Molecular Chirality: Enantiomers

 Enantiomers :

Everything has a mirror image, but not all things are superimposable on their mirror images.

In 1894, William Thomson (Lord Kelvin) coined a word for this property. He defined an object as chiral if it is not superimposable on its mirror image. Applying Thomson’s term to chemistry, we say that a molecule is chiral if its two mirror-image forms are not superimposable in three dimensions. The word chiral is derived from the Greek word cheir, meaning “hand,” and it is entirely appropriate to speak of the “handedness” of molecules. The opposite of chiral is achiral. A molecule that is superimposable on its mirror image is achiral.

In organic chemistry, Chirality most often occurs in molecules that contain a carbon that is attached to four different groups. An example is bromochlorofluoromethane.

Structures A and B are mirror-image representations of bromochlorofluoromethane (BrClFCH).

As shown in Figure, the two mirror images of bromochlorofluoromethane cannot be superimposed on each other. Because the two mirror images of bromochlorofl uoromethane are not superimposable, BrClFCH is chiral.

The mirror images of bromochlorofluoromethane have the same constitution. That is, the atoms are connected in the same order. But they differ in the arrangement of their atoms in space; they are stereoisomers. Stereoisomers that are related as an object and its nonsuperimposable mirror image are classified as enantiomers. The word enantiomer describes a particular relationship between two objects.