Center of chirality, chiral center: "an atom bearing a set
of substituents in a spatial arrangement that is not superposable to its
mirror image arrangement." (Quote from Hellwich,
translated by C.R.)
Examples (apart from the usual asymmetric C atom):
ethyl methyl sulfoxide
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ethyl methyl phenyl phosphanoxide
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N-ethyl-N-methylbenzylamine
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The three substituents and the lone pair at an amine N atom are arranged
tetrahedrically. However, by fast inversion of the N atom the enantiomers
interconvert, such that an amine N atom is not normally considered
a center of chirality.
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