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Understanding Allylic Conjugation in Alkadienes

Explore the concept of allylic conjugation in alkadienes and allylic systems, including allylic carbocations, radicals, and their stability. Learn about allylic halogenation and the selective replacement of allylic hydrogen. Discover how resonance models and orbital overlap shape allylic reactions.

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Understanding Allylic Conjugation in Alkadienes

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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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