Chapter 3, Problem 33P

Interpretation Introduction

Interpretation:The greater percentage of anti-conformation in case of $\text{1,2-dibromoethane}$ relative to simple butane should be explained.

Concept introduction: Various interconvertible forms that result from rotation around the $\text{C–C}$ bind in ethane are termed as conformer. To depict such conformation Newman projections are written. These are obtained by conversion of hashed-wedged line structure simply when molecule is viewed in a plane along the $\text{C–C}$ as illustrated below:

  

Thus in Newman's projection of simple ethane molecule the “front” $\text{C}$ is depicted as intersection of three bonds with three attached $\text{H}$ . The other three bonds connect to the large circle corresponding to “back” carbon in $\text{C–C}$ bond.The potential energy shows peaks and falls. This corresponds to transition from staggered and eclipsed conformation molecule adopts with each $60°$ rotation as illustrated in lateral view below.

  

Staggered conformation that has substituents on both carbons farthest apart is regarded most stable due to least torsional strain, whereas the eclipsed has large amount of torsional strain due to steric repulsions. Therefore in potential-energy diagram, peak corresponds to eclipsed while the valley corresponds to stable staggered conformation.