E–Z notation

E–Z notation specifies the geometry of an alkene by ranking substituents on each double-bonded carbon and stating whether the higher-priority groups lie opposite or on the same side.

Cahn–Ingold–Prelog priority rules: A system for ranking atoms or groups attached to a stereogenic unit by atomic number and subsequent tie-breaking rules. These rules determine which substituent on each alkene carbon controls its E or Z assignment.
Atomic number: The number of protons in an atom’s nucleus, which identifies the element. When directly attached atoms differ, the higher atomic number gives the higher substituent priority.
IUPAC nomenclature: The systematic rules for naming chemical substances established by the International Union of Pure and Applied Chemistry. E or Z is placed in a systematic name to specify an alkene’s configuration.
Cis–trans nomenclature: A naming convention that describes groups as being on the same or opposite sides of a reference structure. Cis–trans names can be ambiguous for alkenes with four different substituents, where E–Z remains precise.
Cis-2-butene: The 2-butene stereoisomer with both methyl groups on the same side of the double bond. Equal substituent priorities make this familiar cis form the Z isomer.
Alkene: A hydrocarbon containing at least one carbon–carbon double bond. E–Z notation applies to the geometry around an alkene double bond.
Constitutional isomerism: Isomerism in which compounds have the same molecular formula but different atom connectivity. E and Z distinguish spatial arrangements without changing the alkene’s connectivity.
Retinal: A vitamin A aldehyde that serves as the light-sensitive chromophore in visual pigments. Light converts retinal between configurations at a double bond, initiating visual signaling.
R–S notation: A system for assigning absolute configuration at stereogenic centers using Cahn–Ingold–Prelog priorities. R–S uses the same priority framework but describes tetrahedral centers rather than alkene geometry.
Trans-2-butene: The 2-butene stereoisomer with its two methyl groups on opposite sides of the double bond. Its opposite-side methyl groups give the E configuration.

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