1. 7. Alkenes: Reactions and Synthesis
Based on McMurry’s Organic Chemistry, 6th edition
©2003 Ronald Kluger
Department of Chemistry
University of Toronto

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)
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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)
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

5. Addition of Br2 to Cyclopentene
Addition is exclusively trans +
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

6. Mechanism of Bromine Addition
Br+ adds to an alkene producing a cyclic ion
Bromonium ion, bromine shares charge with carbon
Gives trans addition
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

9. McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
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)
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

10. Mechanism of Formation of a Bromohydrin
Br2 forms bromonium ion, then water adds
Orientation toward stable C+ species
Aromatic rings do not react
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

13. Oxymercuration Intermediates
For laboratory-scale hydration of an alkene
Use mercuric acetate in THF followed by sodium borohydride
Markovnikov orientation
via mercurinium ion
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

15. McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
BH3 Is a Lewis Acid
Six electrons in outer shell
Coordinates to oxygen electron pairs in ethers
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

16. McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

17. Orientation in Hydration via Hydroboration
Regiochemistry is opposite to Markovnikov orientation
OH is added to carbon with most H’s
H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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 C=C
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

19. Hydroboration: Orientation in Addition Step
Addition in least crowded orientation, syn
Addition also is via most stable carbocation
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

20. Hydroboration, Electronic Effects Give Non-Markovnikov
More stable carbocation is also consistent with steric preferences
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

21. Hydroboration - Oxygen Insertion Step
H2O2, OH- inserts OH in place of B
Retains syn orientation
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

23. Formation of Dichlorocarbene
Base removes proton from chloroform
Stabilized carbanion remains
Unimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

24. Simmons-Smith Reaction
Equivalent of addition of CH2:
Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species
Forms cyclopropanes by cycloaddition
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

25. Reaction of Dichlorocarbene
Addition of dichlorocarbene is stereospecific cis
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

26. 7.7 Reduction of Alkenes: Hydrogenation
Addition of H-H across C=C
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)
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

27. Hydrogen Addition- Selectivity
Selective for C=C. No reaction with C=O, C=N
Polyunsaturated liquid oils become solids
If one side is blocked, hydrogen adds to other
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

28. Mechanism of Catalytic Hydrogenation
Heterogeneous – reaction between phases
Addition of H-H is syn
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

29. 7.8 Oxidation of Alkenes: Hydroxylation and Cleavage
Hydroxylation adds OH to each end of C=C
Catalyzed by osmium tetroxide
Stereochemistry of addition is syn
Product is a 1,2-dialcohol or diol (also called a glycol)
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

30. Osmium Tetroxide Catalyzed Formation of Diols
Hydroxylation - converts to syn-diol
Osmium tetroxide, then sodium bisulfate
Via cyclic osmate di-ester
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

31. Alkene Cleavage: Ozone
Ozone, O3, adds to alkenes to form molozonide
Reduce molozonide to obtain ketones and/or aldehydes
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

32. Examples of Ozonolysis of Alkenes
Used in determination of structure of an unknown alkene
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

33. Structure Elucidation With Ozone
Cleavage products reveal an alkene’s structure
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

34. Permangante Oxidation of Alkenes
Oxidizing reagents other than ozone also cleave alkenes
Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

35. McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

36. Mechanism of Periodic Acid Oxidation
Via cyclic periodate intermediate
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

38. Biological Hydration Example
Fumarate to malate catalyzed by fumarase
Specific for trans isomer
Addition of H, OH is anti
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

40. Free Radical Polymerization of Alkenes
Alkenes combine many times to give polymer
Reactivity induced by formation of free radicals
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

43. Polymerization: Termination
Chain propagation ends when two radical chains combine
Not controlled specifically but affected by reactivity and concentration
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

44. McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
Other Polymers
Other alkenes give other common polymers
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

45. Unsymmetrical Monomers
If alkene is unsymmetrical, reaction is via more highly substituted radical
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

47. McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
Long Chain Branching
In long chains, a hydrogen from another chain is abstracted
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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
McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003