MCQ Detailed View

In organic chemistry, the study of conformational analysis helps us understand how molecules adopt different spatial arrangements and how these affect their stability. Butane (C₄H₁₀) is a simple alkane that provides an excellent example of this concept. Its rotations around the central C–C bond give rise to different conformations, mainly anti and gauche.

In the anti conformation, the two methyl (–CH₃) groups are positioned 180° apart. This is the most stable arrangement because the bulky groups are far from each other, minimizing repulsive interactions. In contrast, in the gauche conformation, the two methyl groups are only 60° apart. This brings them close enough for their electron clouds to repel each other.

This repulsion is not due to bond angle distortion (so angle strain is not involved) and it is also different from torsional strain, which arises when bonds eclipse each other. Instead, the extra energy in the gauche form of butane comes primarily from steric strain. Steric strain occurs when atoms or groups of atoms are forced into close proximity, leading to repulsive interactions between their electron densities.

The energy difference between anti and gauche conformations of butane is about 0.9 kcal/mol, with anti being more stable. While this may seem small, in larger molecules or polymers, steric interactions accumulate and can strongly influence molecular shape, reactivity, and even biological function.

Understanding steric strain is fundamental in organic chemistry because it explains why certain conformations or reactions are favored. For instance, in stereochemistry and enzymatic processes, steric hindrance often determines the outcome of reactions by preventing certain orientations of approach.

Thus, the relatively high energy of the gauche conformation of butane is mainly due to steric strain caused by the close proximity of the two bulky methyl groups.