8.4 Alkene Isomerism For the pi bond to remain intact, rotation around a double bond is prohibited As a result, cis and trans structures are not identical Is there a difference between cis-butane and trans- butane? What specific type of isomers are cis and trans butene? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-1 Klein, Organic Chemistry 2e

8.4 Alkene Isomerism In cyclic alkenes with less than 8 atoms in the ring, only cis alkenes are stable. WHY? Draw the structure for trans-cyclooctene When applied to bicycloakenes, this rule is called Bredt’s rule Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-2 Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Apply Bredt’s rule to the compounds below The carbons of the C=C double bond and the atoms that are directly attached to them must be planar to maintain the pi bond overlap. A handheld model can be used to help visualize the p orbital overlap and resulting geometry Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-3 Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Cis and trans modifiers are strictly used to describe C=C double bonds with identical groups on each carbon. Where are the identical groups in trans-2-pentene? For molecules with different groups attached to the C=C double bond, the E/Z notation is used instead of cis/trans notation Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-4 Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Assigning E or Z to a stereoisomers 1.prioritize the groups attached to the C=C double bond based on atomic number Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-5 Klein, Organic Chemistry 2e

8.4 Alkene Isomerism Assigning E or Z to a stereoisomers 1.prioritize the groups attached to the C=C double bond based on atomic number 2.If the top priority groups are on the same side of the C=C double bond, it is Z (for zussamen, which means together) 3.If the top priority groups are on opposite sides of the C=C double bond, it is E (for entgegen, which means opposite) Practice with SkillBuilder 8.2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-6 Klein, Organic Chemistry 2e

8.5 Alkene Stability Because of steric strain, cis isomers are generally less stable than trans Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-7 Klein, Organic Chemistry 2e

8.5 Alkene Stability The difference in stability can be quantified by comparing the heats of combustion How does heat of combustion relate to stability? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-8 Klein, Organic Chemistry 2e

8.5 Alkene Stability Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 8-9 Klein, Organic Chemistry 2e

8.5 Alkene Stability Consider the following stability trend What pattern do you see? Practice with SkillBuilder 8.3 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

8.5 Alkene Stability List the following molecules in order of increasing heat of combustion 2,3,4-trimethyl-1,3-pentadiene 2-isopropyl-1,4-pentadiene 3,3-dimethyl-1,5-hexadiene 4,5-dimethylcyclohexene Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e