Chapter 5. Alkenes and Alkynes II: Reactions

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

• Addition of Hydrogen

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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

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Mechanism
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Addition of Water to Alkenes

• Acid-Catalyzed Hydration
• Oxymercuration-Demercuration
• Hydroboration-Oxydation

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Acid-Catalyzed Hydration
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Mechanism
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Mechanism
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Markovnikov Addition
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Oxymercuration-Demercuration
Use mercuric acetate in THF followed by sodium
borohydride Markovnikov orientation via
mercurinium ion
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Mechanism
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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

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Hydroboration-Oxidation
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BH3 Is a Lewis Acid

• Six electrons in outer shell
• Coordinates to oxygen electron pairs in ethers

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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

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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)

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Anti-Markovnikov Addition
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Syn Addition
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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

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