What is the least stable conformer of n-butane?

The least stable conformer of n-butane is the fully eclipsed form. In this configuration, the atoms or groups on adjacent carbon atoms are aligned directly on top of one another, creating significant torsional strain. This alignment leads to increased electron repulsion between the bonded atoms, which destabilizes the molecule more than other conformations.

In contrast, the staggered form, which involves groups positioned at an angle (i.e., maximizing the distance between them), is more stable due to reduced electron repulsion. The gauche form, where two methyl groups are at a 60-degree dihedral angle, has some steric strain but is still more stable than the fully eclipsed form. The anti form, characterized by two methyl groups being 180 degrees apart, represents another stable configuration because it minimizes steric interactions.

Overall, the significant torsional strain in the fully eclipsed form makes it the least stable conformation for n-butane.