$word_{count}$: 280

Stereoisomers have the same structural connectivity but differ in spatial arrangement. Geometrical $\frac{E}{Z}$ isomerism requires restricted rotation (C=C, C=N, or ring) AND different groups on each carbon of the restricted bond. CIP priority rules assign E (entgegen — higher priority groups opposite) or Z (zusammen — same side) based on atomic number at the first point of difference. trans isomers are generally more stable than cis due to reduced steric strain. Optical isomerism requires molecular chirality — typically from a carbon bonded to 4 different groups (chiral center). R/S configuration is assigned using CIP priorities and the steering wheel method: orient lowest priority away, clockwise 1-2-3 = R, counterclockwise = S. Enantiomers (non-superimposable mirror images) have identical physical properties except optical rotation direction. Diastereomers (stereoisomers that aren't mirror images) have different physical properties and can be separated by conventional methods. Meso compounds have chiral centers but an internal symmetry plane, making them optically inactive. For n chiral centers with no meso possibility, maximum stereoisomers = 2^n. When meso forms exist (identical substitution at chiral centers), subtract the redundant pairs. Chirality can also arise without a chiral carbon: axial chirality in allenes (R1R2C=C=CR3R4 when R1 ≠ R2 and R3 ≠ R4), restricted rotation biphenyls, and helical chirality in helicenes. Nitrogen is usually NOT a stereocenter due to rapid umbrella inversion except in quaternary salts or constrained rings.