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Chirality (chemistry)

Chirality /kaɪˈrælɪti/ is a geometric property of some molecules and ions. A chiral molecule/ion is non-superposable on its mirror image. The presence of an asymmetric carbon center is one of several structural features that induce chirality in organic and inorganic molecules. The term chirality is derived from the Ancient Greek word for hand, χείρ (cheir). Chirality /kaɪˈrælɪti/ is a geometric property of some molecules and ions. A chiral molecule/ion is non-superposable on its mirror image. The presence of an asymmetric carbon center is one of several structural features that induce chirality in organic and inorganic molecules. The term chirality is derived from the Ancient Greek word for hand, χείρ (cheir). The mirror images of a chiral molecule or ion are called enantiomers or optical isomers. Individual enantiomers are often designated as either right-handed or left-handed. Chirality is an essential consideration when discussing the stereochemistry in organic and inorganic chemistry. The concept is of great practical importance because most biomolecules and pharmaceuticals are chiral. Chiral molecules and ions are described by various ways of designating their absolute configuration, which codify either the entity's geometry or its ability to rotate plane-polarized light, a common technique in studying chirality. Chirality is based on molecular symmetry elements. Specifically, a chiral compound can contain no improper axis of rotation (Sn), which includes planes of symmetry and inversion center. Chiral molecules are always dissymmetric (lacking Sn) but not always asymmetric (lacking all symmetry elements except the trivial identity). Asymmetric molecules are always chiral. In general, chiral molecules have point chirality at a single stereogenic atom, which has four different substituents. The two enantiomers of such compounds are said to have different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism). The stereogenic atom (also known as the stereocenter) is usually carbon, as in many biological molecules. However a stereocenter can coincide with any atom, including metals (as in many chiral coordination compounds), phosphorus, or sulfur. The low barrier of nitrogen inversion make most N-chiral amines (NRR′R″) impossible to resolve, but P-chiral phosphines (PRR′R″) as well as S-chiral sulfoxides (OSRR′) are optically stable. While the presence of a stereogenic atom describes the great majority of chiral molecules, many variations and exceptions exist. For instance it is not necessary for the chiral substance to have a stereogenic atom. Examples include 1-bromo-3-chloro-5-fluoroadamantane, methylethylphenyltetrahedrane, certain calixarenes and fullerenes, which have inherent chirality. The C2-symmetric species 1,1′-bi-2-naphthol (BINOL), 1,3-dichloroallene have axial chirality. (E)-cyclooctene and many ferrocenes have planar chirality. When the optical rotation for an enantiomer is too low for practical measurement, the species is said to exhibit cryptochirality. Even isotopic differences must be considered when examining chirality. Illustrative is the derivative of benzyl alcohol PhCHDOH, which is chiral. The S enantiomer has D = +0.715°. Many biologically active molecules are chiral, including the naturally occurring amino acids (the building blocks of proteins) and sugars.

[ "Stereochemistry", "Photochemistry", "Organic chemistry", "Inorganic chemistry", "Catalysis", "Permethylated beta-cyclodextrin", "Diethyl tartrate", "Optical rotation", "Diphosphines", "Filiformin" ]
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