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Chirality (chemistry)


Chirality /kˈrælɪti/ 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 Greek word for hand, χειρ (kheir).

The mirror images of a chiral molecule/ion are called enantiomers or optical isomers. Individual enantiomers are often designated as either "right-" 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.


Molecular symmetry and chirality
Rotational
axis (Cn)
Improper rotational elements (Sn)
  Chiral
no Sn
Achiral
mirror plane
S1 = σ
Achiral
inversion centre
S2 = i
C1 Chiral sym CHXYZ.svg Chiral sym CHXYRYS.svg Chiral sym CCXRYRXSYS.svg
C2 Chiral sym CCCXYXY.svg Chiral sym CHHXX.svg Chiral sym CCXYXY.svg

  • Any non-racemic chiral substance is called scalemic. Scalemic materials can be enantiopure or enantioenriched.
  • A chiral substance is enantiopure when only one of two possible enantiomers is present so that all molecules within a sample have the same chirality sense. Use of homochiral as a synonym is strongly discouraged.
  • A chiral substance is enantioenriched or heterochiral when its enantiomeric ratio is greater than 50:50 but less than 100:0.
  • Enantiomeric excess or ee is the difference between how much of one enantiomer is present compared to the other. For example, a sample with 40% ee of R contains 70% R and 30% S (70% − 30% = 40%).
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Wikipedia

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