Chapter 5. Alkenes and Alkynes II: Reactions - PowerPoint PPT Presentation

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Chapter 5. Alkenes and Alkynes II: Reactions

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... 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) ... – PowerPoint PPT presentation

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Title: Chapter 5. Alkenes and Alkynes II: Reactions


1
Chapter 5.Alkenes and Alkynes II Reactions
2
Elimination Reactions
3
Addition Reactions
4
Addition of Halogens
5
Examples
6
Mechanism
7
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8
Halohydrin Formation
9
Mechanism
10
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11
Orientation of the OH toward the more stable
cabocation (follows Markovnikov addition)
12
Hydrogenation
  • Addition of Hydrogen

13
Reduction of Alkenes
  • Addition of H-H across CC
  • 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

14
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15
Mechanism
16
Addition of Water to Alkenes
  • Acid-Catalyzed Hydration
  • Oxymercuration-Demercuration
  • Hydroboration-Oxydation

17
Acid-Catalyzed Hydration
18
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19
Mechanism
20
Mechanism
21
Markovnikov Addition
22
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23
Oxymercuration-Demercuration
Use mercuric acetate in THF followed by sodium
borohydride Markovnikov orientation via
mercurinium ion
24
Mechanism
25
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

26
Hydroboration-Oxidation
27
BH3 Is a Lewis Acid
  • Six electrons in outer shell
  • Coordinates to oxygen electron pairs in ethers

28

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

29
Orientation in Hydration via Hydroboration
  • Regiochemistry is opposite to Markovnikov
    orientation (Anti-Markovnikov)
  • OH is added to carbon with most Hs
  • H and OH add with syn stereochemistry, to the
    same face of the alkene (opposite of anti
    addition)

30
Anti-Markovnikov Addition
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 CC

33
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34
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