Some molecules may occur with identical atoms in the same sequence but with different spatial arrangements. These are referred to as stereoisomers. A stereoisomer that is nonsuperimposable on its mirror image is chiral, and an atom with 4 different substituents is a chiral center; the 2 mirror images are enantiomers. An equal mixture of the 2 enantiomers is racemic. Generally only 1 enantiomer is biologically active, as in the case of ibuprofen. In some cases, one enantiomer may be biologically beneficial while the other enantiomer is harmful. For example, one enantiomer of thalidomide is a beneficial drug whereas the other ...
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