configuration

Meaning: Configuration is the fixed spatial arrangement of groups at a stereocenter or a double bond. It distinguishes stereoisomers and can change only if a bond is broken and re-formed. At a chiral center it is labeled R or S: rank the four substituents by Cahn-Ingold-Prelog priority (higher atomic number wins), point the lowest priority away from you, and trace 1 → 2 → 3. Clockwise is R, counterclockwise is S. At an alkene, Z means the higher-priority groups are on the same side and E means they are on opposite sides.

On the MCAT: You may be asked to assign R or S from a drawing, or to classify two structures as identical, enantiomers or diastereomers. Enantiomers are opposite at every stereocenter; diastereomers differ at some but not all. Reactions are tested by what they do to configuration: SN2 inverts it by backside attack, while SN1 goes through a planar carbocation and gives a mixture of both configurations.

Common confusion: Configuration is not conformation. Rotation about single bonds (staggered to eclipsed, chair flips) changes conformation only and breaks no bonds. R/S is also not the sign of optical rotation: an R compound can be (+) or (−), and the sign is found only by measurement.