Ethane's conformations arise from rotation around its central C-C bond. This rotation creates different spatial arrangements of atoms, with staggered and eclipsed conformations being the main types. Understanding these conformations is crucial for grasping molecular behavior.

Staggered conformations are more stable due to reduced electron repulsion, while eclipsed ones have higher energy. The energy difference between these forms, about 12 kJ/mol, is called the rotational barrier. This concept is key to understanding molecular stability and reactivity.

Conformations of Ethane

Conformations in ethane

• Different spatial arrangements of atoms resulting from rotation around single bonds (bond rotation)
  • Not distinct chemical structures but different orientations of the same molecule
• In ethane conformations arise due to rotation around the central C-C single bond allowing for free rotation
  • Results in an infinite number of possible conformations
  • Two main types are staggered and eclipsed conformations (discussed below)

Staggered vs eclipsed ethane conformations

• Staggered conformations have hydrogen atoms on adjacent carbon atoms offset by 60°
  • More stable than eclipsed conformations
  • Minimized steric strain due to reduced repulsion between electron clouds of C-H bonds
  • Energy minima on the potential energy diagram
• Eclipsed conformations have hydrogen atoms on adjacent carbon atoms aligned with each other (0° offset)
  • Less stable than staggered conformations
  • Higher steric strain due to increased repulsion between electron clouds of C-H bonds
  • Energy maxima on the potential energy diagram
• Staggered conformations are more stable by approximately 12 kJ/mol compared to eclipsed conformations
  • Energy difference between staggered and eclipsed conformations is called the rotational barrier
• Stability differences are influenced by steric hindrance and van der Waals forces between atoms

Ethane rotation energy diagram

• Plots the potential energy of the molecule as a function of the dihedral angle (rotation angle around the C-C bond)
  • Sinusoidal shape with three energy minima and three energy maxima per 360° rotation
• Energy minima correspond to staggered conformations
  1. Occur at dihedral angles of 60°, 180°, and 300°
  2. Represent the most stable conformations of ethane
• Energy maxima correspond to eclipsed conformations
  1. Occur at dihedral angles of 0°, 120°, and 240°
  2. Represent the least stable conformations of ethane
• Difference in potential energy between the minima (staggered) and maxima (eclipsed) is the rotational barrier
  • Approximately 12 kJ/mol for ethane

Structural considerations

• Ethane exhibits tetrahedral geometry around each carbon atom
• Molecular symmetry plays a role in the equivalence of staggered and eclipsed conformations
• The tetrahedral arrangement contributes to the overall stability of the molecule