gauche and anti conformation
/ GOHSH /
Ethane's only worry is torsional strain, because all its outer atoms are identical hydrogens. Butane adds a twist: its central C-C bond has a bulky methyl group on each end, so now it matters not just whether bonds eclipse but how close those two big methyls come to each other. Looking along that central bond in a Newman projection, the two methyls can sit at various angles, and three staggered arrangements stand out.
When the two methyls are pointed in opposite directions, 180 degrees apart, that is the anti conformation — the methyls are as far apart as possible, and it is the lowest-energy, most populated shape. Rotate one carbon by 60 degrees and the methyls come to 60 degrees apart: that is the gauche conformation. Gauche is still staggered, so it has no torsional strain, yet it sits a little higher in energy (about 3.8 kJ/mol above anti) because the two methyls are now close enough to bump — this crowding is steric strain.
So butane has two staggered valleys of different depth (anti deeper, gauche shallower) separated by eclipsed hills, the worst hill being where the two methyls eclipse each other. At room temperature a sample of butane is a mixture, mostly anti with a meaningful slice of gauche. The anti-versus-gauche balance is the prototype for steric reasoning everywhere: bulky groups prefer to stay far apart, and this single preference shapes ring conformations, reaction selectivity, and the folding of long chains.
In butane, the anti conformer (methyls 180 degrees apart) is the most populated; the gauche conformer (methyls 60 degrees apart) is present too but rarer, by about 3.8 kJ/mol of steric strain.
Anti = methyls far apart (lowest energy); gauche = methyls crowded at 60 degrees (steric strain).
Gauche is still staggered, so its extra energy is steric (groups crowding), not torsional (bonds eclipsing). Don't conflate the two strains — only eclipsed forms have torsional strain.