Chapter 10 Conjugation in Alkadienes and Allylic Systems

1. Chapter 10Conjugation in Alkadienes andAllylic Systems • conjugare is a Latin verb meaning "to link or yoke together"

2. C C C C C C + • allylic carbocation allylic radical C C C C conjugated diene The Double Bond as a Substituent

3. H H C C H H C H 10.1The Allyl Group

4. allylic carbon vinylic carbons Vinylic versus Allylic C C C

5. Vinylic versus Allylic H C C H C H Vinylic hydrogens are attached to vinylic carbons.

6. H H C C H C Vinylic versus Allylic Allylic hydrogens are attached to allylic carbons.

7. Vinylic versus Allylic X C C X C X Vinylic substituents are attached to vinylic carbons.

8. Vinylic versus Allylic X X C C X C Allylic substituents are attached to allylic carbons.

9. + C C C 10.2Allylic Carbocations

10. CH3 Cl H2C C C CH CH3 Allylic Carbocations • The fact that a tertiary allylic halide undergoessolvolysis (SN1) faster than a simple tertiaryalkyl halide… CH3 Cl CH3 CH3 123 1 relative rates: (ethanolysis, 45°C)

11. CH3 + H2C CH CH3 Allylic Carbocations • provides good evidence that allylic carbocations • are more stable than other carbocations. CH3 + C C CH3 CH3 H2C=CH— stabilizes C+ better than does CH3—

12. Stabilization of Allylic Carbocations • Delocalization of electrons in the doublebond stabilizes the carbocation. • resonance model orbital overlap model

13. CH3 CH3 + + C C H2C H2C CH CH CH3 CH3 CH3 C + + H2C CH CH3 Resonance Model

14. Orbital Overlap Model + +

15. Orbital Overlap Model

16. Orbital Overlap Model

17. Orbital Overlap Model

18. 10.3SN1 Reactions of Allylic Halides

19. CH3 Cl H2C C CH CH3 CH3 CH3 C CH + HOCH2 OH H2C C CH CH3 CH3 (15%) (85%) Hydrolysis of an Allylic Halide H2O Na2CO3

20. CH3 C CH ClCH2 CH3 CH3 CH3 C CH + HOCH2 OH H2C C CH CH3 CH3 (15%) (85%) Corollary Experiment H2O Na2CO3

21. CH3 CH3 C CH Cl ClCH2 H2C C CH CH3 CH3 CH3 CH3 + + C C H2C H2C CH CH CH3 CH3 and Give the same products because they form the same carbocation.

22. CH3 CH3 CH3 CH3 C CH + HOCH2 OH + H2C C CH + C C H2C H2C CH CH CH3 CH3 (15%) CH3 CH3 (85%) More positive charge on tertiary carbon;therefore more tertiary alcohol in product.

23. 10.4SN2 Reactions of Allylic Halides

24. Cl H2C CH2 CH Allylic SN2 Reactions • Allylic halides also undergo SN2 reactions • faster than simple primary alkyl halides. Cl H3C CH2 CH2 1 80 relative rates: (I-, acetone)

25. Cl H2C CH2 CH Allylic SN2 reactions • Two factors: • Steric • Trigonal carbon smaller than tetrahedral carbon. Cl H3C CH2 CH2 1 80 relative rates: (I-, acetone)

26. Cl H2C CH2 CH Allylic SN2 reactions • Two factors: • Electronic • Electron delocalization lowers LUMO energy • which means lower activation energy. Cl H3C CH2 CH2 1 80 relative rates: (I-, acetone)

27. • C C C 10.5Allylic Free Radicals

28. • • C C C C C C Allylic Free Radicals are Stabilized byElectron Delocalization

29. Allylic Free Radicals are Stabilized byElectron Delocalization • Spin density is a measure of the unpaired electron distribution in a molecule. • The picture on the next slide shows the unpaired electron in allyl radical "divides its time" equally between C-1 and C-3.

30. Allylic Free Radicals are Stabilized byElectron Delocalization Spin density in allyl radical

31. CHCH2 CHCH2—H H2C H2C Free Radical Stabilities are Related toBond-dissociation Energies • C—H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane. 410 kJ/mol • + CH3CH2CH2—H CH3CH2CH2 H• • 368 kJ/mol + H•

32. 10.6Allylic Halogenation

33. ClCH2CHCH3 Cl CHCH3 H2C CHCH2Cl H2C Chlorination of Propene addition + Cl2 500 °C + HCl substitution

34. Allylic Halogenation • Selective for replacement of allylic hydrogen • Free radical mechanism • Allylic radical is intermediate

35. H H .. . Cl : C C .. H C Hydrogen-atom Abstraction Step H • Allylic C—H bond is weaker than vinylic. • Chlorine atom abstracts allylic H in propagation step. H 410 kJ/mol 368 kJ/mol H

36. .. Cl : : H .. Hydrogen-atom Abstraction Step H H • C C H H C 410 kJ/mol 368 kJ/mol H

37. Br O O heat + + NBr NH CCl4 O O (82-87%) N-Bromosuccinimide • Reagent used (instead of Br2) for allylic bromination.

38. Limited Scope Allylic halogenation is only used when: • all of the allylic hydrogens are equivalent • and • the resonance forms of allylic radicalare equivalent.

39. H H H H H H • H Example Cyclohexene satisfies both requirements. All allylichydrogens areequivalent. H H • H Both resonance forms are equivalent.

40. CH3CH CHCH3 But • • CH3CH CH CH2 CH3CH CH CH2 Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides. Example All allylichydrogens areequivalent. 2-Butene

41. CH3CH CHCH3 Example All allylichydrogens areequivalent. 2-Butene forms Br Br CH3CH CH CH2 and CH3CH CH CH2 Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.

42. - C C C 10.7Allylic Anions

43. - CH2 CH2 H2C H3C CH CH Acidity of Propene CH3 H3C CH2 pKa ~ 62 pKa ~ 43 - CH3 H2C CH2 Propene is significantly more acidic than propane.

44. - CH2 H2C CH - - CH2 H2C CH Resonance Model - CH2 H2C CH Charge is delocalized to both terminal carbons, stabilizing the conjugate base.

45. 10.8 Classes of Dienes

46. C Classification of Dienes Isolated diene Conjugated diene Cumulated diene

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

48. 10.9Relative Stabilitiesof Dienes

49. 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

50. 126 kJ/mol 111 kJ/mol Heats of Hydrogenation 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol