L. D. Shirtcliff – CHEM 3053 Isomerism Classical Definitions of Isomerism: Isomers are compounds that have identical molecular formulae but differ in the nature or sequence of bonding of their atoms or in the spatial arrangement of their atoms. Constitutional Isomers differ in the nature or sequence of bonding of their atoms. Stereoisomers have the same sequence of bonding, but differ as to the spatial arrangement of their atoms. Symmetry Classification of Isomerism (K. Mislow, Bull. Soc. Chim. Belg. 1975 , 86 , 595.) - see handout Stereochemical Terminology A molecule (or other object) is chiral or dissymmetric if it possesses no alternating symmetry axes (S n- rotation/reflection). (Such a condition makes the chiral molecule nonsuperimposable on its mirror image.) The property is called chirality . (Note that optical activity is a measurable; chirality is not.) An achiral molecule does possess an S n and is superimposable on its mirror image. Classification of Stereoisomers (two separate parallel classification schemes):
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