E/Z isomerism

Meaning: Rotation about a C=C bond is restricted because turning it would break the π bond, so substituents are locked on one side or the other. To label the isomers, rank the two groups on each sp2 carbon by Cahn-Ingold-Prelog priority: the higher atomic number at the first point of difference wins. If the two higher-priority groups are on the same side, the alkene is Z (zusammen, together); if on opposite sides, E (entgegen, opposite). Each sp2 carbon needs two different groups.

On the MCAT: You may assign E or Z from a drawn structure, or compare isomers. E and Z isomers are diastereomers, not mirror images, so they differ in physical properties such as boiling point, melting point, and dipole moment. A Z isomer with polar groups on the same side often has a larger net dipole, and E isomers are often more stable because large groups are farther apart. Interconversion needs enough energy or light to break the π bond.

Common confusion: E/Z is not the same as cis/trans. Cis/trans describes the position of similar groups, while E/Z uses priority, so a cis alkene can be E. Rank by atomic number at the first point of difference, not by chain length.