INTOCHEMISTRY / STEREOCHEMISTRY
Assign E/Z Alkene Stereochemistry
Use substituent priorities on each alkene carbon.
Recognize when E/Z is defined and assign it using CIP priorities on both alkene carbons.
1When E/Z applies
Each atom of the alkene double bond must have two different substituents for an E/Z descriptor to be possible.
2Prioritize each side independently
On each double-bond carbon, determine which substituent has higher CIP priority. Never directly compare groups attached to opposite alkene carbons.
3E: opposite
If the higher-priority substituents are on opposite sides, the configuration is E (entgegen).
4Z: together
If the higher-priority substituents are on the same side, the configuration is Z (zusammen).
5Worked examples
(E)-but-2-ene has methyl groups on opposite sides of the double bond; (Z)-but-2-ene has them on the same side.
6Common error
But-1-ene has two H substituents on its terminal alkene carbon, so it has no E/Z designation.
Concept check
Can but-1-ene receive an E/Z descriptor?
Reveal explanation
No. One double-bond carbon is attached to two identical hydrogens.