1. Chapter 8 Alkenes: Reactions and Synthesis

2. Diverse Reactions of Alkenes
Alkenes react with many electrophiles to give useful products by addition (often through special reagents)

3. Why this chapter?
To begin a systematic description of major functional groups
Begin to focus on general principles and patterns of reactivity that tie organic chemistry

4. 8.1 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)
requires strong acids (sulfuric acid, 50 ºC)

5. Reactions for Making Unsaturateds - Video
1) Reactions for Making Unsaturateds – dehydration and dehydrohalogenation

6. 8.2 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

7. Addition of Br2 to Cyclopentene
Addition is exclusively trans

8. Mechanism of Bromine Addition
Br+ adds to an alkene producing a cyclic ion
Bromonium ion, bromine shares charge with carbon
Gives trans addition

9. 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

10. The Reality of Bromonium Ions
Bromonium ions were postulated more than 60 years ago to explain 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

11. Halogenation of an Alkene - Video

12. 8.3 Halohydrins from Alkenes: Addition of HOX
This is formally the addition of HO-X to an alkene 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)

13. Mechanism of Formation of a Bromohydrin
Br2 forms bromonium ion, then water adds
Orientation toward stable C+ species
Aromatic rings do not react

14. 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

15. Halohydrin Formation - Video

16. 8.4 Hydration of Alkenes: Addition of H2O by Oxymercuration
Hydration of an alkene is the addition of H-OH to give an alcohol
Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

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

18. Hydration of Alkenes - Video
1) Acidic hydration of alkenes
2) Oxymercuration of alkenes for hydration: video 1
3) Oxymercuration of alkenes for hydration: video 2

19. 8.5 Hydration of Alkenes: Addition of H2O by Hydroboration
Borane (BH3) is electron deficient
Borane adds to an alkene to give an organoborane

20. 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

21. 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)

22. Mechanism of Hydroboration
Borane is a Lewis acid
Alkene is Lewis base
Transition state involves anionic development on B
The components of BH3 are added across C=C
More stable carbocation is also consistent with steric preferences

23. Hydroboration-Oxidation - Video

24. 8.6 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)

25. 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

26. Mechanism of Catalytic Hydrogenation
Heterogeneous – reaction between phases
Addition of H-H is syn

27. 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation
Epoxidation results in a cyclic ether with an oxygen atom
Stereochemistry of addition is syn

28. Organic Mechanism - Epoxidation with mCPBA - Video
2) Preparation of Epoxides – halohydrin treated with sodium hydride (strong base)

29. Acid catalyzed epoxide opening to produce diol - Video
1) Opening of epoxides with acid and water to give trans diols

30. Osmium Tetroxide Catalyzed Formation of Diols
Hydroxylation - converts to syn-diol
Osmium tetroxide, then sodium bisulfite
Via cyclic osmate di-ester

31. Osmium Tetroxide Catalyzed Formation of Diols - Video
Syn Dihydroxylation

32. 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds
Ozone, O3, adds to alkenes to form molozonide
Molozonideis converted to ozonide that may be reduced to obtain ketones and/or aldehydes

33. Examples of Ozonolysis of Alkenes
Used in determination of structure of an unknown alkene

34. Example of Ozonolysis of Alkenes - Videos
1) ozonolysis - oxidative cleavage

35. Permangate 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

36. Permanganate Oxidation of Alkenes - Video
1) Oxidative Cleavage Alkenes and Aromatic Side Chains with Potassium Permanganate

37. 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

38. Cleavage of 1,2-diols - Video
1) Periodate Cleavage – END AT 9:36

39. 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis
The carbene functional group is “half of an alkene”
Carbenes are electronically neutral with six electrons in the outer shell
They add symmetrically across double bonds to form cyclopropanes

40. Formation of Dichlorocarbene
Base removes proton from chloroform
Stabilized carbanion remains
Unimolecular elimination of Cl- gives electron deficient species, dichlorocarbene

41. Reaction of Dichlorocarbene
Addition of dichlorocarbene is stereospecific cis

42. Reaction of Dichlorocarbene - Video
1) Addition of Carbenes to Alkenes - START VIDEO AT 24:50 MINS; END AT 36:55

43. Simmons-Smith Reaction
Equivalent of addition of CH2:
Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species
Forms cyclopropanes by cycloaddition

44. Simmons-Smith Reaction - Video
1) Cyclopropane Simmons-Smith reaction - START VIDEO AT 5:55 MINS_END AT 17:55

45. 8.10 Radical Additions to Alkenes: Chain-Growth 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 polyethylene, for example

46. 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

47. Polymerization: Propagation
Radical from initiation adds to alkene to generate alkene derived radical
This radical adds to another alkene, and so on many times

48. Polymerization: Termination
Chain propagation ends when two radical chains combine
Not controlled specifically but affected by reactivity and concentration

49. Other Polymers
Other alkenes give other common polymers

50. Addition Polymerization - Video
1) Radical Addition to Alkenes: Chain-Growth Polymer

51. 8.11 Biological Additions of Radicals to Alkenes
Severe limitations to the usefulness of radical addition reactions in the lab
In contrast to electrophilic additions, reactive intermediate is not quenched so it reacts again and again uncontrollably

52. Biological Reactions
Biological reactions different from in the laboratory
One substrate molecule at a time is present in the active site of an enzyme
Biological reactions are more controlled, more specific than other reactions

53. Pathway of Biosynthesis of Prostaglandins

54. 8.12 and 8.13 Addition of water to achiral and chrial alkanes
As a general rule, the formation of a new chirality center by reaction of achiral reactants always leads to racemic mextures of enantiomeric products.
As a general rule, the formation of a new chirality center by the reaction of a chiral reactant leads to unequal amounts of diasteromeric products.
This is because the carbocation has no plane of symmetry and is chiral, it does not react equally well from the top and bottom faces.
One of the two faces is likely, for steric reasons, to be a bit more accessible than the other face, thus leading to a mixture of R and S products in some ratio other than 50:50.

55. 8.12 and 8.13 Addition of water to achiral and chrial alkanes - Video
Addition of Water (Acid-Catalyzed) Mechanism

56. Let’s Work a Problem
Which Reaction would one predict to be faster, addition of HBr to cyclohexene or to 1-methylcyclohexene?

57. Answer
First, draw out both reactants with HBr. What we should realize at this point is that the formation of the intermediate that is more stabilized via carbocation formation is the one that will form product faster. At this point, we should see that the intermediate formed via the 3˚ intermediate from 1-methylcyclohexene (as opposed to the 2˚ carbocation intermediate in the case of cyclohexene) will proceed faster.