stereoisomers

Meaning: Stereoisomers share the same molecular formula and bonding sequence (unlike constitutional isomers) but differ in the spatial arrangement of atoms. There are two kinds. Enantiomers are non-superimposable mirror images; every stereocenter is inverted, so R becomes S at each one. Diastereomers are stereoisomers that are not mirror images, such as cis/trans (E/Z) alkene pairs and molecules with several stereocenters where only some are inverted. Epimers are diastereomers that differ at exactly one stereocenter.

Equation: Maximum number of stereoisomers = 2ⁿ, where n is the number of stereocenters (a pure count, no unit). The actual number is lower when meso compounds are present.

On the MCAT: You compare two drawn structures, assign R/S at each stereocenter, and classify the pair: all centers opposite means enantiomers, some opposite means diastereomers, none opposite means identical. Enantiomers have identical melting points, boiling points and solubilities in achiral solvents, but they rotate plane-polarized light by equal amounts in opposite directions. Diastereomers differ in physical properties, so they can be separated by crystallization or distillation.

Common confusion: Different R/S labels alone do not make enantiomers; every stereocenter must be opposite. The sign of rotation (+ or −) is measured experimentally and cannot be read from R or S.