Atkins Chapter 5.ppt
- 1. Chapter 5. Alkenes and Alkynes II: Reactions
- 4. Addition of Halogens C C + X2 C C X X
- 5. Examples CH3CH2CH CH2 Br2 CH3CH2CHCH2Br Br CH3CH CHCH2CH3 Cl2 CH3CHCHCH2CH3 Cl Cl
- 6. Mechanism
- 8. Halohydrin Formation C C + X2 C C X OH H2O CH3CH CHCH3 Br2 H2O CH3CHCHCH3 Br OH
- 9. Mechanism CH3CH CHCH3 CH3CHCHCH3 Br OH Br Br CH3CH CHCH3 Br OH2 CH3CHCHCH3 Br OH H
- 11. Orientation of the OH toward the more stable cabocation (follows Markovnikov addition)
- 13. • 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 Reduction of Alkenes C C + H2 C C H H M M = Pt, Pd, or Ni
- 14. C C CH3 H CH3 H H2 Pt CH3CH2CH2CH3 C C H CH3 CH3 H H2 Ni CH3CH2CH2CH3 H2 Ni CH3CH2CH2CH3 CH3CH2CH CH2
- 15. Mechanism
- 16. Addition of Water to Alkenes • Acid-Catalyzed Hydration • Oxymercuration-Demercuration • Hydroboration-Oxydation
- 17. Acid-Catalyzed Hydration C C + H2O C C H OH H
- 20. Mechanism
- 21. Markovnikov Addition CH3CH CH2 H H2O CH3CHCH3 OH
- 23. Oxymercuration-Demercuration Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation via mercurinium ion
- 24. Mechanism
- 25. Hydroboration-Oxidation C C 1. BH3 . THF 2. H2O2 C C H OH •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
- 27. BH3 Is a Lewis Acid • Six electrons in outer shell • Coordinates to oxygen electron pairs in ethers
- 28. • 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 Hydroboration-Oxidation Forms an Alcohol from an Alkene
- 29. 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)
- 30. Anti-Markovnikov Addition C H3C H3C CH2 1. BH3 . THF 2. H2O2 CH3 CH CH2 CH3 OH
- 31. Syn Addition
- 32. 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