1. The Reactions of Alkenes The Stereochemistry of Addition Reactions
Chapter 6
The Reactions of Alkenes
The Stereochemistry of Addition Reactions
Paula Yurkanis Bruice
University of California,
Santa Barbara

2. The Mechanism of the Reaction

3. An Electrophilic Addition Reaction

4. Addition of Hydrogen Halides

5. Which sp2 Carbon Gets the H+?

6. The Mechanism
Carbocation formation is the rate-limiting step.

7. Relative Stabilities of Carbocations

8. Carbocation Stability
Alkyl groups decrease the concentration of positive charge on the carbon.

9. Hyperconjugation Stabilizes a Carbocation

10. Stabilization by Hyperconjugation

11. Hyperconjugation

12. What Does the Transition State Look Like?

13. It Looks Like What it is Closer To
The Hammond postulate says that the transition state is more similar in structure to the species to which it is more similar in energy.

14. Why the Difference in Rate?
The more stable carbocation is formed more rapidly.

15. The Difference in Carbocation Stability Determines the Products

16. What Product Will Be Formed?

17. The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens
A regioselective reaction forms more of one constitutional isomer than another.
Highly regioselective
Completely regioselective
Moderately regioselective

18. Why is the First Reaction More Highly Regioselective?

19. Not Regioselective

20. Acid-Catalyzed Addition of Water

21. Mechanism for the Acid-Catalyzed Addition of Water

22. Acid-Catalyzed Addition of an Alcohol

23. Acid-Catalyzed Addition of an Alcohol
Mechanism for the
Acid-Catalyzed Addition of an Alcohol

24. The Major Product is a Surprise

25. The Major Product is a Surprise

26. Carbocation Rearrangement
(a 1,2-hydride shift)

27. Carbocation Rearrangement
(a 1,2-methyl shift)

28. The Carbocation Does Not Rearrange
(No Improvement in Carbocation Stability)

29. BH3 is an Electrophile

30. Hydroboration–Oxidation

31. The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens
The reagents are numbered because the second set of reagents is not added until the first reaction is over.

32. Mechanism for Hydroboration

33. Addition of BH3 and Addition of HBr Follow the Same Rule

34. BH3 Has Three Potential Hydride Ions So
a Dialkylborane and a Trialkylborane Can Be Formed

35. R2BH Allows Only Monoalkylation
Because of its bulky R groups, it has a stronger preference for the less substituted sp2 carbon.

36. The Mechanism is the Same

37. OH Replaces BR2

38. Mechanism for the Oxidation Reaction

39. No Carbocation Rearrangements

40. Addition of Br2 or Cl2

41. The Product is a Vicinal Dihalide

42. The Mechanism for the Addition of a Halogen
The intermediate is a cyclic bromonium ion.

43. A Cyclic Bromonium Ion
The carbons are the most electrophilic atoms.

44. No Carbocation Rearrangements

45. Alkenes Do Not Add I2

46. Formation of Halohydrins

47. Mechanism for Halohydrin Formation

48. Why Does it Follow the Same Rule?
The electrophile adds to the sp2 carbon
bonded to the most hydrogens.

49. What Alkene Gave the Ozonolysis Products?

50. Addition of Hydrogen
catalytic hydrogenation
a reduction reaction

51. Mechanism for Hydrogen Addition
catalytic hydrogenation

52. Relative Stabilities of Alkenes

53. The Most Stable Alkene Has the Smallest Heat of Hydrogenation

54. Relative Stabilities of Alkenes

55. Trans is More Stable Than Cis

56. The Cis Isomer Has Steric Strain

57. The Relative Stabilities of Disubstituted Alkenes
relative stabilities of dialkyl-substituted alkenes

58. A Regioselective Reaction

59. A Stereoselective Reaction

60. A Stereospecific Reaction

61. The Product Does Not Have Stereoisomers

62. The Product is a Racemic Mixture
When a reactant that does not have an asymmetric center
forms a product that has one asymmetric center,
the product is a racemic mixture.

63. Formation of a Racemic Mixture

64. Each Product Will Be a Racemic Mixture

65. Formation of a Second Asymmetric Center
When a reactant that has an asymmetric center forms a product that has new asymmetric center, the product is a pair of diastereomers.

66. Two Asymmetric Centers
Reactions that Form
Two Asymmetric Centers
the stereoisomers obtained as products depend on the mechanism

67. Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate
syn and anti addition

68. Only Syn Addition

69. Syn Addition to a Cis Isomer Forms Only the Erythro Stereoisomers

70. If the Substituents are the Same,
the Erythro Stereoisomer is a Meso Compound

71. Syn Addition to a Trans Isomer Forms Only the Threo Stereoisomers

72. Syn Addition to a Cis Isomer Forms Only the Cis Stereoisomers

73. If the Substituents are the Same,
the Cis Stereoisomer is a Meso Compound

74. Unless They Have at Least Eight Ring Atoms
Cyclic Alkenes are Cis,
Unless They Have at Least Eight Ring Atoms

75. Addition of a Halogen is an Anti Addition

76. Anti Addition to a Cis Isomer Forms Only the Threo Stereoisomers

77. If the Substituents are the Same,
the Cis Stereoisomer is a Meso Compound

78. If the Substituents are the Same,
the Erythro Stereoisomer is a Meso Compound

79. Anti Addition to a Cis Isomer Forms Only the Trans Stereoisomers

80. CIS-SYN-(ERYTHRO or CIS)
Can change two terms but not one
CIS-ANTI-(THREO or TRANS) OK
TRANS-ANTI-(ERYTHRO or CIS) OK
CIS-ANTI-(ERYTHRO or CIS) not OK

82. An Enzyme Forms One Stereoisomer; It is Completely Stereoselective

83. An Enzyme Can Block One Side of the Reactant

84. An Enzyme Reacts with Only One Stereoisomer;
It is Completely Stereospecific

85. The Stereospecificity of an Enzyme-Catalyzed Reaction Allows Enantiomers to Be Separated

86. Enzymes and Receptors are Chiral
Later we will see that an enzyme reacts with only one enantiomer.
A chiral molecule binds only one of a pair of enantiomers.
A right-handed glove fits only a right hand.

87. A Receptor Binds One Enantiomer

88. When A Reacts, B is Synthesized

89. Compounds That Can Be Formed from an Alkene