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)
• cyclopropane (add CH2)

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7.1 Preparation of Alkenes A Preview of
Elimination Reactions

• Dehydrohalogenation

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7.2 Addition of Halogens to Alkenes

• addition is stereoselective
• Stereoselective reaction a reaction in which one
  stereoisomer is formed or destroyed in preference
  to all others

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Mechanism

• Step 1 formation of a bridged bromonium ion
  intermediate
• Step 2 attack of halide ion from the opposite
  side of the three-membered ring

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• For a cyclohexene, anti coplanar addition
  corresponds to trans diaxial addition

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7.3 Halohydrin Formation

• The actual reagent is the dihalogen (Br2 or Cl2
  in water in an organic solvent)

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Reaction is both regiospecific (anti addition)
and stereoselective (OH to the more substituted
carbon)
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Mechanism of Formation of a Bromohydrin
Step 1
Step 2
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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
Oxymercuration/Reduction
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Addition of Hg(II) and oxygen is anti
stereoselective
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7.5 Addition of Water to Alkenes Hydroboration

• Herbert Brown (HB) invented hydroboration (HB)
• Borane (BH3) is a Lewis acid
• 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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Hydroboration, Electronic Effects give
Non-Markovnikov

• More stable carbocation is consistent with steric
  preferences

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7.6 Addition of Carbenes to Alkenes

• Carbenes are neutral with six electrons in the
  outer shell
• They symmetrically add across double bonds to
  form cyclopropanes

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Formation of Dichlorocarbene
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Reaction of Dichlorocarbene

• Addition of dichlorocarbene is stereospecific

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Simmons-Smith Reaction
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7.7 Reduction of Alkenes Hydrogenation

• Most alkenes react with H2 in the presence of a
  transition metal catalyst to give alkanes
• commonly used catalysts are Pt, Pd, Ru, and Ni
• The process is called catalytic reduction or,
  alternatively, catalytic hydrogenation

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Selectivity in Hydrogen Addition

• 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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7.8 Oxidation of Alkenes Hydroxylation and
Cleavage

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

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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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Permanganate Oxidation of Alkenes

• Oxidizing reagents other than ozone also cleave
  alkenes

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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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Cis-Trans Interconversion in Vision