Dienes

1. Dienes Chapter 8 Adapted from Turro & Breslow, Columbia University andIrene Lee, Case Western Reserve University

2. Unsaturated hydrocarbons containing: two double bonds: diene three double bonds: triene four double bonds: tetraene many double bonds: polyene

3. C Classification of Dienes • isolated diene • conjugated diene • cumulated diene

4. C Nomenclature • (2E,5E)-2,5-heptadiene • (2E,4E)-2,4-heptadiene • 3,4-heptadiene

8. If there is a tie, the double bond gets the lowest number

10. Relative Stabilitiesof Dienes

11. Heats of Hydrogenation • 1,3-pentadiene is 26 kJ/mol more stable than 1,4-pentadiene, but some of this stabilization is because it also contains a more highly substituted double bond 252 kJ/mol 226 kJ/mol

12. Heats of Hydrogenation 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol

13. Heats of Hydrogenation 126 kJ/mol 111 kJ/mol 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol

14. Heats of Hydrogenation • when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated 126 kJ/mol 111 kJ/mol

15. Heats of Hydrogenation • this extra 15 kJ/mol is known by several termsstabilization energydelocalization energyresonance energy 126 kJ/mol 111 kJ/mol

16. H2C CH2CH3 Heats of Hydrogenation Cumulated double bonds have relatively high heats of hydrogenation + CH3CH2CH3 C 2H2 H2C CH2 DH° = -295 kJ + CH3CH2CH3 H2 DH° = -125 kJ

19. Bondingin Conjugated Dienes

20. Isolated diene 1,4-pentadiene 1,3-pentadiene Conjugated diene

21. Isolated diene p bonds are independent of each other 1,3-pentadiene Conjugated diene

22. Isolated diene p bonds are independent of each other p orbitals overlap to give extended p bond encompassing four carbons Conjugated diene

23. Isolated diene less electron delocalization; less stable more electron delocalization; more stable Conjugated diene

24. H H H H H H H H H H H H Conformations of Dienes • s prefix designates conformation around single bond • s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) s-trans s-cis

25. H H H H H H H H H H H H Conformations of Dienes • s prefix designates conformation around single bond • s prefix is lower case (different from Cahn-Ingold-Prelog S which designates configuration and is upper case) s-trans s-cis

26. Conformations of Dienes • Both conformations allow electron delocalization via overlap of p orbitals to give extended p system s-trans s-cis

27. s-trans is more stable than s-cis • Interconversion of conformations requires two p bonds to be at right angles to each other and prevents conjugation 12 kJ/mol

29. 16 kJ/mol 12 kJ/mol

30. Bonding in Allenes

31. A cumulated diene is less stable than an isolated diene

32. C C C Cumulated Dienes • cumulated dienes are less stable thanisolated and conjugated dienes

33. Structure of Allene • linear arrangement of carbons • nonplanar geometry 118.4° 131 pm

34. Structure of Allene • linear arrangement of carbons • nonplanar geometry 118.4° 131 pm

35. Bonding in Allene sp sp 2 sp 2

36. Bonding in Allene

37. Bonding in Allene

38. Bonding in Allene

39. C C C Chiral Allenes • Allenes of the type shown are chiral X A Y B A πB; X πY Have a stereogenic axis

40. Stereogenic Axis • analogous to difference between: • a screw with a right-hand thread and one with a left-hand thread a right-handed helix and a left-handed helix