Understanding Allylic Systems: Conjugation & Reactivity in Alkadienes

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. Allylic Carbocations • Allylic carbocations are more stable than other carbocations. CH3 CH3 + + H2C C C CH CH3 CH3 CH3 Formed faster

11. CH3 + H2C CH CH3 Allylic Carbocations • 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 modelOrbital 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. 10.3 & 10.4Nucleophilic Substitution Reactionsof Allylic Halides SN1 (and SN2) reactions are faster for allylic halides than corresponding nonallylic halides.

19. CH3 Cl H2C C C CH CH3 Allylic Carbocations • A tertiary allylic halide undergoes solvolysis (SN1) faster than a simple tertiary alkyl halide. CH3 Cl CH3 CH3 123 1 Relative rates (ethanolysis, 45°C)

20. 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

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

22. 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.

23. (85%) (15%) CH3 CH3 + C CH OH HOCH2 H2C C CH CH3 CH3 More positive charge on tertiary carbon;therefore, more tertiary alcohol in product. CH3 CH3 + + C C H2C H2C CH CH CH3 CH3

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 Reason: steric and electronic effects.

25. • C C C 10.5Allylic Free Radicals

26. • • C C C C C C Allylic Free Radicals Are Stabilized byElectron Delocalization

27. Allylic Free Radicals Are Stabilized byElectron Delocalization • Spin density is a measure of the unpaired electron distribution in a molecule. • The unpaired electron in allyl radical "divides its time" equally between C-1 and C-3.

28. Allylic Free Radicals Are Stabilized byElectron Delocalization Spin density in allyl radical

29. 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•

30. 10.6Allylic Halogenation

31. ClCH2CHCH3 Cl CHCH3 H2C CHCH2Cl H2C Chlorination of Propene Addition + Cl2 500°C + HCl Substitution

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

33. H H .. . Cl : C C .. H C Hydrogen-Atom Abstraction Step H • Allylic C—H bond weaker thanvinylic. • Chlorineatom abstracts allylic H in propagation step. H 410 kJ/mol 368 kJ/mol H

34. .. Cl : : H .. Hydrogen-Atom Abstraction Step H H • C C H H C 410 kJ/mol 368 kJ/mol H

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

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

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

38. 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

39. 10.8 Classes of Dienes

40. C Classification of Dienes • Isolated diene • Conjugated diene • Cumulated diene

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

42. 10.9Relative Stabilitiesof Dienes

43. 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

44. 126 kJ/mol 111 kJ/mol Heats of Hydrogenation 126 kJ/mol 115 kJ/mol 252 kJ/mol 226 kJ/mol

45. 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

46. Heats of Hydrogenation • This extra 15 kJ/mol is known by several terms: • Stabilization energy • Delocalization energy • Resonance energy 126 kJ/mol 111 kJ/mol

47. H° = -295 kJ/mol H2C CH2CH3 H° = -125 kJ/mol Heats of Hydrogenation Cumulated double bonds have relatively high heats of hydrogenation. + CH3CH2CH3 C 2H2 H2C CH2 + CH3CH2CH3 H2

48. C Stabilities of Dienes • Conjugated diene = most stable • Isolated diene • Cumulated diene = least stable

49. 10.10Bondingin Conjugated Dienes

50. Isolated diene 1,4-Pentadiene 1,3-Pentadiene Conjugated diene