1. Reactions and Synthesis
Chapter 7
Alkenes:
Reactions and Synthesis

2. Introduction
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)

4. I. Synthesis of Alkenes Dehydrohalogenation of Alkyl Halides
Dehydration of Alcohols

5. Preparation of Alkenes: A Preview of Elimination Reactions
Alkenes are commonly made by elimination reactions
dehydrohalogenation
dehydration

6. A. Dehydrohalogenation of Alkyl Halides
Dehydrohalogenation is elimination (loss) of HX from an alkyl halide
uses strong base (KOH)

7. B. Dehydration of Alcohols
Dehydration is elimination of water (H-OH) from an alcohol
requires strong acids (sulfuric acid, 50 ºC)

8. Practice Problem: One problem with elimination reactions is that
Practice Problem: One problem with elimination reactions is that mixtures of products are often formed. For example, treatment of 2-bromo methylbutane with KOH in ethanol yields a mixture of two alkene products. What are their likely structures?

9. Practice Problem: How many alkene products, including E,Z
Practice Problem: How many alkene products, including E,Z isomers, might be obtained by dehydration of methyl-3-hexanol (3-methyl hydroxyhexane) with aqueous sulfuric acid?

10. II. Addition Reactions of Alkenes
Addition of Halogens
Halohydrin Formation
Addition of Water: Oxymercuration
Addition of Water: Hydroboration

11. II. Addition Reactions of Alkenes
Addition of Carbenes
Reduction of Alkenes: Hydrogenation
Oxidation of Alkenes: Hydroxylation and Cleavage

12. II. Addition Reactions of Alkenes
Biological Alkene Addition Reactions
Addition of Radicals

13. A. Addition of Halogens
Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process
F2 is too reactive and I2 does not add
Cl2 reacts as Cl+ Cl- and Br2 reacts similarly

14. Addition of Halogens is Anti
Addition of halogens occurs with anti stereochemistry.
The two ends of the double bonds are attacked from different sides
Only the trans isomer is produced

15. Possible Mechanism of Halogen Addition ?
Addition of Br2 to cyclopentene is exclusively trans +
Formation of a carbocation does not explain the anti stereochemistry

16. Mechanism of Halogen Addition
In 1937, Kimball and Roberts proposed the reaction intermediate to be a bromonium ion, R2Br+, and not a carbocation.
This is a one-step process

17. Br+ adds to an alkene producing a cyclic ion
Bromonium ion: Bromine shares charge with carbon
Br “shields” one side and gives trans addition

18. 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 +

19. 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

20. Addition of Halogens: Summary
Bromine and chlorine add to alkenes to give 1,2-dihalides
via three-membered ring bromonium ion or chloronium ion intermediates
anti stereochemistry

21. Practice Problem: What product would you expect to obtain from
Practice Problem: What product would you expect to obtain from addition of Cl2 to 1,2-dimethylcyclohexene? Show the stereochemistry of the product.

22. Practice Problem: Addition of HCl to 1,2-dimethylcyclohexene
Practice Problem: Addition of HCl to 1,2-dimethylcyclohexene yields a mixture of two products. Show the stereochemistry of each, and explain why a mixture is formed.

23. B. Halohydrin Formation
HO-X adds to an alkene to give a 1,2-halo alcohol, called a halohydrin
The actual reagent is the dihalogen (Br2 or Cl2) in the presence of water in an organic solvent
H2O acts as the nucleophile

24. Mechanism of Bromohydrin formation

25. 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

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

27. Halohydrin Formation: Summary
HO-X adds to an alkene to give a 1,2-halo alcohol, called a halohydrin
Bromonium ion or chloronium ion intermediates
Anti stereochemistry
Markovnikov regiochemistry (X ~ H)

28. Practice Problem: What product would you expect from the
Practice Problem: What product would you expect from the reaction of cyclopentene with NBS and water? Show the stereochemistry.

29. Practice Problem: When an unsymmetrical alkene such as
Practice Problem: When an unsymmetrical alkene such as propene is treated with N-bromosuccinimide in aqueous dimethyl sulfoxide, the major product has the bromine atom bonded to the less highly substituted carbon atom. Is this Markovnikov or non-Markovnikov orientation? Explain.

