Ethane: Staggered (0 kJ/mol, most stable) vs Eclipsed (12.5 kJ/mol, torsional strain). Barrier to rotation = 12.5 kJ/mol.

Butane (looking along C2-C3 bond):

• Anti: 0 kJ/mol (most stable — CH3 groups 180° apart)
• Gauche: +3.8 kJ/mol (CH3 groups 60° apart — steric strain)
• Eclipsed $\frac{CH3}{H}$: +16 kJ/mol
• Fully eclipsed $\frac{CH3}{CH3}$: +19 kJ/mol (least stable — maximum steric + torsional strain)

JEE application: Know how to draw Newman projections and identify the most/least stable conformers. Cyclic conformations $\frac{chair}{boat cyclohexane}$ are related but more complex.