Reactions of Alkenes: Addition Reactions

1. Reactions of Alkenes: Addition Reactions Disparlure: sex attractant of the female gypsy moth. (A type of pheromone.)

2. Addition Reactions I. Hydrogenation of Alkenes oleic acid (unsaturated) stearic acid (saturated)

3. I. Hydrogenation of Alkenes A. Heats of hydrogenation DHº ~ -30 kcal/mol DHº -30.1 kcal -28.1 -27.2 -27.8 -26.7 More substituted  more stable.

4. I. Hydrogenation of Alkenes A. Heats of hydrogenation less stable more stable

5. I. Hydrogenation of Alkenes B. Stereochemistry of hydrogenation anti addition syn addition Catalytic hydrogenation proceeds by syn addition:

6. CheckAnswer I. Hydrogenation of Alkenes Question 6-1. Give the product of the following reaction. Click on the arrow to check your answer.

7. I. Hydrogenation of Alkenes Answer 6-1. Give the product of the following reaction. Click on the arrow to check your answer.

8. II. Electrophilic Addition “Loose” p electrons are nucleophilic (Lewis bases), react with electrophiles (Lewis acids).

9. II. Electrophilic Addition A. Addition of hydrogen halides (X = Cl, Br, I) Reactivity: HI > HBr > HCl >> HF (stronger acid = better electrophile)

10. CheckAnswer II. Electrophilic Addition A. Addition of hydrogen halides 1. Markovnikov’s rule In the addition of HX to an alkene, the H goes to the carbon with more H’s. Question 6-2. Draw the products. Click on the arrow to check answers.

11. II. Electrophilic Addition A. Addition of hydrogen halides 1. Markovnikov’s rule In the addition of HX to an alkene, the H goes to the carbon with more H’s. Answer 6-2.

12. II. Electrophilic Addition A. Addition of hydrogen halides 2. mechanism Mechanistic interpretation of Markovnikov’s rule: The reaction proceeds through the more stable carbocation intermediate.

13. II. Electrophilic Addition A. Addition of hydrogen halides 2. mechanism lower Ea faster rate of formation

14. II. Electrophilic Addition A. Addition of hydrogen halides 3. carbocation rearrangements

15. CheckAnswer II. Electrophilic Addition A. Addition of hydrogen halides 3. carbocation rearrangements Question 6-3. Give the expected major product for each reaction. Click on the arrow to check answers.

16. II. Electrophilic Addition A. Addition of hydrogen halides 3. carbocation rearrangements Answer 6-3. Give the expected major product for each reaction. Click on the arrow to check answers. forms tertiary carbocation- no need for rearrangement. forms secondary carbocation, rearranges to a tertiary carbocation via a hydride shift forms a secondary carbocation, rearranges to a tertiary carbocation via a methide shift

17. II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Markovnikov orientation antiMarkovnikov orientation (peroxide effect)

18. II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Free radical chain mechanism: Initiation Propagation

19. II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Reaction proceeds through more stable radical intermediate.

20. II. Electrophilic Addition A. Addition of hydrogen halides 4. free-radical addition of HBr Compare: addition of HBr with and without peroxides Markovnikov orientation More stable intermediate in both cases. antiMarkovnikov orientation Regiochemical control

21. CheckAnswer II. Electrophilic Addition A. Addition of hydrogen halides Question 6-5. Give the major products of the following reactions. Click on the arrow to check your answer.

22. II. Electrophilic Addition A. Addition of hydrogen halides Answer 6-5. Give the major products of the following reactions. goes Markovnikov via electrophilic reaction goes anti-Markovnikov via radical reaction rearranges to more stable carbocation goes anti-Mark via free radical, no rearrangement

23. II. Electrophilic Addition B. Addition of sulfuric acid (industrial) alkyl hydrogen sulfate overall: hydration Markovnikov orientation

24. II. Electrophilic Addition C. Acid-catalyzed hydration reverse of dehydration (Le Châtelier) Markovnikov Principle of microscopic reversibility

25. II. Electrophilic Addition D. Addition of halogens (X = Cl or Br) a vicinaldihalide Stereoselective anti addition: trans only

