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Chapter 10 Conjugation in Alkadienes and Allylic Systems

Chapter 10 Conjugation in Alkadienes and Allylic Systems. conjugare is a Latin verb meaning "to link or yoke together". C. C. C. +. The Double Bond as a Substituent. allylic carbocation. C. C. C. C. C. C. +. •. The Double Bond as a Substituent. allylic carbocation.

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Chapter 10 Conjugation in Alkadienes and Allylic Systems

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  1. Chapter 10Conjugation in Alkadienes andAllylic Systems conjugare is a Latin verb meaning "to link or yoke together" Dr. Wolf's CHM 201 & 202

  2. C C C + The Double Bond as a Substituent allylic carbocation Dr. Wolf's CHM 201 & 202

  3. C C C C C C + • The Double Bond as a Substituent allylic carbocation allylic radical Dr. Wolf's CHM 201 & 202

  4. C C C C C C + • C C C C The Double Bond as a Substituent allylic carbocation allylic radical conjugated diene Dr. Wolf's CHM 201 & 202

  5. H H C C H H C H The Allyl Group Dr. Wolf's CHM 201 & 202

  6. Vinylic versus Allylic C C C allylic carbon vinylic carbons Dr. Wolf's CHM 201 & 202

  7. Vinylic versus Allylic H C C H C H vinylic hydrogens are attached to vinylic carbons Dr. Wolf's CHM 201 & 202

  8. H H C C H C Vinylic versus Allylic allylic hydrogens are attached to allylic carbons Dr. Wolf's CHM 201 & 202

  9. Vinylic versus Allylic X C C X C X vinylic substituents are attached to vinylic carbons Dr. Wolf's CHM 201 & 202

  10. Vinylic versus Allylic X X C C X C allylic substituents are attached to allylic carbons Dr. Wolf's CHM 201 & 202

  11. + C C C AllylicCarbocations Dr. Wolf's CHM 201 & 202

  12. 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) Dr. Wolf's CHM 201 & 202

  13. Allylic Carbocations provides good evidence for the conclusion thatallylic carbocations are more stable thanother carbocations CH3 CH3 + + H2C C C CH CH3 CH3 CH3 formed faster Dr. Wolf's CHM 201 & 202

  14. CH3 + H2C CH CH3 Allylic Carbocations provides good evidence for the conclusion thatallylic carbocations are more stable thanother carbocations CH3 + C C CH3 CH3 H2C=CH— stabilizes C+ better than CH3— Dr. Wolf's CHM 201 & 202

  15. Stabilization of Allylic Carbocations Delocalization of electrons in the doublebond stabilizes the carbocation resonance modelorbital overlap model Dr. Wolf's CHM 201 & 202

  16. CH3 + + C C H2C H2C CH CH CH3 CH3 Resonance Model CH3 Dr. Wolf's CHM 201 & 202

  17. CH3 + + C C H2C H2C CH CH CH3 CH3 Resonance Model CH3 CH3 d+ d+ C H2C CH CH3 Dr. Wolf's CHM 201 & 202

  18. Orbital Overlap Model + + Dr. Wolf's CHM 201 & 202

  19. Orbital Overlap Model Dr. Wolf's CHM 201 & 202

  20. Orbital Overlap Model Dr. Wolf's CHM 201 & 202

  21. Orbital Overlap Model Dr. Wolf's CHM 201 & 202

  22. SN1 Reactions of Allylic Halides Dr. Wolf's CHM 201 & 202

  23. CH3 Cl H2C C CH CH3 CH3 OH H2C C CH CH3 Hydrolysis of an Allylic Halide H2O Na2CO3 CH3 C CH + HOCH2 CH3 (15%) (85%) Dr. Wolf's CHM 201 & 202

  24. CH3 C CH ClCH2 CH3 CH3 CH3 C CH HOCH2 OH H2C C CH CH3 CH3 Corollary Experiment H2O Na2CO3 + (15%) (85%) Dr. Wolf's CHM 201 & 202

