Problem 98 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Among the following conformers, which has highest potential energy for \(\mathrm{n}\) -butane (along \(\mathrm{C}_{2}-\mathrm{C}_{3}\) bond rotation)? (a) Skew (b) Fully eclipsed (c) Staggered (d) Partially eclipsed

Step-by-Step Solution

Answer

The fully eclipsed conformer has the highest potential energy.

Step 1: Understanding Energy in Conformers

In conformational analysis, different spatial arrangements of molecules (conformers) can exist. These result from rotation around single bonds. The potential energy of a conformer is indicative of how stable or unstable it is. Generally, staggered conformers are more stable (lower potential energy) whereas eclipsed conformers are less stable (higher potential energy).

Step 2: Identifying Fully Eclipsed

In a fully eclipsed conformation, all of the bonds at the dihedral angle overlap each other. This causes a maximum torsional strain due to steric hindrance and electrostatic repulsion between the electron clouds of the atoms involved, leading to the highest possible potential energy among simple conformers.

Step 3: Potential Energy Ranking for n-butane

For \(\)-butane rotating around the \(\mathrm{C}_{2}-\mathrm{C}_{3}\) bond, the fully eclipsed conformer is expected to have the highest potential energy. This is followed by partially eclipsed and skew. The most stable is the staggered conformer, which has the lowest potential energy.

Step 4: Selecting the Highest Potential Energy

Given the potential energy ranking, the fully eclipsed conformer has the highest potential energy among the options provided. This is because of the significant steric strain and electronic repulsion when atoms on adjacent carbons are directly aligned.

Key Concepts

Torsional Strain

Torsional strain is a fundamental concept in conformational analysis. It occurs when atoms situated around a bond are in staggered versus eclipsed positions, affecting the molecule's energy level. In an eclipsed conformation, atoms are aligned such that electron clouds overlap, causing repulsion. This creates significant torsional strain due to increased potential energy. In contrast, staggered conformations, where atoms are positioned with minimal overlap, experience reduced torsional strain, resulting in lower potential energy. In n-butane, torsional strain becomes especially apparent in the differing energy of its conformers due to varied atomic interactions during bond rotation. Understanding how torsional strain impacts molecular stability is crucial in analyzing conformational energy landscapes.

n-butane

n-Butane is a simple hydrocarbon with the chemical formula C₄H₁₀. In terms of molecular structure, it consists of four carbon atoms arranged in a straight chain with associated hydrogen atoms. The chemistry of n-butane is often explored through its conformational analysis. n-Butane can rotate around the C₂-C₃ bond, leading to various spatial arrangements, known as conformers. These include staggered, skew, and eclipsed forms. Each conformer has a distinct potential energy due to interactions between atoms. n-Butane's fully eclipsed and staggered conformers exemplify the effect of torsional strain, with staggered being inherently more stable due to lower strain. This makes understanding n-butane's conformations essential for grasping the nuanced energy variations in organic molecules.

Eclipsed Conformer

The eclipsed conformer is a specific spatial arrangement of a molecule where atoms across a bond are in direct alignment. This conformation is crucial to understanding molecular energy and stability. In the case of n-butane's fully eclipsed conformer, all constituent atoms around the C₂-C₃ bond align in parallel rows, maximizing torsional strain. The electron clouds create significant steric hindrance and electrostatic repulsion. This interaction results in the highest potential energy among n-butane's conformers. Despite their instability, eclipsed conformers are essential in learning how molecules adjust spatially. The transition between staggered and eclipsed states in molecules like n-butane helps chemists understand chemical reactivity and conformational flexibility.