The more complex stereochemistry of highly substituted alkenes (especially 3 or 4 substituents) cannot be defined using the cis-trans notation. For this purpose a new notation known as E-Z notation is used. Following a set of defined rules (Cahn-Ingold-Prelog priority rule), each substituent on a double bond is assigned a priority.
If the two groups of higher priority are on opposite sides of the double bond, the bond is assigned the configuration E, (from entgegen, the German word for "opposite").
If the two groups of higher priority are on the same side of the double bond, the bond is assigned the configuration Z, (from zusammen, the German word for "together").
The difference between E and Z isomers
The above diagram cleanly indicates where the constituent groups would be for the E or Z isomers, however it is not a good example of molecules that would be officially called E or Z. When each carbon in a double bond contains a hydrogen and another constituent then the cis/trans notation should be used. E/Z notation is for more complex alkenes with constituents that are not hydrogen (e.g. an alkane and a halogen) attached to each of the carbon atoms.
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