Lecture on Conformation of Alkanes

Introduction to Ethane Structure

• Ethane: Two carbon atoms and six hydrogen atoms.
• 2D structure bond angle appears as 90 degrees.
• True 3D structure bond angle for sp3 carbons should be 109.5 degrees.

3D Structure and Conformation

• Rotation about carbon-carbon single (sigma) bonds.
• Formation of conformers through rotation.
• Conformational isomers arise from different spatial arrangements.

Representations of Conformers

• Sawhorse Projections: A linear view of the molecule.
• Newman Projections: Viewed along the axis of the bond of interest.
  • Front carbon shown at the center of a circle.
  • Back carbon shown as the outer circle.

Stability of Newman Projections

• Least Stable: Eclipsed conformation.
• Most Stable: Staggered conformation.

Butane Conformers

• Extreme Conformers:
  • Staggered conformation
  • Eclipsed conformation
• Dihedral angle influences stability (e.g., 0° for eclipsed).
• Anti and Gauche conformations: Different stability and angles.

Energy Considerations and Torsional Strain

• Rotations are not free: electron repulsions prevent close proximity in eclipsed forms.
• Torsional Strain: Repulsion between bonding electrons.
  • Staggered conformers: free from torsional strain.
  • Eclipsed conformers: high torsional strain.

Energy Diagrams for Ethane and Butane

• Ethane: Alternating energy levels between staggered (low energy) and eclipsed (high energy).
• Butane: Energy levels affected by eclipsing of methyl groups.
  • Calculating strain energy for conformers using torsional strain values.

Calculation Example

• Strain Energy Calculation:
  • Use given torsional strain values to calculate total strain energy.
  • Example conformations: methyl-methyl eclipsed (11 kJ/mol), methyl-hydrogen (6 kJ/mol).

Summary

• Conformation plays a critical role in molecular stability.
• Understanding conformational isomerism is essential for predicting molecular behavior and reactivity.
• Energy diagrams illustrate stability variations and guide conformational analysis.