Chapter 4.9, Problem 14P

Interpretation Introduction

Interpretation:

Why separation of the two enantiomers of $\text{5,5'-dinitrobiphenyl-3,3'-dicarboxylic acid}$ would be extremely difficult is to be explained.

Concept introduction:

Some compounds are chiral because they contain an axis around which spatial arrangement is asymmetric, making the structure and its mirror image non-superimposable.

The two rings of $\text{1,1'-biphenyl}$ rotate about the bond joining the rings so as to relieve steric strain between the atoms/groups in $\text{2, 6}$ and $\text{2',6'}$ positions. This introduces asymmetry (chirality) about the axis that includes this bond.

Rotation about this axis and, therefore, the equilibrium between the two enantiomers depends on the size of atoms/groups in these positions.

If the steric strain (energy barrier to rotation) is high, the equilibration between the two isomers can be slow enough to permit separation of the two isomers.