30. C. Addition of Water: Oxymercuration
Hydration of an alkene is the addition of H-OH to an alkene to give an alcohol
Acid catalysts are used at high temperature in industrial processes: ethylene is converted to ethanol

31. Mechanism of Acid-catalyzed Hydration
Protonation of the alkene gives a carbocation intermediate

32. Mechanism of Acid-catalyzed Hydration: Example

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

34. Mechanism of Oxymercuration

35. Mechanism of Oxymercuration: Example

36. Oxymercuration-Reduction: Summary
HO-H adds to an alkene to give an alcohol
Markovnikov regiochemistry (OH attaches to the more highly substituted carbon)
Mecurinium ion intermediates

37. Practice Problem: What products would you expect from
Practice Problem: What products would you expect from oxymercuration of the following alkenes?

38. Practice Problem: What alkenes might the following alcohols
Practice Problem: What alkenes might the following alcohols have been prepared from?

39. D. Addition of Water: Hydroboration
In 1959, Herbert Brown (HB) invented hydroboration (HB)
Borane adds to an alkene to give an organoborane

40. Borane (BH3) is a Lewis acid
Borane (BH3) is electron deficient
Boron has six electrons in its outer shell
BH3 coordinates to oxygen electron pairs in ethers (THF)

41. 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

42. Orientation in Hydration via Hydroboration
H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)
Regiochemistry is opposite to Markovnikov orientation
OH is added to carbon with most H’s

43. Mechanism of Hydroboration
Borane is a Lewis acid
Alkene is a Lewis base
Transition state involves anionic development on B
Four-center, cyclic transition state
The components of BH3 are across C=C

44. Mechanism of Hydroboration

45. Hydroboration: Orientation in Addition Step
Addition in least crowded orientation, syn
Addition also is via most stable carbocation

46. Hydroboration: Electronic Effects Give Non-Markovnikov
More stable carbocation resembles a 3o carbocation

47. Hydroboration: Steric Effects Give Non-Markovnikov
More stable carbocation is also consistent with steric preferences

48. Oxidation: Oxygen Insertion Step
H2O2, OH- inserts OH in place of B
Retains syn orientation

49. Hydroboration-oxidation: Summary
Borane adds to an alkene to give an organoborane which is then oxidized by alkaline H2O2 to give an alcohol
Syn stereochemistry
Non-Markovnikov regiochemistry

50. Practice Problem: Show the structures of the products you
Practice Problem: Show the structures of the products you would obtain by hydroboration/oxidation of the following alkenes:

51. Practice Problem: What alkenes might be used to prepare the
Practice Problem: What alkenes might be used to prepare the following alcohols by hydroboration/oxidation?

52. Practice Problem: The following cycloalkene gives a mixture of
Practice Problem: The following cycloalkene gives a mixture of two alcohols on hydroboration followed by oxidation. Draw the structures of both, explain the result.

53. E. Addition of Carbenes
A carbene (R2C:) adds to an alkene to give a cyclopropane
A carbene is electrically neutral with six electrons in the outer shell
The carbene functional group is “half of an alkene”
A carbene is electron-deficient and acts as an electrophile

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

55. Formation of Dichlorocarbene

56. Structure of Dichlorocarbene
Dichlorocarbene is sp2 hybridized.
It has a vacant p orbital

57. Reaction of Dichlorocarbene
Addition of dichlorocarbene is stereospecific
Only a single stereoisomer is formed

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

59. Addition of Carbenes: Summary
A carbene (R2C:) adds to an alkene to give a cyclopropane
Stereospecific
No intermediate

60. Practice Problem: What products would you expect from the
Practice Problem: What products would you expect from the following reactions?

