Molecules of \(6,6^{\prime}\)-dinitrobiphenyl-2,2'-dicarboxylic acid have no tetrahedral chiral center, and yet they can be resolved to a pair of enantiomers. Account for this chirality.

Enantiomers are molecules that are non-superimposable mirror images of one another, just like a person’s left and right hands. While they may contain the same components, the spatial arrangement of these parts differs in a way that mirror imaging cannot reconcile. Due to this unique spatial structure, each enantiomer possesses distinct properties; for instance, they usually have opposite effects when interacting with other chiral substances, such as biological molecules. In pharmaceuticals, this is crucial as one enantiomer can be therapeutically active while the other may be inert or even harmful.

The intriguing case of the 6,6'-dinitrobiphenyl-2,2'-dicarboxylic acid guides us to discover that enantiomers are not limited to molecules with tetrahedral chiral centers. The molecule shows that enantiomers can exist due to other types of chiral axes or planes, broadening the spectrum of chirality in molecular structures.

The phenomenon of restricted rotation plays a significant role in the chirality of certain molecules. It occurs when the rotation around a bond is hindered by the presence of bulky substituents, preventing the free rotation that is typically associated with single bonds.

This restricted rotation creates a barrier that keeps the molecule in one of multiple stable conformations. In the case at hand, the 6,6'-dinitrobiphenyl-2,2'-dicarboxylic acid molecule experiences restricted rotation around the single bond connecting the two benzene rings. The bulky nitro and carboxylic acid groups act as spatial hindrances, locking the molecule into one of two stable orientations. These orientations can be enantiomers, showcasing that chirality can be the result of factors beyond the presence of chiral centers.

Optical activity is an intriguing property of chiral molecules where they rotate plane-polarized light in either a clockwise (dextrorotatory) or counterclockwise (levorotatory) direction. The direction and degree of this rotation are specific to the enantiomer, and it provides a practical method to distinguish between two enantiomers.

Even without a tetrahedral chiral center, the 6,6'-dinitrobiphenyl-2,2'-dicarboxylic acid enantiomers exhibit optical activity due to their spatial arrangement, caused by the restricted rotation. This property is essential in fields such as pharmacology and analytical chemistry, as it allows for the identification and separation of enantiomers, which could have vastly different biological activities or properties.

A chiral center, traditionally considered to be a carbon atom with four different substituents, is a pivotal point in a molecule from which the nonsuperimposable mirror images - the enantiomers - emerge.

Chirality Without a Chiral Center

However, chirality is not confined to molecules with such asymmetric carbon atoms. The 6,6'-dinitrobiphenyl-2,2'-dicarboxylic acid example elucidates that molecules can exhibit chirality due to other features, such as restricted rotation around a bond.

This expands the understanding of chirality beyond the limited scope of the classic chiral center. It emphasizes the rich diversity of molecular structures and reminds us that the search for sources of chirality can lead to surprising and educational discoveries about the three-dimensional nature of molecules.