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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"

  2. C C C + The Double Bond as a Substituent allylic carbocation

  3. C C C C C C + • The Double Bond as a Substituent allylic carbocation allylic radical

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

  5. H H C C H H C H 10.1The Allyl Group

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

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

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

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

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

  11. + C C C 10.2Allylic Carbocations

  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)

  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

  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—

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

  16. CH3 + + C C H2C H2C CH CH CH3 CH3 Resonance Model CH3

  17. CH3 + + C C H2C H2C CH CH CH3 CH3 Resonance Model CH3 CH3 d+ d+ C H2C CH CH3

  18. Orbital Overlap Model d+ d+

  19. Orbital Overlap Model

  20. Orbital Overlap Model

  21. Orbital Overlap Model

  22. 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%)

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

  24. CH3 CH3 C CH Cl ClCH2 H2C C CH CH3 CH3 and give the same products because they form the same carbocation

  25. 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

  26. more positive charge on tertiary carbon;therefore more tertiary alcohol in product CH3 CH3 + + C C H2C H2C CH CH CH3 CH3

  27. 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

  28. • C C C 10.3Allylic Free Radicals

  29. • • C C C C C C Allylic free radicals are stabilized byelectron delocalization

  30. 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•

  31. 10.4Allylic Halogenation

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

  33. Allylic Halogenation • selective for replacement of allylic hydrogen • free radical mechanism • allylic radical is intermediate

  34. 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

  35. .. Cl : : H .. Hydrogen-atom abstraction step H H • C C H H C 410 kJ/mol 368 kJ/mol H

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

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

  38. H H H H Example Cyclohexene satisfies both requirements All allylichydrogens areequivalent

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

  40. 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.

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