Single-bond rotation produces different conformations, but not all are equally stable. Newman projections let you look down a C–C bond to compare torsional strain and steric clashes in acyclic alkanes.

Newman Projections and Dihedral Angles

• View straight down a C–C bond: front carbon as a dot, back carbon as a circle.
• Dihedral angle: angle between a front substituent and a back substituent.
• Staggered (≈60° offsets) minimizes torsional strain; eclipsed (0°) maximizes it.

Butane: Anti vs Gauche (and Eclipsed)

• Look down C2–C3.
• Anti (staggered, 180°): CH₃ groups opposite → lowest energy.
• Gauche (staggered, 60°): CH₃ groups closer → ~0.9 kcal/mol higher (steric bump).
• Eclipsed: highest energy, especially when CH₃ eclipses CH₃ (torsional + steric).

Rotation energy profile: anti (min) → eclipsed (H/CH₃) → gauche → eclipsed (CH₃/CH₃, max).

Why It Matters

Preferred conformations dictate dominant shapes, affect reaction approach trajectories, and rationalize observed selectivity (e.g., anti-periplanar requirements in E2).

Summary

Use Newman projections to spot torsional and steric strain. Staggered beats eclipsed; in butane, anti is most stable, gauche slightly higher, eclipsed forms are transient. Mastering these views helps predict favored shapes and reactivity.