1. 8. Alkenes: Reactions and Synthesis
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

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

3. 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)
require strong acids (sulfuric acid, 50 ºC)

4. Preview of Elimination Reactions
Dehydrohalogenation: Loss of HX
With D and strong base (KOH)

5. Preview of Elimination Reactions
Dehydration: Loss of HOH
With strong acid cat (H2SO4)

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. Biological Halogenation
Occurs in marine organisms.
“Br1+” or “Cl1+” like compounds made by oxidizing Br1- or Cl1- with H2O2
p. 218

12. Which product will result from the following reaction?
Learning Check:
Which product will result from the following reaction? 1. 2. 5. 4. 3. 1 2 3 4 5

13. Which product will result from the following reaction?
Solution:
Which product will result from the following reaction? 1. 2. 5. 4. 3. 1 2 3 4 5

14. 8.3 Addition of Hypohalous Acids to Alkenes: Halohydrin Formation
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)

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

16. 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

17. Learning Check:
Which of the compounds will result from the reaction of NBS/H2O/DMSO with 1-methylcyclopentene? 2. 1. 5. 3. 4. 1 2 3 4 5

18. Solution:
Which of the compounds will result from the reaction of NBS/H2O/DMSO with 1-methylcyclopentene? 2. 1. 5. 3. 4. 1 2 3 4 5

19. 8.4 Addition of Water to Alkenes:
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

20. Figure 7.2: MECHANISM: Mechanism of the acid-catalyzed hydration of an alkene to yield an alcohol. Protonation of the alkene gives a carbocation intermediate that reacts with water.
Fig. 7-2, p. 221

21. Biological Addition of Water
p. 222

22. Oxymercuration
p. 222

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

24. 8.5 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

25. BH3-THF Complex
THF = Tetrahydrofuran; a common ether solvent
p. 223

26. Hydroboration-Oxidation: Alkene  Alcohol
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

27. 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)

28. 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 consistent with steric preferences

29. Example:
p. 225

30. Learning Check
p. 225

31. Solution:
Options are:
So:
p. 226

32. 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)

33. 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

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

35. Examples:
p. 232

36. Examples:
p. 232

37. Learning Check:
What product would you obtain from catalytic hydrogenation of the following alkenes?
p. 232

38. Biological Example: Fig. 7-8, p. 233
Figure 7.8: Catalytic hydrogenation of polyunsaturated fats leads to saturated products, along with a small amount of isomerized trans fats.
Fig. 7-8, p. 233

39. Learning Check:
Which compound is the best starting material for hydrogenation to decalin shown below? 1. 2. 3. 5. 4. 1 2 3 4 5

40. Solution:
Which compound is the best starting material for hydrogenation to decalin shown below? 1. 2. 3. 5. 4. 1 2 3 4 5

41. 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation
Epoxidation results in a cyclic ether with an oxygen atom
Stereochemistry of addition is syn

42. Ways to Make Epoxides
p. 234

43. Use Epoxides to make Diols

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

45. 8.8 Oxidaton of Alkenes: Cleavage to Carbonyl Compounds
Ozone, O3, adds to alkenes to form molozonide
Reduce molozonide to obtain ketones and/or aldehydes

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

47. Compound X reacts with O3 (followed by Zn/AcOH) to give I and II.
Learning Check:
Compound X reacts with O3 (followed by Zn/AcOH) to give I and II.
Which of the following could be X? 1. 2. 5. 3. 4. 1 2 3 4 5

48. Compound X reacts with O3 (followed by Zn/AcOH) to give I and II.
Solution:
Compound X reacts with O3 (followed by Zn/AcOH) to give I and II.
Which of the following could be X? 1. 2. 5. 3. 4. 1 2 3 4 5

49. 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

50. 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

51. What is the major product of the following reaction?
Learning Check:
What is the major product of the following reaction? 2. 1. 5. 3. 4. 1 2 3 4 5

52. What is the major product of the following reaction?
Solution:
What is the major product of the following reaction? 2. 1. 5. 3. 4. 1 2 3 4 5

53. Learning Check:
Which of the following will react with periodic acid (HIO4) to give an aldehyde? 1. 2. 5. 3. 4. 1 2 3 4 5

54. Solution:
Which of the following will react with periodic acid (HIO4) to give an aldehyde? 1. 2. 5. 3. 4. 1 2 3 4 5

55. 8.9 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 add symmetrically across double bonds to form cyclopropanes

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

57. Reaction of Dichlorocarbene
Addition of dichlorocarbene is stereospecific cis

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

59. 8.10 Radical Addition to Alkenes: Polymers
A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization

60. Polymers

61. Free Radical Polymerization: Initiation
Alkenes react with radical catalysts to undergo radical polymerization
Ethylene is polymerized to poyethylene, for example
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

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

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

64. Other Polymers
Other alkenes give other common polymers

65. Other Polymers
Table 7-1, p. 242

66. Which monomer would give the polymer shown below?
Learning Check:
Which monomer would give the polymer shown below? 2. 1. 5. 4. 3. 1 2 3 4 5

67. Which monomer would give the polymer shown below?
Solution:
Which monomer would give the polymer shown below? 2. 1. 5. 4. 3. 1 2 3 4 5

68. 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

69. Biological Reactions
Biological reactions different than 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

71. Natural Rubber
Natural rubber is obtained from the bark of the rubber tree, Hevea brasiliensis, grown on enormous plantations in Southeast Asia.
p. 245

