Topic overview

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Enantiomers have the same connectivity and share physical properties in achiral environments, but rotate plane-polarized light in opposite directions. The measured rotation sign is empirical and does not determine whether a configuration is R or S. CIP rules establish group priorities, beginning with atomic number and resolving ties by comparing atoms farther from the stereocenter; isotopes are ranked by mass, and multiple bonds are treated as duplicated attachments. With the lowest-priority group directed away, tracing priorities 1 to 2 to 3 assigns R or S, with an inversion needed when that group faces the observer. Fischer projections use vertical bonds directed away and horizontal bonds toward the observer. Meso compounds, racemic mixtures, diastereomers, and enantiomeric excess provide further ways to describe stereochemical relationships and mixtures.