1. Chapter 4 Reactions of Alkenes Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee, Case Western Reserve University

2. Electrophilic Additions: Alkenes

3. Addition of Hydrogen Halides

4. What is the product?

5. Which carbocation is more stable? Carbocation formation is the rate-limiting step

6. Carbocation Stabilities

7. Alkyl groups decrease the concentration of positive charge in the carbocation

8. Delocalization of Electrons

9. Molecular Orbital Diagram in a Hyperconjugation System

10. Hammond postulate: the transition state will be more similar to the species that it is closer to energetically Exergonic reaction: early transition state resembles reactants (I). Endergonic reaction: late transition state resembles products (II).

11. I: early transition state (Like reactants) II: mid-transition stateIII: later transition state (Like products)

12. In a regioselective reaction, one constitutional isomer is the major or the only product. Markovnikov’s Rule The electrophile adds to the sp 2 carbon that is bonded to the greater number of hydrogens

13. Explained by the intermediates, for example: tert-butyl cation is formed faster and it is more stable than isobutyl.

14. Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule

15. When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents. Markovnikov's Rule

16. acetic acid Br CH 3 CH 2 CHCH 3 Example 1 Markovnikov's Rule CH 2 CH 3 CH 2 CH HBr (80%)

17. CH 3 C Br (90%) C C Markovnikov's Rule Example 2 acetic acid HBr CH 3 H H

18. 0°C Cl (100%) Markovnikov's Rule Example 3 HCl

19. Mechanistic Basis for Markovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations.

20. Br CH 3 CH 2 CHCH 3 Mechanistic Basis for Markovnikov's Rule: Example 1 CH 2 CH 3 CH 2 CH acetic acid HBr

21. Br CH 3 CH 2 CHCH 3 Mechanistic Basis for Markovnikov's Rule: Example 1 CH 2 CH 3 CH 2 CH HBr CH 3 CH 2 CH—CH 3 + Br – +

22. Br CH 3 CH 2 CHCH 3 Mechanistic Basis for Markovnikov's Rule: Example 1 CH 2 CH 3 CH 2 CH HBr CH 3 CH 2 CH—CH 3 + Br – + CH 3 CH 2 CH 2 —CH 2 + primary carbocation is less stable: not formed

23. Cl CH 3 H Mechanistic Basis for Markovnikov's Rule: Example 3 0°CHCl

24. Cl CH 3 H HH + Cl – Mechanistic Basis for Markovnikov's Rule: Example 3 HCl

25. Cl CH 3 H H H + +H H Cl – Mechanistic Basis for Markovnikov's Rule: Example 3 HCl secondary carbocation is less stable: not formed

27. Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

28. HCl, 0°C CH 3 CHCH(CH 3 ) 2 + Cl (40%) CH 3 CHC(CH 3 ) 2 + H CH 3 CH 2 C(CH 3 ) 2 Cl (60%) Rearrangements sometimes occur H2CH2CH2CH2C CHCH(CH 3 ) 2

29. Rearrangement of Carbocation 1,2-hydride shift a more stable carbocation

30. a more stable carbocation Rearrangement of Carbocation 1,2-methyl shift

31. Carbocation Rearrangement a more stable carbocation Ring Expansion

32. Carbocation does not always rearrange …

33. Addition of Halogens to Alkene

34. Addition of Water to Alkene (alcohols)

35. Acid-Catalyzed Addition of Alcohol (ethers)

36. Addition of Halogens in the Presence of Water (halohydrins)

37. Oxymercuration and Mercuration of Alkene (alcohols w/o carbocation rearrangement)

38. Addition of Borane Hydroboration–Oxidation Anti-Markovnikov’s rule in product formation (less substituted alcohols) Vs. Markovnikov’s rule in product formation (more substituted alcohols)

39. Formation of Alkyl Boranes Anti-Markovnikov Addition Boron adds to least hindered carbon

41. Anti-Markovnikov Addition Boron adds to least hindered carbon and is replaced w/ -OH by oxidation Markovnikov Addition Formation of the most stable carbocation (A type of pericyclic reaction; important reaction and mechanism in directing reactions both regio- and stereo- selectively.)

42. Examples of Anti-Markovnikov Addition of an OH Group

43. Carbene: another reactive intermediate Reaction with an Alkene

44. Synthesis of Bromobutane Isomers

45. Generation of Free Radicals Using 1/2 arrows for the movement of one electron

46. Addition of Radicals to Alkenes Initiation  Propagation  Termination

47. Relative Stabilities of Alkyl Radicals

48. Addition of Hydrogen to Alkenes

49. Catalytic Hydrogenation of an Alkene