Chapter 12 Alkene Reactions Part 2

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Chapter 12 Alkene Reactions Part 2

• HydroborationOxidation
• Hydroboration
• Borane, BH3, is stable in ether or THF because
  they stabilize the Lewis acid by bonding as Lewis
  bases
• BH3 will add to alkenes
• Mechanism BH3 electrophile, p-bond nucleophile

Alkylborane
Filled p-orbitals
Empty p-orbital
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• Boron binds to the less hindered Carbon
  (regioselective and syn addition) due primarily
  to steric factors
• Oxidation of Alkylborane gives an Alcohol
• Oxidation Mechanism
• Result of HydroborationOxidation is
  Anti-Markovnikov H2O addition
• Example

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• Oxidations of Alkenes
• Peroxycarboxylic Acids are useful oxidation
  reagents for alkenes
• Generic
• MCPBA (metachloroperbezoic acid) is often used in
  small scale reactions
• Epoxidation is the common name for the oxidation
  of an alkene to an oxacyclopropane (epoxide)
• The reaction is syn stereospecific
• Mechanism
• More substituted alkenes react faster

MCPBA
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• Hydrolysis of Oxacyclopropanes leads to Vicinal
  Anti Dihydroxylation
• D. Osmium Tetroxide oxidation gives Vicinal syn
  Dihydroxylation
• Overall Reaction

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• Mechanism
• KMnO4 reacts identically and is used as a test
  for CC
• E. Ozonolysis
• Ozone O3 Reacts with Alkenes to give two
  carbonyl products

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• Mechanism
• Reduction step is complex
• Example

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• Radical Additions Anti-Markovnikov Products
• Radical Hydrobromination
• HBr addition to alkenes is cleanly Markovnikov
• If peroxides are present (ROOR), the addition is
  Anti-Markovnikov
• Peroxides provide entry into a Radical Addition
  Mechnism

Initiation
Propagation
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• The bromine radical attacks least substituted
  carbon, yielding the carbon radical that is most
  stabilizedthe most substituted carbon
• Only HBr works, HCl and HI reactions are
  endothermic by radicals
• Popular Initiating peroxides
• Other Anti-Markovnikov Radical Additions
• Thiols can add to alkenes by a radical mechanism
  initiated by peroxide
• Halomethanes can add to alkenes by a radical
  mechanism initiated by peroxide

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• Polymerization producing plastics, and synthetic
  materials
• A. Acid catalyzed polymerization
• Alkenes can react with themselves if catalytic H
  is present
• Cations (even carbocations) attack the p-system
• An acid is needed to initiate the polymerization
  H2SO4, HF, BF3, etc
• Repeated attack leads to polymerization

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• Controlled reactions can be stopped at dimers or
  trimers or oligomers
• Radical Polymerization is initiated by a radical
  instead of an acid

Oligomer or Polymer
Initiation
Propagation
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• Polyethylene has a branched structure and a MW of
  1,000,000
• Polyvinylchloride (PVC) is made with
  regioselective polymerization
• Radicals only add to unsubstituted end
• Radicals next to Cl are very stable, so they are
  formed preferentially
• A head-to-tail structure results
• Anionic Polymerization
• 1) Strong bases can also initiate polymerization
  of alkenes