Chirality


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chirality

[kī′ral·əd·ē]
(chemistry)
The handedness of an asymmetric molecule.
(particle physics)
The characteristic of particles of spin ½ ℏ that are allowed to have only one spin state with respect to an axis of quantization parallel to the particle's momentum; if the particle's spin is always parallel to its momentum, it has positive chirality; antiparallel, negative chirality.
(physics)
The characteristic of an object that cannot be superimposed upon its mirror image.

Chirality

 

a concept in chemistry characterizing the property of nonsuperimposability of an object with its mirror image. It was first formulated in 1884 by Lord Kelvin (W. Thomson), but it gained common acceptance only after 1966, when it was introduced into stereochemistry by V. Prelog.

In addition to configuration and conformation, chirality is a basic concept of modern stereochemistry. A distinction is made between centric, axial, and planar chirality, to which chiral elements correspond: center, axis, and plane (see Figure 1). For enantiomorphic crystals and some types of molecules, the consideration of chiral space is worthwhile. A chiral center is a broader

Figure 1. Examples of molecules with different chiral elements: (a) center, (b) axis, (c) plane. A, B, C, and D represent different groupings of atoms.

concept than an asymmetric atom, since molecules exist in which the chiral centers do not coincide with any of their atoms. Planar chirality is characteristic of metallo-organic compounds, for example, π-complexes of olefins and arenes. Chirality is a necessary condition for natural molecular optical activity, since chiral objects exist as pairs of enantiomorphs.

The absence of chirality is indicated by the term “achirality.” Achiral molecules may demonstrate induced optical activity. A molecule is prochiral if it may be converted into a chiral molecule by the replacement of a single atom, for example, the replacement of a hydrogen atom in CH2BrCl by a fluorine atom. When chiral and prochiral fragments are combined in one molecule, the phenomenon of nuclear diastereotopy arises, which is observed in nuclear magnetic resonance spectra. The newest method of determining molecular chirality is based on this effect.

REFERENCE

Sokolov, V. I. Novoe v stereokhimii. Moscow, 1975.

V. I. SOKOLOV

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