stereochemistry

Meaning: Stereochemistry is the study of molecules in three dimensions: compounds with the same connectivity can differ in how their atoms are arranged in space. Such compounds are stereoisomers. Enantiomers are non-superimposable mirror images, usually arising from a chiral center (a carbon with four different groups). Diastereomers are stereoisomers that are not mirror images, including cis/trans alkenes and molecules that differ at some but not all chiral centers. Enantiomers share melting point, boiling point and solubility in achiral conditions but rotate plane-polarized light in opposite directions by equal amounts. Diastereomers differ in physical properties and can be separated by ordinary methods.

On the MCAT: Expect to assign R or S from Cahn-Ingold-Prelog priorities, assign E or Z for alkenes, count stereoisomers (up to 2ⁿ for n chiral centers), spot meso compounds, and predict outcomes. SN2 inverts a stereocenter, SN1 gives a mixture that is often close to racemic, and enzymes typically act on one enantiomer.

Common confusion: R and S do not tell you the sign of optical rotation (+ or −); that is measured experimentally. Also, a molecule with chiral centers can still be achiral if it is meso, meaning it has an internal mirror plane.