Cue Column | Notes Column

What is a Newman projection? | A view along a C–C bond axis — front carbon shown as a dot, back carbon as a circle. Substituents drawn as lines from each.

Ethane conformations and energies? | Staggered: dihedral 60°, most stable, 0 torsional strain. Eclipsed: dihedral 0°, ~12 kJ/mol strain from H–H orbital repulsion.

What causes torsional strain? | Electron–electron repulsion between filled C–H bonding orbitals on adjacent carbons when they eclipse each other (0° dihedral).

Butane Newman projection (C2–C3 bond)? |

• Anti (180°): $CH_{3}$ groups farthest apart — most stable, 0 kJ/mol
• Gauche (60°): ~3.8 kJ/mol above anti — van der Waals $CH_{3}$–$CH_{3}$ interaction
• Eclipsed (120°): ~16 kJ/mol above anti — $CH_{3}$ eclipses H
• Fully eclipsed (0°): ~19 kJ/mol above anti — $CH_{3}$ eclipses $CH_{3}$ (maximum strain)

What is steric strain vs torsional strain? | Steric strain = van der Waals repulsion between non-bonded groups that are spatially close (bulky groups). Torsional strain = electronic repulsion between eclipsing bonds. Fully eclipsed butane has BOTH.

Summary

Newman projections reveal how substituent arrangement around a C–C bond determines stability. Staggered ethane and anti butane are preferred. The energy cost of "rotating" through eclipsed conformations explains why molecules prefer staggered arrangements in solution.