Stereoisomerism Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms. Alkenes can exhibit a type of isomerism called E-Z stereoisomerism E-Z isomers exist due to restricted rotation about the C=C bond Single carbon-carbon covalent bonds can easily rotate E-Z stereoisomers arise when: (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of the double bond. C C H H C H C H H H H H C C H C H C H H H H H H But-1-ene two identical groups attached to one end of the restricted double bond – no E-Z isomers two different groups attached either end of the restricted double bond- leads to EZ isomers C H H C C H H C H H H H Z- but-2-ene E -but-2-ene These are two isomers as the lack of rotation around the double bonds means one cannot be switched to the other. Naming E-Z stereoisomers Priority Group: The atom with the bigger atomic number is classed as the priority atom Priority group side 1 Priority group side 2 If the priority atom is on the same side of the double bond it is labelled Z from the german zusammen (The Zame Zide!) If the priority atom is on the opposite side of the double bond it is labelled E from the german entgegen (The Epposite side!) Z-1,2-dichloroethene E-1,2-dichloroethene But-1-ene is a structural isomer of But-2- ene but does not show E-Z isomerism. 8 First determine the priority groups on both sides of the double bond Cl C C Cl H H Cl C C H H Cl N Goalby chemrevise.org 1. Compare the atomic number of the atoms directly attached to each side of the double bond; the group having the atom of higher atomic number receives higher priority. Cl C C Br H Cl priority priority 2. If there is a tie, consider the atoms at distance 2 from the double bond. Make a list for each group of the atoms bonded to the one directly attached to the double bond. Arranged list in order of decreasing atomic number. Compare the lists atom by atom; at the earliest difference, the group containing the atom of higher atomic number receives higher priority H C3
E–Z isomerism is a form of stereoisomerism and occurs as a result of restricted rotation about the planar carbon– carbon double bond.
Cahn–Ingold–Prelog (CIP) priority rules.
Students should be able to:
• define the term stereoisomer
• draw the structural formulas of E and Z isomers
• apply the CIP priority rules to E and Z isomers.