Case Study
Passage with linked questions
Case Set 1
Case AnalysisPassage
Rahul is a Class 11 student performing an experiment on conformational isomers. His teacher explains that ethane (C2H6) can exist in infinite spatial arrangements due to rotation around its C-C single bond. The teacher demonstrates two extreme conformations using molecular models: one where the hydrogen atoms on adjacent carbons are as close together as possible, and another where they are as far apart as possible. She explains that rotation around the C-C bond is not completely free due to a small energy barrier caused by repulsive interactions between adjacent bond electron clouds. The energy difference between the two extreme forms is approximately 12.5 kJ mol-1. Rahul is asked to draw both conformations using Newman projections and explain which form is more stable and why.
Question 1: What are the two extreme conformations of ethane? Define each briefly.
- The eclipsed conformation is the arrangement in which the hydrogen atoms on adjacent carbons are as close together as possible, with a dihedral angle of 0 degrees between C-H bonds on opposite carbons.
- The staggered conformation is the arrangement in which hydrogen atoms on adjacent carbons are as far apart as possible, with a dihedral angle of 60 degrees between corresponding C-H bonds.
- Any intermediate conformation between these two extremes is called a skew conformation; both eclipsed and staggered conformations can be represented using Sawhorse or Newman projections.
Question 2: What is torsional strain, and how does it differ between the eclipsed and staggered conformations of ethane?
- Torsional strain is the repulsive interaction between electron clouds of adjacent C-H bonds on neighbouring carbon atoms, caused by electrostatic repulsion between the bond electron clouds.
- In the eclipsed conformation, C-H bond electron clouds on adjacent carbons are as close as possible, maximising torsional strain and making it the least stable conformation with highest potential energy.
- In the staggered conformation, the electron clouds are maximally separated, minimising torsional strain and resulting in the lowest potential energy and highest stability; the magnitude of torsional strain depends on the dihedral (torsional) angle.
Question 3: Despite the staggered conformation being more stable, conformational isomers of ethane cannot be isolated at room temperature. Explain why, and discuss what this implies about the nature of C-C single bond rotation.
- The energy difference between the staggered and eclipsed conformations of ethane is only approximately 12.5 kJ mol-1, which is very small compared to the average thermal (kinetic) energy available at room temperature.
- At ordinary temperatures, ethane molecules gain sufficient thermal energy through intermolecular collisions to continuously overcome this small energy barrier, allowing rapid interconversion between staggered, skew, and eclipsed conformations.
- Because interconversion is extremely rapid and the energy barrier is too low to trap any individual conformer, it is not possible to separate and isolate different conformational isomers of ethane; consequently, rotation about the C-C single bond is considered almost free for all practical purposes.