Chpt' 7' Alkenes: Reactions and Synthesis

1
Chpt. 7. Alkenes Reactions and Synthesis
2
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)

3
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)

4
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

5
Addition of Br2 to Cyclopentene

• Addition is exclusively trans

6
Mechanism of Bromine Addition

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

7
Bromonium Ion Mechanism

• 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

8
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

9
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 in an organic solvent)

10
Mechanism of Formation of a Bromohydrin

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

11
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

12
7.4 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

13
Oxymercuration Intermediates

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

14
7.5 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

15
BH3 Is a Lewis Acid

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

16

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

17
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)

18
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

19
Hydroboration Orientation in Addition Step

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

20
Hydroboration, Electronic Effects Give
Non-Markovnikov

• More stable carbocation is also consistent with
  steric preferences

21
Hydroboration - Oxygen Insertion Step

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

22
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

23
Formation of Dichlorocarbene

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

24
Simmons-Smith Reaction

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

25
Reaction of Dichlorocarbene

• Addition of dichlorocarbene is stereospecific cis

26
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)

27
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

28
Mechanism of Catalytic Hydrogenation

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

29
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)

30
Osmium Tetroxide Catalyzed Formation of Diols

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

31
Alkene Cleavage Ozone

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

32
Examples of Ozonolysis of Alkenes

• Used in determination of structure of an unknown
  alkene

33
Structure Elucidation With Ozone

• Cleavage products reveal an alkenes structure

34
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

35
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

36
Mechanism of Periodic Acid Oxidation

• Via cyclic periodate intermediate

37
7.9 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

38
Biological Hydration Example

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

39
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

40
Free Radical Polymerization of Alkenes

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

41
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

42
Polymerization Propagation

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

43
Polymerization Termination

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

44
Other Polymers

• Other alkenes give other common polymers

45
Unsymmetrical Monomers

• If alkene is unsymmetrical, reaction is via more
  highly substituted radical

46
Chain Branching During Polymerization

• During radical propagation chain can develop
  forks leading to branching
• One mechanism of branching is short chain
  branching in which an internal hydrogen is
  abstracted

47
Long Chain Branching

• In long chains, a hydrogen from another chain is
  abstracted

48
Cationic Polymerization

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