Lecture Notes on Butane Conformations

Introduction to Butane Molecule

• Butane consists of 4 carbon atoms labeled as carbon 1 through carbon 4.
• Focus on the carbon 2-3 bond and its role in different conformations.

Conformations of Butane

Staggered and Eclipsed Conformations

• Staggered Conformation

  • Achieved by rotating the front carbon while keeping the back carbon stationary.
  • More stable due to lower potential energy.

• Eclipsed Conformation

  • Achieved by rotating the front carbon 60 degrees from staggered.
  • Higher in potential energy, hence less stable.
  • Steric hindrance occurs when atoms are close enough to physically interfere.

Energy Considerations

• Energy Diagram
  • Staggered conformations have lower potential energy than eclipsed ones.
  • Energy to shift from staggered to eclipsed is approximately 16 kJ/mol.
• Energy Differences
  • Eclipsed conformation: 19 kJ/mol, higher than other eclipsed states.
  • Staggered conformations differ by approximately 3.8 kJ/mol.
  • Degenerate conformations: equal energy levels for staggered and eclipsed states.

Detailed Analysis of Conformations

Anti Conformation (Staggered)

• Lowest potential energy, most stable.
• Methyl groups are 180 degrees apart.
• All bonds are staggered, minimizing steric hindrance.

Gauche Conformation (Staggered)

• Dihedral angle: 60 degrees between methyl groups.
• Higher energy than anti due to closer proximity of groups causing steric hindrance.
• Stable but less so than anti-conformation.

Eclipsed Conformations

• Identifying Energy Costs
  • Hydrogen-hydrogen eclipsing: 4 kJ/mol.
  • Methyl-hydrogen eclipsing: 6 kJ/mol.
  • Methyl-methyl eclipsing: 11 kJ/mol.
• Effects of Steric Hindrance
  • Methyl groups close enough to touch, causing destabilization.
  • Highest potential energy found in methyl-methyl eclipsed state.

Summary

• Anti conformation is the most stable due to minimal steric hindrance.
• Eclipsed conformations are less stable due to higher potential energy and steric hindrance.
• The balance between steric hindrance and torsional strain dictates stability of the conformations.