Rotation Around Single Bonds
Unlike double bonds which are rigid, single C-C bonds allow free rotation. However, not all rotational orientations (conformations) are equally favorable. The energy landscape as a function of dihedral angle reveals periodic barriers that molecules must overcome as they spin around each bond. Understanding these barriers is essential for predicting molecular shape, reactivity, and NMR coupling constants.
Newman Projection Visualization
The Newman projection provides an intuitive view by looking straight down the bond axis. Front-carbon substituents appear as lines from a central point, while rear-carbon substituents emerge from behind a circle. At 0° (eclipsed), substituents overlap; at 60° (staggered), they alternate in the gaps. This simulation animates the rotation in real time, showing how substituent positions change with dihedral angle.
Energy Barriers and Strain
The torsional energy follows a cosine function with threefold symmetry for ethane-like molecules: E = (V₀/2)(1 - cos 3φ). Energy maxima occur at eclipsed conformations (0°, 120°, 240°) and minima at staggered positions (60°, 180°, 300°). For substituted ethanes, steric interactions between bulky groups create additional energy differences between gauche and anti staggered conformations.
Boltzmann Populations
At any given temperature, molecules distribute among accessible conformations according to the Boltzmann distribution. At room temperature (298 K), the 12.5 kJ/mol barrier of ethane is easily overcome, resulting in rapid rotation (about 10 billion rotations per second). Larger barriers in substituted systems can lead to significant population differences between conformers, affecting average molecular properties and reaction selectivity.