Enantioomerism

Enantiomerism is a type of stereoisomerism in which a molecule and its mirror image are non-superimposable. These two forms are called enantiomers. A molecule that exhibits enantiomerism is chiral. Chirality is often associated with the presence of a stereocenter, commonly a carbon atom bonded to four different groups. However, chirality can arise from other structural features as well.

Enantiomers share many physical properties, such as melting point, boiling point, and solubility. However, they differ in their interaction with plane-polarized light. One enantiomer will rotate plane-polarized light in a clockwise direction (dextrorotatory, denoted by + or d), while the other will rotate it in a counterclockwise direction (levorotatory, denoted by - or l) by an equal amount. This property is known as optical activity.

In chemical reactions, enantiomers often react identically with achiral reagents. However, they can exhibit different reaction rates or produce different stereoisomers when reacting with other chiral molecules or catalysts. This difference is crucial in biological systems, where enzymes and receptors are chiral. Consequently, two enantiomers of a drug can have vastly different pharmacological effects, with one being therapeutic and the other inactive or even harmful. A mixture of equal amounts of two enantiomers is called a racemic mixture or racemate.