Lecture Notes on Cyclohexane Conformations

Overview

• Discussion on confirmation and physical properties of six-membered ring systems, specifically cyclohexane.

Key Conformations of Cyclohexane

1. Planar Structure
   • Basic structure of cyclohexane.
2. Chair Conformation
   • Most stable conformation of cyclohexane.
3. Boat Conformation
   • Less stable compared to chair.

Stability Comparison

• Chair Conformation: More stable
• Boat Conformation: Less stable

Conversion Between Conformations

From Planar to Chair Conformation

• Transitioning from planar to chair involves substituents on the six-membered ring.
• Molecular formula of cyclohexane: C6H12.
• Hydrogen Positions:
  • 12 Hydrogens can be categorized as:
    • Beta Hydrogens: Above the plane (6 total)
    • Alpha Hydrogens: Below the plane (6 total)

Classification of Hydrogens

• Beta Hydrogens:
  • 3 Equatorial
  • 3 Axial
• Alpha Hydrogens:
  • 3 Equatorial
  • 3 Axial

Stability Order of Conformations

1. Chair Conformation: Most stable
2. Twisted Boat: More stable than boat
3. Boat Conformation: Less stable than twisted boat
4. Half Chair: Highly unstable

Energy Profile Diagram for Conversion

1. Chair Conformation
2. Half Chair
3. Twisted Boat
4. Boat Conformation
5. Another Chair Conformation

Substituted Cyclohexane

Monosubstituted Cyclohexane

• Example: Methylcyclohexane.

Conformation Representation

• Wedge Lines: Above the plane (axial)
• Dotted Lines: Below the plane (equatorial)

Stability of Substituted Structures

• Methyl group can be either axial or equatorial.
• Axial Position: Unstable due to 1,3-diaxial interactions with adjacent hydrogens.
• Equatorial Position: More stable as it avoids steric hindrance.

Conclusion

• Equatorial methyl (structure B) is more stable than axial methyl (structure A).

Next Topic

• Upcoming discussion on di-substituted cyclohexane.