61. F. Reduction of Alkenes: Hydrogenation
H-H adds to an alkene to give an alkane
Reduction in general is addition of H2 or its equivalent
It requires metal catalyst, Pt or Pd as powders, on carbon and H2
Hydrogen is first adsorbed on catalyst
Reaction is heterogeneous (process is not in solution)

62. Catalytic Hydrogenation is syn
Reaction is heterogeneous (process is not in solution)
Reaction is syn
Hydrogens add to the double bond from the same face

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

64. 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

66. Polyunsaturated liquid oils become solids upon catalytic hydrogenation

67. Hydrogenation: Summary
H-H adds to an alkene to give an alkane
Syn stereochemistry

68. Practice Problem: What product would you obtain from catalytic
Practice Problem: What product would you obtain from catalytic hydrogenation of the following alkenes?

69. G. Oxidation of Alkenes Alkene Hydroxylation
Oxidative Cleavage of Alkenes

70. 1. Alkene Hydroxylation
HO-OH adds to an alkene to give a 1,2-dialcohol or diol (also called a glycol)
It is catalyzed by osmium tetroxide OsO4
Stereochemistry of addition is syn

71. Osmium Tetroxide Catalyzed Formation of Diols
Hydroxylation of alkenes
converts alkenes to cis-1,2-diol
is catalyzed by osmium tetroxide OsO4, then sodium bisulfite NaHSO3
occurs via cyclic osmate di-ester intermediate

72. Alkene Hydroxylation: Summary
HO-OH adds to an alkene to give a 1,2-dialcohol or diol (also called a glycol)
Syn stereochemistry
Osmate intermediate

73. 2. Oxidative Cleavage of Alkenes
Ozonolysis
Permanganate Oxidation
Oxidative Cleavage of 1,2-diols

74. a. Ozonolysis
Ozone, O3, adds to alkenes to form molozonide which is then reduced by zinc, Zn, to give ketones and/or aldehydes

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

76. Structure Elucidation With Ozone
Cleavage products reveal an alkene’s structure

77. b. Permanganate Oxidation
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

78. c. Oxidative 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

79. Mechanism of Periodic Acid Oxidation
It occurs via cyclic periodate intermediate

80. Oxidative Cleavage: Summary

81. Practice Problem: What alkene would you start with to prepare
Practice Problem: What alkene would you start with to prepare each of the following compounds?

82. Practice Problem: What products would you expect from
Practice Problem: What products would you expect from reaction of 1-methylcyclohexene with the following reagents?
Aqueous acidic KMnO4
O3, followed by Zn, CH3CO2H

83. Practice Problem: Propose structures for alkenes that yield the
Practice Problem: Propose structures for alkenes that yield the following products on reaction with ozone followed by treatment with Zn:
(CH3)2C=O + H2C=O
2 equiv CH3CH2CH=O

84. H. 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
They are usually more specific for reactant and stereochemistry

85. Example of Biological Hydration
Fumarase catalyzes fumaric acid to malic acid
It is specific for trans isomer
Addition of H-OH is anti

86. I. Addition of Radicals to Alkenes: Polymers
A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization

87. Alkenes react with radical catalysts to undergo radical polymerization
For example: Ethylene is polymerized to polyethylene

88. Free Radical Polymerization of Alkenes
Alkenes combine many times to give polymer
Reactivity is induced by formation of free radicals

89. Free Radical Polymerization: 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

90. Free Radical Polymerization: Propagation
Radical from intiation step adds to another alkene to generate alkene-derived radical
This radical adds to another alkene, and so on many times

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

92. Other Polymers
Other alkenes give other common polymers

93. Unsymmetrical Monomers
If alkene is unsymmetrical, reaction is via more highly substituted radical

95. Practice Problem: Show the monomer units you would use to
Practice Problem: Show the monomer units you would use to prepare the following polymers:

96. 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

97. Long Chain Branching
In long chain-branching, a hydrogen from another chain is abstracted

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

99. Practice Problem: One of the chain-termination steps that
Practice Problem: One of the chain-termination steps that sometimes occurs to interrupt polymerization is the following reaction between radicals:
Propose a mechanism for this reaction, using fishbook arrows to indicate electron flow

100. Practice Problem: tert-Butyl vinyl ether is polymerized
Practice Problem: tert-Butyl vinyl ether is polymerized commercially for use in adhesives by cationic process. Draw a segment of poly(tert-butyl vinyl ether), and show the mechanism of the chain-carrying step:

101. Chapter 7
The End