Conformational Analysis of n-Butane

Introduction
Conformational analysis is the study of different spatial arrangements or conformations that a molecule can adopt by rotating around single bonds. In the case of n-butane, the conformational analysis focuses on the rotation about the central C-C bond.

Predominant Conformers
It is believed that the predominant conformers of n-butane about the C-C bond are the anti and gauche conformers. These two conformers have the lowest energy and are most stable, while the remaining conformers are very unstable and contribute negligibly to the equilibrium mixture at room temperature.

Anti Conformer
The anti conformer is the most stable conformer of n-butane. In this conformation, the two methyl groups are positioned opposite to each other, resulting in maximum separation and minimal steric hindrance. The energy of the anti conformer is the lowest among all possible conformers.

Gauche Conformer
The gauche conformer is the second most stable conformer of n-butane. In this conformation, the two methyl groups are positioned next to each other, resulting in some steric hindrance. However, the energy of the gauche conformer is still lower than the remaining conformers.

Equilibrium Constant
The equilibrium constant for the conformational interconversion between the anti and gauche conformers of n-butane can be determined experimentally. The value of 0.25 indicates that the equilibrium mixture contains more anti conformers compared to gauche conformers.

Explanation
The preference for the anti conformer over the gauche conformer can be explained by the lower energy of the anti conformer. The anti conformer has a staggered arrangement, which minimizes the steric interactions between the methyl groups. On the other hand, the gauche conformer has a slightly eclipsed arrangement, resulting in some steric hindrance.

At room temperature, the molecules of n-butane are constantly rotating about the C-C bond. Due to the lower energy of the anti conformer, it is more likely to be populated compared to the gauche conformer. Therefore, the equilibrium mixture at room temperature contains a higher proportion of anti conformers.

The stability of the anti and gauche conformers can also be explained by the concept of torsional strain. Torsional strain arises from the repulsion between electron clouds of adjacent atoms, which is minimized in the anti conformer and slightly increased in the gauche conformer.

In conclusion, the conformational analysis of n-butane about the C-C bond reveals that the predominant conformers are the anti and gauche conformers. These conformers have the lowest energy and are most stable, while the remaining conformers are very unstable and contribute negligibly to the equilibrium mixture at room temperature. The equilibrium constant of 0.25 indicates a higher proportion of anti conformers in the equilibrium mixture.