Q.Describe the conformers of n-butane.

Step 1. Set-up. n-Butane can be viewed as ethane with one H on each carbon replaced by a methyl group; rotating about the CENTRAL C2-C3 bond (keeping C1 and C4, the two terminal methyls, as the reference substituents) generates the same family of conformers ethane has, but now distinguished further by how close the two BULKY methyl groups (not just H's) sit to each other.

Step 2. Eclipsed conformation. When the two methyl groups sit directly in front of one another (dihedral angle 0 degrees), the distance between them is at a MINIMUM, giving maximum steric (and torsional) repulsion -- this is the LEAST stable conformer, and in fact the highest-energy point on the whole rotational profile (since it combines torsional strain with the worst possible methyl-methyl steric clash).

Step 3. Gauche conformation. At a dihedral angle of about 60 degrees, the two methyl groups are staggered relative to all the H's (so torsional strain from eclipsing bonds is gone) but are still relatively close to each other -- giving a LOCAL energy minimum that is more stable than any eclipsed form but less stable than the anti form, owing to residual steric (gauche) interaction between the two methyls.

Step 4. Anti (staggered) conformation. At a dihedral angle of 180 degrees, the two methyl groups sit as far apart from each other as geometry allows, with no torsional strain and minimal steric interaction -- this is the GLOBAL energy minimum, the single most stable conformer of n-butane. …

Track the C2-C3 dihedral angle through a full rotation and identify, at each named conformer, exactly how close the two BULKY methyl substituents (not just the smaller H's) are to each other -- steric clash between the methyls, on top of ordinary torsiona …

• Treating n-butane's conformational analysis as identical to ethane's simple eclipsed-versus-staggered picture, and missing the further GAUCHE-versus-ANTI distinction that only appears once bulky (non-H) substituents like the two methyls are present. …