trans: Groups in opposite directions (one up, one down).
• Cis: Groups in the same direction (both up or both down).

Ring Flips

• Change the direction of parallel lines to draw a mirror image.
• Move carbon numbering in clockwise or counterclockwise direction.
• Axial and equatorial positions switch during a flip.

Example Problems

• Bromine Atom Position Change: Axial to equatorial during a ring flip.
• Stable conformations have bulky groups in equatorial positions.

Predicting Stability

• Stability determined by the position of bulky groups.
• Equilibrium direction indicated by arrow size in reaction notation.

Converting Structures to Chair Conformations

• Maintain directionality based on solid/dashed wedges (up/down positions).
• Consistently count carbon numbers to maintain correct positioning.

Summary

• Recognize the lower energy form of chair conformations.
• Understand the conversion process between different forms.
• Apply knowledge to interpret and predict molecular behavior.