Conformations of Ethane and Propane

The staggered conformation of ethane occurs when C-H groups are positioned at 60° dihedral angles, placing hydrogen atoms as far apart as possible. This arrangement minimizes steric repulsion between electron-rich bonds and is stabilized by favorable interactions between bonding and antibonding molecular orbitals, making it the lowest energy and most stable form of ethane.

In the eclipsed conformation, C-H bonds align at 0° dihedral angles, causing increased steric repulsion and eliminating stabilizing orbital interactions. This results in 12 kJ/mol higher energy than the staggered form, with each H-H eclipsing interaction contributing 4 kJ/mol. This energy difference is called torsional strain or torsional barrier.

At room temperature, approximately 99% of ethane molecules exist in the lowest energy staggered conformation. Molecular collisions provide energy to overcome the torsional barrier, allowing molecules to rotate through high-energy eclipsed states and interconvert between different staggered conformations continuously throughout the sample.

Propane has two major conformers: staggered (low energy, stable) and eclipsed (high energy, unstable). The eclipsed conformer has 14 kJ/mol torsional strain. Each eclipsing H-H pair contributes 4 kJ/mol, while CH3-H eclipsing interactions contribute 6 kJ/mol, making propane's torsional barrier slightly higher than ethane's.

Dihedral angle measures the spatial relationship between C-H bonds on adjacent carbons. At 60° angles, ethane adopts the staggered, low-energy conformation. Rotating to 0° creates the eclipsed, high-energy form. Continuous rotation from 0° to 360° generates alternating staggered and eclipsed states with varying energy levels.

In staggered ethane, the occupied bonding molecular orbital of one C-H bond interacts favorably with the unoccupied antibonding molecular orbital of another C-H bond. This orbital overlap provides additional stabilization beyond simple steric effects, contributing to the staggered conformation's lower energy and greater stability.

Propane's eclipsed conformation has 14 kJ/mol torsional strain compared to ethane's 12 kJ/mol, making propane slightly less stable in its eclipsed form. The difference arises because propane contains CH3-H eclipsing interactions worth 6 kJ/mol each, in addition to H-H interactions, increasing overall strain significantly.