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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. The Mechanism of the Reaction. An Electrophilic Addition Reaction. Addition of Hydrogen Halides. Which sp 2 Carbon Gets the H + ?.

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The Reactions of Alkenes The Stereochemistry of Addition Reactions

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  1. 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 decreasethe 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. • Completely regioselective • Highly 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. 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 areduction reaction

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