© S. Lievens, University of California, Davis March 2012 For use in UCDavis Chemistry 8/118 Series Assigning Stereochemistry VI E and Z in Alkenes Alkenes can have multiple geometric isomers (non-superimposable, nonmirror images) If there are exactly two substituents and two hydrogens attached an alkene the isomer may be labeled as cis- or trans-. H H a trans alkene H opposite R' R vs. H R R' H a cis alkene H on same side If there are two or more substituents attached to an alkene the isomer may be labeled as E or Z o All cis- molecules are Z, but not all Z molecules are cis-; all transmolecules are E, but not all E molecules are trans-. Cis- and transare accepted when appropriate, but E and Z work for all asymmetrical alkenes. Assigning Relative Configuration: E vs. Z o For each sp2 carbon in the alkene identify the two substituents and prioritize them 1 and 2 using the Cahn-Ingold-Prelog Rules. See Assigning Stereochemistry II o If both high priority groups are on the same side of the alkene the molecule is Z. (similar to a cis) This can be remembered by Z is on the Zame Zide. o If the high priority groups are on opposite sides of the alkene the molecule is E. (similar to trans) o If the there is no difference between the groups then the molecule is symmetrical and thus E/Z is unnecessary. (e.g. monosubstituted alkenes) an E alkene 1st priority opposite 2 Y 1 1 W W Z 2 vs. 2 X1 Y X Z 2 21 1 a Z alkene 1st priority on the same side © S. Lievens, University of California, Davis March 2012 For use in UCDavis Chemistry 8/118 Series Examples: Compound A: Br assign priority Br H 2 2 O 1 1 High priority H H (3Z)-4-bromo-3-pentenal on the same side O Compound B: 2 assign priority 1 High priority 1 2 (3E)-3-methyl-3-hexene on opposite sides H Compound C: 2 assign priority Cl High priority (3E)-1-chloro-4-ethyl-5-methyl-3-hexene Cl on opposite sides 1 1 2 H Compound D: O 2H assign priority O OH High priority 2 1 OH Br Br 1 (2Z)-2-bromo-3-butenoic acid on the same side Compound E: H B 1 assign priority 1 2 H (2Z,4E)-2,4-heptadiene or cis,trans-2,4-heptadiene A A 2 H H B 22 © S. Lievens, University of California, Davis March 2012 For use in UCDavis Chemistry 8/118 Series Practice Examples: Assign E or Z to each molecule. Br OCH3 OH Cl Br Cl A CHO B COOH Br C D Br SCH3CH2NH2 CN Br F E G H CN Cl HOOC I OH OCH3 L K J CN CHO N M O P OH O Br OHC Br Cl Cl Cl R Q CBr3 Cl I Cl U T S W V 23 X © S. Lievens, University of California, Davis March 2012 For use in UCDavis Chemistry 8/118 Series Practice Examples Key: Br (Z) OCH3 OH (E) (E) Cl (Z) Br Cl A CHO B COOH (E) Br C D Br SCH3CH2NH2 (E) CN Br F E (Z) (Z) G H CN (E) (Z) (Z) Cl HOOC (E) I OH OCH3 L K J CN (E) CHO (Z) (Z) (E) N (E) M O OH O Br (E) OHC P Br (Z) Cl Cl Cl (E) (E) R Q T S CBr3 Cl (E) (E) (E) Cl U W V 24 I (Z) X