Enantiomer
Mirror-image molecules that are non-superposable and rotate polarized light.
An enantiomer is one of a pair of molecular entities that are mirror images of each other and non-superposable, a relationship known as chirality. This mirror-image relationship is permanent, based on the three-dimensional arrangement of atoms; no amount of reorientation or conformational change can convert one enantiomer into the other. Enantiomers rotate plane-polarized light in opposite directions: a dextrorotatory enantiomer rotates the plane clockwise and is denoted (+), while a levorotatory enantiomer rotates it counterclockwise and is denoted (−). A mixture of equal amounts of each enantiomer, called a racemic mixture or racemate, does not rotate light. The naming conventions for specifying which enantiomer is which include the R/S system, based on the molecule’s geometry and the Cahn–Ingold–Prelog priority rules; the (+)/(−) system, based on optical rotation; and the D/L system, based on the molecule’s relationship to glyceraldehyde. The Latin words for left (*laevus*, *sinister*) and right (*dexter*, *rectus*) give rise to the prefixes levo- and dextro- in common names, as well as the ar- prefix (from *recto*) for the right-handed version and es- (from *sinister*) for the left-handed version, as seen with ketamine, arketamine, and esketamine. A compound with exactly one asymmetric atom is always chiral, but compounds with an even number of asymmetric atoms can be achiral if they possess a mirror symmetry plane, as in meso compounds like meso-tartaric acid. Chirality can also arise without asymmetric atoms, such as axial, planar, or helical chirality. The practical importance of enantiomers is evident in pharmacology: the sedative thalidomide had one enantiomer providing the desired sedative effect while the other caused birth defects; the herbicide mecoprop is a racemic mixture, with only the (R)-(+)-enantiomer possessing herbicidal activity; and the antidepressant citalopram is a racemate, while its pure (S)-enantiomer, escitalopram, is a chiral switch requiring half the dosage. Enantiopure compounds, consisting of only one enantiomer, often have improved therapeutic efficacy.
- field
- Chemistry
- known_for
- Chirality, optical isomerism, and the R/S, (+)/(-), and D/L naming conventions
- key_concept
- Mirror-image, non-superposable molecular pairs
- related_terms
- Racemic mixture, diastereomer, chirality center
Lore & Background
Enantiomers are defined by their chirality, a permanent three-dimensional relationship that prevents one enantiomer from being superposed onto its mirror image without conversion. This property is analogous to left and right hands. Chemical structures with chirality rotate plane-polarized light, with dextrorotatory enantiomers rotating it clockwise and levorotatory ones counterclockwise. The naming conventions for enantiomers include the R/S system based on geometry and Cahn–Ingold–Prelog priority rules, the (+)/(-) system based on optical rotation, and the D/L system based on relationship to glyceraldehyde.
Reader's Guide
The significance of enantiomers extends deeply into chemistry and pharmacology. Because enantiomers can have vastly different biological effects, as seen with thalidomide—where one enantiomer provided sedative effects while the other caused birth defects—the separation or selective synthesis of a single enantiomer is often crucial for drug safety and efficacy. This has led to the practice of chiral switches, where a racemic drug is replaced by an enantiopure version, such as escitalopram replacing citalopram. Enantiopure compounds can improve therapeutic efficacy and may be separately patentable. While enantiomers are nearly identical in energy, theoretical physics predicts a minute energy difference due to parity violation of the weak nuclear force, though this is chemically inconsequential. Methods to obtain pure enantiomers include asymmetric synthesis, enantioconvergent synthesis, and, in rare cases, mechanical separation of crystals, as pioneered by Louis Pasteur with sodium ammonium tartrate.
Did You Know?
- The trade names of the enantiomeric pair propoxyphene—Darvon and Novrad—reflect their chemical mirror-image relationship.
- Meso compounds, such as meso tartaric acid, have an even number of asymmetric atoms but lack chirality due to a mirror symmetry plane.
- The prefix 'ar-' (from Latin recto, right) is applied to the right-handed version of a molecule, and 'es-' (from Latin sinister, left) to the left-handed version, as in arketamine and esketamine.
- Amines with three distinct substituents are chiral but often rapidly racemize at room temperature via umbrella inversion, making them effectively achiral.
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