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In chemistry, an enantiomer (/ɪˈnænti.əmər, ɛ-, -oʊ-/ ih-NAN-tee-ə-mər), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities which are mirror images of each other and non-superposable.
Enantiomer molecules are like right and left hands: one cannot be superposed onto the other without first being converted to its mirror image. It is solely a relationship of chirality and the permanent three-dimensional relationships among molecules or other chemical structures: no amount of re-orientation of a molecule as a whole or conformational change converts one chemical into its enantiomer. Chemical structures with chirality rotate plane-polarized light. A mixture of equal amounts of each enantiomer, a racemic mixture or a racemate, does not rotate light.
Stereoisomers include both enantiomers and . Diastereomers, like enantiomers, share the same molecular formula and are also non-superposable onto each other; however, they are not mirror images of each other.
The R/S system is based on the molecule's geometry with respect to a chiral center. The R/S system is assigned to a molecule based on the priority rules assigned by Cahn–Ingold–Prelog priority rules, in which the group or atom with the largest atomic number is assigned the highest priority and the group or atom with the smallest atomic number is assigned the lowest priority.
The (+) or (−) symbol is used to specify a molecule's optical rotation — the direction in which the polarization of light rotates as it passes through a solution containing the molecule. When a molecule is denoted dextrorotatory, it rotates the plane of polarized light clockwise and can also be denoted as (+). When it is denoted as levorotatory, it rotates the plane of polarized light counterclockwise and can also be denoted as (−).
The Latin words for left are laevus and sinister, and the word for right is dexter (or rectus in the sense of correct or virtuous). The English word right is a cognate of rectus. This is the origin of the D/L and R/S notations, and the employment of prefixes levo- and dextro- in Systematic name.
The prefix ar-, from the Latin recto (right), is applied to the right-handed version; es-, from the Latin sinister (left), to the left-handed molecule. Example: ketamine, arketamine, esketamine.
Compounds that contain exactly one (or any odd number) of asymmetric atoms are always chiral. However, compounds that contain an even number of asymmetric atoms sometimes lack chirality because they are arranged in mirror-symmetric pairs, and are known as Meso compound. For instance, meso tartaric acid (shown on the right) has two asymmetric carbon atoms, but it does not exhibit enantiomerism because there is a mirror symmetry plane. Conversely, there exist forms of chirality that do not require asymmetric atoms, such as Axial chirality, Planar chirality, and helical chirality.
Even though a chiral molecule lacks reflection (Cs) and rotoreflection symmetries (S2 n), it can have other molecular symmetries, and its symmetry is described by one of the chiral point groups: C n, D n, T, O, or I. For example, hydrogen peroxide is chiral and has C2 (two-fold rotational) symmetry. A common chiral case is the point group C1, meaning no symmetries, which is the case for lactic acid.
The herbicide mecoprop is a racemic mixture, with the ( R)-(+)-enantiomer ("Mecoprop-P", "Duplosan KV") possessing the herbicidal activity.
Another example is the antidepressant drugs escitalopram and citalopram. Citalopram is a racemate 1:1; escitalopram ( S)-citalopram is a pure enantiomer. The dosages for escitalopram are typically 1/2 of those for citalopram. Here, (S)-citalopram is called a chiral switch of Citalopram.
In his pioneering work, Louis Pasteur was able to isolate the isomers of tartaric acid because the individual enantiomers crystallize separately from solution. To be sure, equal amounts of the enantiomorphic crystals are produced, but the two kinds of crystals can be separated with tweezers. This behavior is unusual. A less common method is by enantiomer self-disproportionation.
The second strategy is asymmetric synthesis: the use of various techniques to prepare the desired compound in high enantiomeric excess. Techniques encompassed include the use of chiral starting materials (chiral pool synthesis), the use of chiral auxiliaries and chiral catalysts, and the application of asymmetric induction. The use of enzymes (biocatalysis) may also produce the desired compound.
A third strategy is Enantioconvergent synthesis, the synthesis of one enantiomer from a racemic precursor, utilizing both enantiomers. By making use of a chiral catalyst, both enantiomers of the reactant result in a single enantiomer of product.
Enantiomers may not be isolable if there is an accessible pathway for racemization (interconversion between enantiomorphs to yield a racemic mixture) at a given temperature and timescale. For example, amines with three distinct substituents are chiral, but with few exceptions (e.g. substituted ), they rapidly undergo "umbrella inversion" at room temperature, leading to racemization. If the racemization is fast enough, the molecule can often be treated as an achiral, averaged structure.
Though not considered actual enantiomers, the naming convention for quasi-enantiomers also follows the same trend as enantiomers, when looking at ( R) and ( S) configurations - which are considered from a geometrical basis (see Cahn–Ingold–Prelog priority rules).
Quasi-enantiomers have applications in parallel kinetic resolution.G.S. Coumbarides, M. Dingjan, J. Eames, A. Flinn, J. Northen and Y. Yohannes, Tetrahedron Lett. 46 (2005), p. 2897er
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