7. Alkenes: Reactions and Synthesis

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7. Alkenes Reactions and Synthesis

• Based on McMurrys Organic Chemistry, 6th edition

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Diverse Reactions of Alkenes

• Alkenes react with many electrophiles to give
  useful products by addition (often through
  special reagents)
• alcohols (add H-OH)
• alkanes (add H-H)
• halohydrins (add HO-X)
• dihalides (add X-X)
• halides (add H-X)
• diols (add HO-OH)
• cyclopropanes (add CH2)

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7.1 Preparation of Alkenes A Preview of
Elimination Reactions

• Alkenes are commonly made by
• elimination of HX from alkyl halide
  (dehydrohalogenation)
• Uses heat and KOH
• elimination of H-OH from an alcohol (dehydration)
  
• require strong acids (sulfuric acid, 50 ºC)

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7.2 Addition of Halogens to Alkenes

• Bromine and chlorine add to alkenes to give
  1,2-dihaldes, an industrially important process
• F2 is too reactive and I2 does not add
• Cl2 reacts as Cl Cl-
• Br2 is similar

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Addition of Br2 to Cyclopentene

• Addition is exclusively trans

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Mechanism of Bromine Addition

• Br adds to an alkene producing a cyclic ion
• Bromonium ion, bromine shares charge with carbon
• Gives trans addition

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The Reality of Bromonium Ions

• Bromonium were postulated more than 60 years ago
  to expain the stereochemical course of the
  addition (to give the trans-dibromide from a
  cyclic alkene
• Olah showed that bromonium ions are stable in
  liquid SO2 with SbF5 and can be studied directly

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7.3 Halohydrin Formation

• This is formally the addition of HO-X to an
  alkene (with OH as the electrophile) to give a
  1,2-halo alcohol, called a halohydrin
• The actual reagent is the dihalogen (Br2 or Cl2
  in water or in an organic solvent)

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Mechanism of Formation of a Bromohydrin

• Br2 forms bromonium ion, then water adds
• Orientation toward stable C species
• Aromatic rings do not react

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An Alternative to Bromine

• Bromine is a difficult reagent to use for this
  reaction
• N-Bromosuccinimide (NBS) produces bromine in
  organic solvents and is a safer source

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7.4 Addition of Water to Alkenes

• Hydration of an alkene is the addition of H-OH
  to to give an alcohol
• Acid catalysts are used in high temperature
  industrial processes ethylene is converted to
  ethanol

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Oxymercuration Intermediates

• For laboratory-scale hydration of an alkene
• Use mercuric acetate in THF followed by sodium
  borohydride
• Markovnikov orientation
• via mercurinium ion

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7.5 Addition of Water to Alkenes Hydroboration

• Herbert Brown invented hydroboration (HB)
• Borane (BH3) is electron deficient is a Lewis
  acid
• First step Borane adds to an alkene to give an
  organoborane

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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
• 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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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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Hydroboration Orientation in Addition Step

• Addition in least crowded orientation, syn
• Addition also is via most stable carbocation

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Hydroboration - Oxygen Insertion Step

• H2O2, OH- inserts OH in place of B
• Retains syn orientation

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7.6 Addition of Carbenes to Alkenes

• The carbene functional group is half of an
  alkene
• Carbenes are electrically neutral with six
  electrons in the outer shell
• They symmetrically across double bonds to form
  cyclopropanes

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Formation of Dichlorocarbene

• Base removes proton from chloroform
• Stabilized carbanion remains
• Unimolecular Elimination of Cl- gives electron
  deficient species, dichlorocarbene

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Simmons-Smith Reaction

• Equivalent of addition of CH2
• Reaction of diiodomethane with zinc-copper alloy
  produces a carbene
• Forms cyclopropanes by cycloaddition

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Reaction of Dichlorocarbene

• Addition of dichlorocarbene is stereospecific cis

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7.7 Reduction of Alkenes Hydrogenation

• Addition of H-H across CC
• Reduction in general is addition of H2 or its
  equivalent
• Requires Pt or Pd as powders on carbon and H2
• Hydrogen is first adsorbed on catalyst
• Reaction is heterogeneous (process is not in
  solution)

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Hydrogen Addition- Selectivity

• Selective for CC. No reaction with CO, CN
• Polyunsaturated liquid oils become solids
• If one side is blocked, hydrogen adds to other

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Mechanism of Catalytic Hydrogenation

• Heterogeneous reaction between phases
• Addition of H-H is syn

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7.8 Oxidation of Alkenes Hydroxylation and
Cleavage

• Hydroxylation adds OH to each end of CC
• Stereochemistry of addition is syn
• Product is a 1,2-dialcohol or diol (also called a
  glycol)

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Osmium Tetroxide Catalyzed Formation of Diols

• Hydroxylation - converts to syn-diol
• Osmium tetroxide, then sodium bisulfate
• Via cyclic osmate di-ester

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Alkene Cleavage Ozone

• Ozone, O3, adds to alkenes to form molozonide
• Reduce molozonide to obtain ketones and/or
  aldehydes

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Examples of Ozonolysis of Alkenes

• Used in determination of structure of an unknown
  alkene

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Structure Elucidation With Ozone

• Cleavage products reveal an alkenes structure

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Permangante Oxidation of Alkenes

• Oxidizing reagents other than ozone also cleave
  alkenes
• Potassium permanganate (KMnO4) can produce
  carboxylic acids and carbon dioxide if Hs are
  present on CC

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Cleavage of 1,2-diols

• Reaction of a 1,2-diol with periodic (per-iodic)
  acid, HIO4 , cleaves the diol into two carbonyl
  compounds
• Sequence of diol formation with OsO4 followed by
  diol cleavage is a good alternative to ozonolysis

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Mechanism of Periodic Acid Oxidation

• Via cyclic periodate intermediate

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Permanganate Oxidation Revisited

• Via cyclic intermediate, syn mechanism
• Diol can be isolated if oxidation proceeds in
  cold, aqu. media (often basic conditions), but
  yields often not good
• Carboxylic acids form under more drastic (hot,
  often acidic) conditions

• Overall
• Alkene hot aqu. permanganate produces
  carboxylic acids
• Alkene cold aqu. permanganate can produce
  diols, but over-oxidation tends to be a problem.
  Osmium tetroxide is much cleaner for this purpose!

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7.10 Addition of Radicals to Alkenes Polymers

• A polymer is a very large molecule consisting of
  repeating units of simpler molecules, formed by
  polymerization
• Alkenes react with radical catalysts to undergo
  radical polymerization
• Ethylene is polymerized to poyethylene, for
  example

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Free Radical Polymerization of Alkenes

• Alkenes combine many times to give polymer
• Reactivity induced by formation of free radicals

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Free Radical Polymerization Initiation

• Initiation - a few radicals are generated by the
  reaction of a molecule that readily forms
  radicals from a nonradical molecule
• A bond is broken homolytically

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Polymerization Propagation

• Radical from intiation adds to alkene to generate
  alkene derived radical
• This radical adds to another alkene, and so on
  many times

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Polymerization Termination

• Chain propagation ends when two radical chains
  combine
• Not controlled specifically but affected by
  reactivity and concentration

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Other Polymers

• Other alkenes give other common polymers

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