The Three-Dimensional Structure of Molecules - Isomerism
(E/Z) Isomerism of Alkenes
In compounds containing a double bond, the free rotation around the bond axis is prevented. Therefore, in compounds with carbon-carbon double bonds (alkenes) for example, so-called (E/Z) isomers exist. The isomers differ in the positions of the substituents on the double-bonded atoms.
In the past these alkene isomers have been named with the prefixes cis and trans. The prefix cis is used if substituents of the same kind are on the same side of the molecule, while the prefix trans is used if they are on opposite sides. Cis/trans isomers or (E/Z) isomers are diastereomers, because they are stereoisomers of the same constitution, that is, the connections between every atom are the same, and the molecules do not have a mirror-image relationship. Thus, they have different physical and chemical properties.
Today the CIP (Cahn-Ingold-Prelog) nomenclature (E/Z nomenclature) is preferred, because it is more generally applicable and it is unequivocal in case of different substituents. According to the CIP nomenclature, substituents on the double bond are ordered by the CIP priority rules. An isomer is called a (Z) isomer (Z: zusammen = together) if the substituents with higher priority are on the same side of the double bond. If they are on opposite sides the isomer is called an (E) isomer (E: entgegen = apart).
The priority order of 1-bromo-1,2-dichloroethene, for example, is bromine > chlorine on the C1 atom and chlorine > hydrogen on the C2 atom.