Interconversion of Cyclohexane Conformations

Cyclohexane and Its Stability

• Cyclohexane consists of six carbon atoms connected by carbon-carbon single bonds.
• It typically exhibits a stable chair conformation due to minimal torsional strain.
• The carbon-carbon single bonds can freely rotate, leading to interconversion between different chair conformations via a process called a ring flip.
• In a ring flip, equatorial bonds become axial and vice versa.

Drawing Ring Flip Chair Conformations

• Three Rules:
  1. Draw the Mirror Image: Pull down one carbon, push up the opposite carbon.
  2. Shift Substituents: Move each substituent one position clockwise or counterclockwise.
  3. Adjust Bond Orientation: If a substituent was axial, it becomes equatorial, and if it was equatorial, it becomes axial. Maintain the relative position (up remains up, down remains down).

Stability of Axial vs Equatorial Positions

• Axial positions are less stable due to steric hindrance from 1,3-diaxial interactions (interactions with axial hydrogens two carbons away).
• Larger substituents increase instability in axial positions due to stronger repulsive forces.
  • Example: Methyle group in axial position interacts with adjacent axial hydrogens, causing steric strain.
  • 1,3-Diaxial interaction energies: Methyl group: 3.6 kJ/mol, Ethyl group: 4.0 kJ/mol, Tertiary Butyl group: 11.4 kJ/mol.

Energy Calculations for Cyclohexane Conformations

• Gauche Interactions: Occur when substituents are 60° apart, leading to increased steric strain.
• Equilibrium Mixtures: Conformations with equal energy appear in equal ratios.

Examples

• Cis-1,2-dimethylcyclohexane: Both chair conformations have equal energy (10.9 kJ/mol) due to equal diaxial and gauche interactions.
• Trans-1,2-dimethylcyclohexane: One conformation is more stable (3.6 kJ/mol) as both methyl groups are equatorial, minimizing strain.

Factors Affecting Stability

• Larger substituents in equatorial positions increase stability.
• Calculate strain energy by considering diaxial and gauche interactions.

Practice Problem

• Problem: Calculate strain energy in different chair conformations.
• Solution Strategy:
  1. Identify axial and equatorial positions.
  2. Calculate diaxial and gauche interactions.
  3. Compare energy levels to determine the most stable conformation.

Conclusions

• Equatorial positions provide more stability.
• Larger substituents should occupy equatorial positions whenever possible.
• The stability of cyclohexane derivatives can be significantly affected by the position and size of substituents.