Chapter 7. Alkenes: Reactions and Synthesis

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Chapter 7. Alkenes Reactions and Synthesis
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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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Reactions of Alkenes
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Preparation of Alkenes
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Part 1 - Synthesis of Alkenes

• These reactions are used to produce alkenes.

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Synthesis of Alkenes Synthesis 1

• Alkenes are commonly made by elimination of HX
  from alkyl halide (dehydrohalogenation)
• Uses heat and KOH

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Synthesis of Alkenes Synthesis 2

• elimination of H-OH from an alcohol (dehydration)
  
• require strong acids (sulfuric acid, 50 ºC)

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Part 2 - Reaction of Alkenes

• These reactions react alkenes to form a series of
  alkane products.

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Addition of Halogens to Alkenes

• Bromine and chlorine add to alkenes to give
  1,2-dihaldes
• F2 is too reactive and I2 does not add.

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Example Mechanism of Bromine Addition

• Electrophilic addition of bromine to give a
  cation is followed by cyclization to give a
  bromonium ion.
• This bromoniun ion is a reactive electrophile and
  bromide ion is a good nucleophile.
• Gives trans addition.

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Example Addition of Br2 to Cyclopentene

• Addition is exclusively trans

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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 in an organic solvent)

Alkene
Halohydrin
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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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Addition of Water to Alkenes Oxymercuration

• 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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Addition of Water to Alkenes Oxymercuration
Hg(OAc)2 is used as an electrophillic sink. The
double bond is then attacked by the water
creating an alchohol. This is then REDUCED by
NaBH4 that adds an H to the molecule.
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Addition of Water to Alkenes Hydroboration

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

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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 Alcohol Formation from
Alkenes

• 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, Electronic Effects Give
Non-Markovnikov

• More stable carbocation is also consistent with
  steric preferences

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Hydroboration - Oxygen Insertion Step

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

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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 carbenoid species
• Forms cyclopropanes by cycloaddition

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Reaction of Dichlorocarbene

• Addition of dichlorocarbene is stereospecific cis

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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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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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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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Section 3 Breakdown of Alkenes

• These Reactions are used to breakdown alkenes
  into two products.

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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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Permanganate 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
  compinds
• 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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Biological Alkene Addition Reactions

• Living organisms convert organic molecules using
  enzymes as catalysts
• Many reactions are similar to organic chemistry
  conversions, except they occur in neutral water
• Usually much specific for reactant and
  stereochemistry

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Biological Hydration Example

• Fumarate to malate catalyzed by fumarase
• Specific for trans isomer
• Addition of H, OH is anti

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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 non-radical 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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Cationic Polymerization

• Vinyl monomers react with Brønsted or Lewis acid
  to produce a reactive carbocation that adds to
  alkenes and propagates via lengthening
  carbocations

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Take Home Message

• Learn the REACTIONS (ALL OF THEM)

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Synthesis of Alkenes
1) dehydrohalogenation
2) dehydration
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Part 2 - Reaction of Alkenes
1) Addition of Halogens to Alkenes
2) Halohydrin Formation
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Part 2 - Reaction of Alkenes
3) Addition of Water to Alkenes
4) Hydroboration-Oxidation Alcohol Formation
5) Carbene Formation Cyclopropane synthesis
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Part 2 - Reaction of Alkenes
6) Catalytic Hydrogenation
7) Hydroxylation and Cleavage
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Part 3 - Breakdown of Alkenes
1) Ozonolysis
2) Permangante Oxidation
3) Periodic Acid Oxidation, Cleavage of 1,2-diols