E–Z notation

Lore & Background

The E–Z convention assigns each substituent on a double bond a priority according to the Cahn–Ingold–Prelog rules. If the two higher-priority groups are on opposite sides of the double bond, the configuration is E (from German entgegen, meaning 'opposite'); if they are on the same side, it is Z (from German zusammen, meaning 'together'). The letters are printed in italic type, within parentheses, and separated from the rest of the name with a hyphen, always as full capitals but not constituting the first letter for English capitalization rules.

Reader's Guide

E–Z notation is significant because it provides a systematic way to describe the absolute stereochemistry of double bonds, overcoming limitations of cis–trans notation for complex substituents. The CIP rules assign priority based on atomic number, as in the example where bromine outranks chlorine, which outranks hydrogen. For molecules with multiple double bonds, locants specify the geometry of each, such as in alitretinoin: (2E,4E,6Z,8E)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)nona-2,4,6,8-tetraenoic acid. Uncertainty in E or Z isomers is indicated by the prefix 'E/Z-', and wavy single bonds represent unknown or unspecified stereochemistry; a crossed double-bond is no longer considered acceptable by IUPAC but may still be required by computer software.

Did You Know?

• E–Z notation is the IUPAC's preferred method for describing absolute stereochemistry of double bonds.
• The letters E and Z come from German words entgegen ('opposite') and zusammen ('together').
• E–Z notation can only be used when neither atom of the double bond has two identical substituents.
• For multiple double bonds, locants specify the geometry of each, as in (2E,4E,6Z,8E)-alitretinoin.