26. II. Electrophilic Addition D. Addition of halogens Mechanism: halonium ions cyclic bromonium ion trans product anti addition

27. II. Electrophilic Addition E. Formation of halohydrins vicinal halohydrin anti addition

28. II. Electrophilic Addition E. Formation of halohydrins Regioselective:

29. II. Electrophilic Addition E. Formation of halohydrins resonance hybrid: more substituted C carries greater d+, has stronger attraction for nucleophile

30. II. Electrophilic Addition E. Formation of halohydrins Other nucleophiles:

31. CheckAnswer II. Electrophilic Addition E. Formation of halohydrins Question 6-8. Give the products, showing stereochemistry where applicable. Click on the arrow to check your answers.

32. II. Electrophilic Addition E. Formation of halohydrins Answer 6-8. Give the products, showing stereochemistry where applicable. Click on the arrow to check your answers. Br- is nucleophile H2O is nucleophile Ethanol is nucleophile Interesting! Br+ is electrophile, since it is less electronegative; Cl- is nucleophile.

33. III. Other Reactions of Alkenes A. Hydroboration-oxidation a boron hydride an organoborane antiMarkovnikov orientation stereoselective syn addition no rearrangements )3 anti-Markovnikov syn addition

34. III. Other Reactions of Alkenes A. Hydroboration-oxidation Mechanism:

35. III. Other Reactions of Alkenes A. Hydroboration-oxidation Regiochemical control in synthesis: Markovnikov antiMarkovnikov

36. CheckAnswer III. Other Reactions of Alkenes A. Hydroboration-oxidation Question 6-9. Give the products of the following reactions. Click on the arrow to check your answers.

37. III. Other Reactions of Alkenes A. Hydroboration-oxidation Answer 6-9. Give the products of the following reactions. Click on the arrow to check your answers. anti-Markovnikov Markovnikov

38. III. Other Reactions of Alkenes B. Epoxidation epoxyethane (ethylene oxide) 1,2-epoxypropane (propylene oxide) 1,2-epoxycyclohexane (cyclohexene oxide)

39. III. Other Reactions of Alkenes B. Epoxidation Stereospecific syn addition: cis-2,3-epoxybutane trans-2,3-epoxybutane

40. III. Other Reactions of Alkenes C. Ozonolysis oxidative cleavage

41. CheckAnswer III. Other Reactions of Alkenes C. Ozonolysis Synthesis: Analysis: Question 6-10. What is the structure of the alkene if ozonolysis produced the following:

42. + + III. Other Reactions of Alkenes C. Ozonolysis Answer 6-10. What is the structure of the unknown alkene if ozonolysis produced the following: The fragments are colored to make it easier to see how they go together. The fragment with two carbonyls must contain two double bonds. The alkene is 2,7-dimethyl-2,4-octadiene. It is impossible to determine whether the diene is E or Z.

43. III. Other Reactions of Alkenes D. Polymerization n = 1000’s – 10,000’s monomer polymer radical initiators: O2 di-t-butyl peroxide benzoyl peroxide

44. III. Other Reactions of Alkenes D. Polymerization monomer polymer polyethylene polypropylene (“poly”) poly(vinyl chloride) (PVC, “vinyl”) polystyrene

45. III. Other Reactions of Alkenes D. Polymerization monomer polymer “Saran” “Orlon” polytetrafluoroethylene (PTFE, “Teflon”)

46. III. Other Reactions of Alkenes D. Polymerization Free-radical chain polymerization: Initiation Propagation “head to tail” polymerization

47. CheckAnswer Summary: Reactions of Alkenes Question 6-11. Draw the structures of the products in the following reactions, including stereochemistry where appropriate.

48. Summary: Reactions of Alkenes Answer 6-11. Draw the structures of the products in the following reactions, including stereochemistry where appropriate.

49. CheckAnswer Summary: Reactions of Alkenes Question 6-12. What reagents would be used to carry out the following conversions?

50. Summary: Reactions of Alkenes Answer 6-12. What reagents would be used to carry out the following conversions?