enantiomere

Enantiomere, or enantiomer in English, is one of two stereoisomers that are non-superimposable mirror images of each other. Enantiomeres arise when a molecule contains a stereogenic element, such as a chiral center, or other forms of chirality like axial or planar chirality. Each pair of enantiomeres forms an enantiomeric relationship.

In achiral environments, enantiomeres typically have identical physical properties, including melting point, boiling point, and solubility. They interact differently with other chiral substances, however, and with polarized light. Enantiomeres rotate plane-polarized light in opposite directions; the terms dextrorotatory and levorotatory describe the direction of rotation, not the absolute configuration. The absolute arrangement around a stereogenic center is commonly described using the Cahn–Ingold–Prelog system as R or S.

A racemate is a 1:1 mixture of enantiomers. Enantiomeric composition is often expressed as enantiomeric excess (ee) or as an enantiomeric ratio. Separation of enantiomers—chiral resolution or asymmetric synthesis—can be achieved by techniques such as chiral chromatography, the use of chiral auxiliaries, or by enzymatic methods.

Enantiomeres frequently display markedly different biological activities because most biomolecules are chiral. This leads to enantioselectivity in pharmacology and metabolism; a drug may be active, inactive, or even toxic in one enantiomer but not in the other. Consequently, enantiomeric purity and specific labeling are important considerations in drug development, regulation, and quality control.

Methods to analyze enantiomeres include polarimetry to measure optical rotation and chiral chromatographic or spectroscopic techniques, often employing chiral stationary phases or chiral solvating agents.