E–Z notation
IUPAC method for double-bond stereochemistry using priority rules.
The E–Z convention is the IUPAC's preferred system for specifying the absolute stereochemistry of double bonds in organic chemistry. It builds on cis–trans isomer notation, which only indicates relative stereochemistry, and can be applied to double bonds with two, three, or four substituents. This notation is only usable when neither carbon of the double bond carries two identical atoms or substituent groups.
Using the Cahn–Ingold–Prelog (CIP) priority rules, each substituent on the double bond is ranked by priority. The positions of the two higher-priority substituents—one on each carbon—are then compared. If these two groups lie on opposite sides of the double bond (trans to each other), the configuration is called E (from the German *entgegen*, meaning "opposite"). If they are on the same side (cis to each other), the configuration is Z (from the German *zusammen*, meaning "together").
The letters E and Z are conventionally printed in italic type, enclosed in parentheses, and separated from the rest of the name by a hyphen. They are always written as full capitals, but do not count as the first letter of the name for English capitalization rules. For example, the CIP rules assign bromine a higher priority than chlorine, and chlorine a higher priority than hydrogen, which can lead to names that seem counterintuitive.
For molecules with multiple double bonds, locants are added to specify the geometry of each. For instance, the chemical name (2E,4E,6Z,8E)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)nona-2,4,6,8-tetraenoic acid indicates that the alkenes starting at positions 2, 4, and 8 are E, while the one at position 6 is Z.
When the stereochemistry of a double bond is unknown or unspecified, the prefix 'E/Z-' can be used. In graphical representations, wavy single bonds are the standard way to show unknown or unspecified stereochemistry or a mixture of isomers (similar to tetrahedral stereocenters). A crossed double-bond has sometimes been used for this purpose, but IUPAC no longer considers it an acceptable general style, though it may still be required by computer software.
- field
- Organic chemistry
- known_for
- Describing absolute stereochemistry of double bonds
- method
- Cahn–Ingold–Prelog priority rules (CIP rules)
- notation
- E (entgegen, 'opposite') and Z (zusammen, 'together')
Lore & Background
The E–Z notation is the IUPAC-preferred method for describing the absolute stereochemistry of double bonds in organic chemistry, serving as an extension of the older cis–trans isomer notation, which only describes relative stereochemistry. This system can be applied to double bonds with two, three, or four substituents, but it is only usable when neither atom of the double bond has two identical atoms or substituent groups attached. The configuration is determined by applying the Cahn–Ingold–Prelog priority rules (CIP rules) to assign a priority to each substituent on the double bond. The positions of the two higher-priority substituents on each carbon are then compared: if they lie on opposite sides of the double bond (trans to each other), the bond is assigned the configuration E (from the German *entgegen*, meaning "opposite"); if they are on the same side (cis to each other), it is assigned Z (from *zusammen*, meaning "together"). The letters E and Z are conventionally 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. For molecules with multiple double bonds, locants are included to specify the geometry of each, as in the chemical name (2E,4E,6Z,8E)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)nona-2,4,6,8-tetraenoic acid. When stereochemistry is unknown or unspecified, the prefix 'E/Z-' can indicate uncertainty, and wavy single bonds are the standard graphical representation; a crossed double-bond was used historically but is no longer considered acceptable by IUPAC for general use, though it may still be required by computer software.
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.
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