7' Alkenes: Reactions and Synthesis

1
7. Alkenes Reactions and Synthesis
2
Topics to Discuss

• Eliminations (forms alkenes)
• alkene preparation
• Additions (to alkenes)
• halogen addition to alkenes
• halohydrin formation
• oxymercuration
• hydroboration
• carbenes
• Reductions (to alkenes)
• hydrogenation
• Oxidations (to alkenes)
• hydroxylation
• Biological Reactions
• Radical Reactions (Polymers)

3
Diverse Reactions of Alkenes

• Alkenes react with many electrophiles to give
  useful products by addition (often through
  special reagents)
• alcohols
• alkanes
• halohydrins
• dihalides
• halides
• diols
• cyclopropane

4
Preparation of Alkenes Elimination Reactions

• Alkenes are commonly made by
• Dehydrohalogenation

5
Preparation of Alkenes Elimination Reactions

• Alkenes are commonly made by
• Dehydration

6
Alkene Addition Reactions

• Addition of Halogens to alkenes

7
Addition of Br2 to Cycloalkenes

• Q Why do we see trans- addition only?

8
Mechanism of Br2 Addition

• 1. p e- attack partially positive bromine atom,
  forming a cyclic bromonium ion, where the bromine
  atom shares electrons with carbons. Electrons
  from Br-Br bond stay with the partially negative
  bromine atom.
• 2. Bromine anion attacks carbon from the
  opposite side of the bromonium ion, which acts as
  a shield to one side of the molecule, and a new
  C-Br bond is formed.
• 3. Other carbon atom keeps the first bromine
  atom, so the two Br atoms are added across the
  CC

9
Halohydrin Formation

• Addition of HO-X to an alkene to give
• The actual reagent is
• When water is added, it competes with
• Follows

10
Mechanism of Bromohydrin Formation

• 1. Alkene reacts with Br2 to form bromonium ion.
• 2. Water opens up ring and forms a C-O bond.
• 3. Since oxygen donated its e-, it has a
  positive charge.
• 4. Loss of H to water gives H3O and the
  neutral product.

11
An Alternative to Bromine

• Bromine is a difficult reagent to use for this
  reaction

12
Addition of Water Oxymercuration

• Q What is hydration?
• Industrial Preparation
• Laboratory Preparation
• NOTE

13
Oxymercuration Example
14
Mechanism of Oxymercuration

• 1. Electrophilic addition of Hg(OAc)2 produces
  mercurinium ion.
• 2. Water acts as a nucleophile, attacks the more
  substituted carbon atom to form a C-O bond.
• Acetate ion removes hydrogen, forming a neutral
  product, but the HgOAc is still attached.
• NaBH4 reduces the HgOAc, replacing it with a
  hydrogen atom to form the alcohol product.

15
Addition of Water Hydroboration

• Hydroboration
• NOTE

16
Hydroboration Example
17
Mechanism of Hydroboration

• Q Why does hydroboration form the less
  substituted alcohol?
• Q Why do we see syn addition?

18
Addition of Carbenes

• Q What are carbenes?
• Q How do we make carbenes?
• NOTE

19
Carbene Addition example
20
Formation of Dichlorocarbene

• 1. Base removes proton from chloroform
• 2. Stabilized carbanion remains
• 3. Elimination of Cl- gives electron deficient
  species, dichlorocarbene.

21
Simmons-Smith Reaction

• Forms
• Equivalent of addition of
• Formed by reaction of

22
Simmons-Smith Reaction example
23
Reduction Hydrogenation

• Q What is a reduction?
• Q How does it work?
• NOTE

24
Hydrogenation example
25
Mechanism of Hydrogenation

• Heterogeneous
• Addition of H-H is

26
Oxidation Hydroxylation

• Q What is hydroxylation?
• Q How does it work?
• NOTE

27
Hydroxylation example
28
Oxidation Cleavage w/Ozone

• Q What is it?
• Q How does it work?

29
Examples of Ozonolysis
30
Permanganate Oxidation

• Q What is it?
• Q How does it work?

31
Examples of Permanganate Oxidation
32
Periodic Acid Cleavage of 1,2-diols

• Q What is it?
• Q How does it work?

33
Examples of Periodic Acid Oxidation
34
Biological Alkene Addition Reactions

• Living organisms convert
• Many reactions are similar to organic chemistry
  conversions, except
• Usually very much specific for

35
Biological Hydration Example

• Fumarate to malate catalyzed by
• Specific for
• Addition of H, OH is

36
Addition of Radicals to Alkenes Polymers

• A polymer is a very large molecule consisting of
• Alkenes react with radical catalysts to undergo
• Ethylene is polymerized to

37
Free Radical Polymerization of Alkenes

• Alkenes combine many times to give polymers.

38
Free Radical Polymerization Initiation

• Initiation
• A bond is broken

39
Polymerization Propagation

• Radical from initiation adds to
• This radical adds to

40
Polymerization Termination

• Chain propagation ends when
• Not controlled specifically but affected by

41
Other Polymers

• Other alkenes give other common polymers

42
Summary

• Eliminations (forms alkenes)
• alkene preparation
• Additions (to alkenes)
• halogen addition to alkenes
• halohydrin formation
• oxymercuration
• hydroboration
• carbenes
• Reductions (to alkenes)
• hydrogenation
• Oxidations (to alkenes)
• hydroxylation
• Biological Reactions
• Radical Reactions (Polymers)