Section 3.1 Stereochemistry is concerned with molecular shape and the relationships between molecules with identical atom connectivity but different shapes.
Section 3.2 Chirality is a property of objects that are not the same thing as their mirror image. Objects (or molecules) that meet this criteria are said to be chiral. Those that are identical to their mirror image are said to be achiral.
Section 3.3 Constitutional isomers have different connectivity of atoms whereas stereoisomers have the same atom connectivity but different shapes. Stereoisomers can be further classified as enantiomers (mirror-image stereoisomers) or diastereomers (non-mirror image stereoisomers) (see also Box 3.1).
Section 3.4 The property of chirality should not be confused with the descriptive terms enantiomer and diastereomer. All chiral molecules have one, and only one, enantiomer. Achiral molecules will never have an enantiomer but may have one or more diastereomers (see Figures 3.5, 3.6, and 3.11).
Section 3.5 The molecule 1,3-dimethylcyclohexane has three distinct stereoisomers—a pair of enantiomers and an achiral diastereomer.
Section 3.6 A chirality center is a center (point) from which chirality originates in a molecule. The most common type of chirality center in organic molecules is an sp3 hybridized carbon atom attached two four distinct substituents.
Section 3.7 A chirality center can have one of two configurations, either R or S. The Cahn–Ingold–Prelog (CIP) rules are employed to assign the configuration of chirality centers (see Box 3.2).
Section 3.8 The assignment of configuration at chirality centers is a powerful strategy for determining stereochemical relationships between molecules. Enantiomers have the opposite configuration at all chirality centers. Diastereomers have opposite configuration(s) at one or more chirality centers, but not at all chirality centers (or they would be enantiomers).
Section 3.9 Meso compounds are achiral members of a set of stereoisomers that includes at least one chiral member. Meso compounds have two or more chirality centers related by an internal symmetry element such as a mirror plane. Configurational assignment by CIP rules cannot identify meso compounds—visual inspection for symmetry elements is required.
Section 3.10 Any configurationally stable atom with tetrahedral (sp3) geometry and four different substituents can be considered a chirality center. The most common non-carbon chirality center in drugs is tetrahedral sulfur in the sulfoxide oxidation state.
Section 3.11 Chirality can also originate from a chirality axis. Certain allenes and rotationally constrained biaryl systems are chiral by virtue of a chirality axis.
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