  25. CH3 CH3 C CH Cl ClCH2 H2C C CH CH3 CH3 and give the same products because they form the same carbocation Dr. Wolf's CHM 201 & 202

  26. CH3 CH3 C CH Cl ClCH2 H2C C CH CH3 CH3 and give the same products because they form the same carbocation CH3 CH3 + + C C H2C H2C CH CH CH3 CH3 Dr. Wolf's CHM 201 & 202

  27. more positive charge on tertiary carbon;therefore more tertiary alcohol in product CH3 CH3 + + C C H2C H2C CH CH CH3 CH3 Dr. Wolf's CHM 201 & 202

  28. H2C C CH (85%) (15%) CH3 CH3 + C CH OH HOCH2 CH3 CH3 more positive charge on tertiary carbon;therefore more tertiary alcohol in product CH3 CH3 + + C C H2C H2C CH CH CH3 CH3 Dr. Wolf's CHM 201 & 202

  29. SN2 Reactions of Allylic Halides Dr. Wolf's CHM 201 & 202

  30. 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) Dr. Wolf's CHM 201 & 202

  31. 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) Dr. Wolf's CHM 201 & 202

  32. 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) Dr. Wolf's CHM 201 & 202

  33. • C C C Allylic Free Radicals Dr. Wolf's CHM 201 & 202

  34. • • C C C C C C Allylic free radicals are stabilized byelectron delocalization Dr. Wolf's CHM 201 & 202

  35. 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• Dr. Wolf's CHM 201 & 202

  36. AllylicHalogenation Dr. Wolf's CHM 201 & 202

  37. ClCH2CHCH3 Cl CHCH3 H2C CHCH2Cl H2C Chlorination of Propene addition + Cl2 500 °C + HCl substitution Dr. Wolf's CHM 201 & 202

  38. Allylic Halogenation selective for replacement of allylic hydrogen free radical mechanism allylic radical is intermediate Dr. Wolf's CHM 201 & 202

  39. H H .. . Cl : C C .. H C Hydrogen-atom abstraction step H allylic C—H bond weaker than vinylic chlorine atom abstracts allylic H in propagation step H 410 kJ/mol 368 kJ/mol H Dr. Wolf's CHM 201 & 202

  40. .. Cl : : H .. Hydrogen-atom abstraction step H H • C C H H C 410 kJ/mol 368 kJ/mol H Dr. Wolf's CHM 201 & 202

  41. O O NBr NH O O N-Bromosuccinimide reagent used (instead of Br2) for allylic bromination Br heat + + CCl4 (82-87%) Dr. Wolf's CHM 201 & 202

  42. Limited Scope Allylic halogenation is only used when: all of the allylic hydrogens are equivalent andthe resonance forms of allylic radicalare equivalent Dr. Wolf's CHM 201 & 202

  43. H H H H Example Cyclohexene satisfies both requirements All allylichydrogens areequivalent Dr. Wolf's CHM 201 & 202

  44. H H H H H H H H • • H H Example Cyclohexene satisfies both requirements All allylichydrogens areequivalent Both resonance forms are equivalent Dr. Wolf's CHM 201 & 202

  45. CH3CH CHCH3 • • CH3CH CH CH2 CH3CH CH CH2 Example All allylichydrogens areequivalent 2-Butene But Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides. Dr. Wolf's CHM 201 & 202

  46. Allylic Anions Dr. Wolf's CHM 201 & 202

  47. CH3 - - C C H2C H2C CH CH CH3 CH3 Allylic anions are stabilized byelectron delocalization CH3 Dr. Wolf's CHM 201 & 202

  48. - 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. Dr. Wolf's CHM 201 & 202

  49. - CH2 H2C CH - - CH2 H2C CH Resonance Model - CH2 H2C CH Charge is delocalized to both terminal carbons, stabilizing the conjugate base. Dr. Wolf's CHM 201 & 202

  50. Classes of Dienes Dr. Wolf's CHM 201 & 202

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