The relative energies and therefore the relative stabilities of the various conformers depends on the proximity of electron clouds on atoms that are not bonded to one another.
For butane this means the proximity of the methyl groups and hydrogens bonded to C-2 to the methyl groups and hydrogens bonded to C-3.
The reason that the proximity of the groups bonded to the adjacent atoms is the important factor is that there are repulsive forces between electron clouds of atoms/groups that are NOT bonded to one another.
The graphics show the electron cloud for anti, gauche and one eclipsed conformation of butane. These show that in the
All eclipsed conformations are less stable (higher energy) than any staggered conformation.
The least stable (highest energy) eclipsed conformation has the largest groups directly opposite one another.
The anti staggered conformations are the more stable (lower energy) than the gauche conformations.
The reason that the proximity of the groups bonded to the adjacent atoms is the important factor is that there are repulsive forces between electron clouds of atoms/groups that are NOT bonded to one another.
The conformations where the repulsive forces are greater are of higher energy (less stable).
The graphics show the electron cloud for anti, gauche and one eclipsed conformation of butane. These show that in the
anti conformation, the electron clouds on the two CH3 groups on C-2 and C-3 are farthest from one another.
gauche conformation, the electron clouds on the CH3 groups on C-2 and C-3 are closer than in the anti conformation
eclipsed conformation shown, the CH3 groups on C-2 and C-3 are closer than any other conformations,
gauche conformation, the electron clouds on the CH3 groups on C-2 and C-3 are closer than in the anti conformation
eclipsed conformation shown, the CH3 groups on C-2 and C-3 are closer than any other conformations,
 | rotate C-4 120° out of screen  |  |
| anti | gauche |
| rotate C-4 60° toward screen |  |
| eclipsed |
All eclipsed conformations are less stable (higher energy) than any staggered conformation.
The least stable (highest energy) eclipsed conformation has the largest groups directly opposite one another.
The anti staggered conformations are the more stable (lower energy) than the gauche conformations.