72. 8.12 Stereochemistry of Reactions: Addition of H2O to Alkenes
Many reactions can produce new chirality centers from compounds without them
What is the stereochemistry of the chiral product?
What relative amounts of stereoisomers form?
Example addition of H2O to 1-butene

73. Achiral Intermediate Gives Racemic Product
Addition via carbocation
Top and bottom are equally accessible

74. 8.13 Stereochemistry of Reactions: Addition of H2O to a Chiral Alkene
What is the sterochemical result of the addition of H2O to a chiral alkene R-4- methyl-1-hexene
Product has 2 chiral centers

75. Figure 9.16: Stereochemistry of the addition of H2O to the chiral alkene, (R)-4-methyl-1-hexene. A mixture of diastereomeric 2R,4R and 2S,4R products is formed in unequal amounts because reaction of the chiral carbocation intermediate is not equally likely from top and bottom. The product mixture is optically active.
Fig. 9-16, p. 313

76. Summary

77. Learning Check:
Which sequence of reagents is necessary to carry out the following synthesis?
Br2/hv; NaOH; RCO3H; H3O+
Cl2/heat; KOH; OsO4; NaHSO3/H2O
H2/Pd; NBS/H2O/DMSO; H3O+
KOH; OsO4; NaHSO3/H2O
NaOH; Br2/H2O; KOH; H3O+

78. Solution:
Which sequence of reagents is necessary to carry out the following synthesis?
Br2/hv; NaOH; RCO3H; H3O+
Cl2/heat; KOH; OsO4; NaHSO3/H2O
H2/Pd; NBS/H2O/DMSO; H3O+
KOH; OsO4; NaHSO3/H2O
NaOH; Br2/H2O; KOH; H3O+

79. Learning Check:
Which alkene would produce two identical ketones when reacted with KMnO4/H+? 2. 1. 3. 4. 5. 1 2 3 4 5

80. Solution:
Which alkene would produce two identical ketones when reacted with KMnO4/H+? 2. 1. 3. 4. 5. 1 2 3 4 5

81. Learning Check:
What kind of intermediate is involved in the biosynthesis of prostaglandins from arachidonic acid?
carbocations
carbanions
carbon-base radicals
bromonium ions
carbenes

82. Solution:
What kind of intermediate is involved in the biosynthesis of prostaglandins from arachidonic acid?
carbocations
carbanions
carbon-base radicals
bromonium ions
carbenes

83. What is the best reagent to carry out the following reaction?
Learning Check:
What is the best reagent to carry out the following reaction?
CH2Cl2/Zn(Cu)
CH2l2/Zn(Cu); Cl2
CH2Cl2/KOH
CHCl3/KOH
Cl2/H2O, CH3I/KOH

84. What is the best reagent to carry out the following reaction?
Solution:
What is the best reagent to carry out the following reaction?
CH2Cl2/Zn(Cu)
CH2l2/Zn(Cu); Cl2
CH2Cl2/KOH
CHCl3/KOH
Cl2/H2O, CH3I/KOH

85. Learning Check:
Which of the following reactions would not yield at least one alcohol functional group?
hydroboration of an internal alkene, followed by H2O2/NaOH
reaction of a mono-substituted alkene with NBS/H2O/DMSO
osmium tetroxide oxidation of a tetrasubstituted cyclic alkene, followed by NaHSO3
oxymercuration/demercuration of terminal alkene
oxidative cleavage of a tetrasubstituted alkene using KMnO4/H+

86. Solution:
Which of the following reactions would not yield at least one alcohol functional group?
hydroboration of an internal alkene, followed by H2O2/NaOH
reaction of a mono-substituted alkene with NBS/H2O/DMSO
osmium tetroxide oxidation of a tetrasubstituted cyclic alkene, followed by NaHSO3
oxymercuration/demercuration of terminal alkene
oxidative cleavage of a tetrasubstituted alkene using KMnO4/H+

87. Learning Check:
Which of the following compounds would yield trialkylborane shown below when treated with BH3/THF?
2-methylbut-1-ene
2-methylbut-2-ene
3-methylbut-1-ene
2-methylpropene
(E)-2-pentene

88. Solution:
Which of the following compounds would yield trialkylborane shown below when treated with BH3/THF?
2-methylbut-1-ene
2-methylbut-2-ene
3-methylbut-1-ene
2-methylpropene
(E)-2-pentene

89. Learning Check:
What is the product of reaction of 1-methylcyclohexene with Hg(OAc)2/H2O/THF followed by NaBD4? (D = 2H) 1. 2. 3. 4. 5. 1 2 3 4 5

90. Solution:
What is the product of reaction of 1-methylcyclohexene with Hg(OAc)2/H2O/THF followed by NaBD4? (D = 2H) 1. 2. 3. 4. 5. 1 2 3 4 5

91. Learning Check:
Which reaction sequence on an alkene will accomplish the same end result as treating the alkene with O3 followed by Zn/AcOH?
RCO3H followed by KMnO4/H3O+
BH3/THF followed by OsO4 followed by NaHSO3/H2O
OsO4/NMO followed by HIO4
Hg(OAc)2/H2O/THF followed by NaBH4 followed by KOH
Br2/H2O followed by KOH, followed by H3O+

92. Solution:
Which reaction sequence on an alkene will accomplish the same end result as treating the alkene with O3 followed by Zn/AcOH?
RCO3H followed by KMnO4/H3O+
BH3/THF followed by OsO4 followed by NaHSO3/H2O
OsO4/NMO followed by HIO4
Hg(OAc)2/H2O/THF followed by NaBH4 followed by KOH
Br2/H2O followed by KOH, followed by H3O+