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Chapter 5. Alkenes and Alkynes II: Reactions

Chapter 5. Alkenes and Alkynes II: Reactions. Elimination Reactions. Addition Reactions. Addition of Halogens. Examples. Mechanism. Halohydrin Formation. Mechanism. Orientation of the OH toward the more stable cabocation ( f ollows Markovnikov addition). Hydrogenation.

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Chapter 5. Alkenes and Alkynes II: Reactions

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  1. Chapter 5.Alkenes and Alkynes II: Reactions

  2. Elimination Reactions

  3. Addition Reactions

  4. Addition of Halogens

  5. Examples

  6. Mechanism

  7. Halohydrin Formation

  8. Mechanism

  9. Orientation of the OH toward the more stable cabocation (follows Markovnikov addition)

  10. Hydrogenation Addition of Hydrogen

  11. Reduction of Alkenes • Addition of H-H across C=C • Reduction in general is addition of H2 or its equivalent • Requires Pt, Pd, or Ni as powders on carbon and H2 • Hydrogen is first adsorbed on catalyst • Reaction is heterogeneous

  12. Mechanism

  13. Addition of Water to Alkenes • Acid-Catalyzed Hydration • Oxymercuration-Demercuration • Hydroboration-Oxydation

  14. Acid-Catalyzed Hydration

  15. Mechanism

  16. Mechanism

  17. Markovnikov Addition

  18. Oxymercuration-Demercuration • Use mercuric acetate in THF followed by sodium borohydride • Markovnikov orientation • via mercurinium ion

  19. Mechanism

  20. Hydroboration-Oxidation • Herbert Brown invented hydroboration (received the 1979 Nobel Chemistry Prize.) • Borane (BH3) is electron deficient is a Lewis acid • Borane adds to an alkene to give an organoborane

  21. Hydroboration-Oxidation

  22. BH3 Is a Lewis Acid • Six electrons in outer shell • Coordinates to oxygen electron pairs in ethers

  23. Hydroboration-Oxidation Forms an Alcohol from an Alkene • Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane • Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

  24. Orientation in Hydration via Hydroboration • Regiochemistry is opposite to Markovnikov orientation (Anti-Markovnikov) • OH is added to carbon with most H’s • H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

  25. Anti-Markovnikov Addition

  26. Syn Addition

  27. Mechanism of Hydroboration • Borane is a Lewis acid • Alkene is Lewis base • Transition state involves anionic development on B • The components of BH3 are across C=C

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