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Chapter 4 Reactions of Alkenes

Chapter 4 Reactions of Alkenes. Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee, Case Western Reserve University. Electrophilic Additions: Alkenes. Addition of Hydrogen Halides. What is the product?. Carbocation formation is the rate-limiting step.

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Chapter 4 Reactions of Alkenes

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

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

  12. Markovnikov’s Rule The electrophile adds to the sp2 carbon that is bonded to the greater number of hydrogens In a regioselective reaction, one constitutional isomer is the major or the only product.

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

  14. Regioselectivity ofHydrogen Halide Addition:Markovnikov's Rule

  15. Markovnikov's Rule 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.

  16. CH3CH2CH CH3CH2CHCH3 CH2 Br Markovnikov's Rule HBr acetic acid (80%) Example 1

  17. CH3 CH3 CH3 C C C Br Markovnikov's Rule CH3 H HBr acetic acid CH3 H (90%) Example 2

  18. CH3 CH3 Cl Markovnikov's Rule HCl 0°C (100%) Example 3

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

  20. HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br Mechanistic Basis for Markovnikov's Rule:Example 1

  21. + CH3CH2CH—CH3 + Br – CH3CH2CH CH3CH2CHCH3 CH2 Br Mechanistic Basis for Markovnikov's Rule:Example 1 HBr

  22. + CH3CH2CH—CH3 + Br – CH3CH2CH CH3CH2CHCH3 CH2 Br Mechanistic Basis for Markovnikov's Rule:Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed HBr

  23. H CH3 HCl CH3 Cl 0°C Mechanistic Basis for Markovnikov's Rule:Example 3

  24. H H CH3 + H CH3 CH3 Cl Mechanistic Basis for Markovnikov's Rule:Example 3 Cl – HCl

  25. H H + CH3 H CH3 CH3 Cl Mechanistic Basis for Markovnikov's Rule:Example 3 secondary carbocation is less stable: not formed H H Cl – CH3 + HCl

  26. Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

  27. CHCH(CH3)2 H2C + CH3CHCH(CH3)2 CH3CH2C(CH3)2 CH3CHCH(CH3)2 Cl Cl Rearrangements sometimes occur HCl, 0°C H + CH3CHC(CH3)2 (40%) (60%)

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

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

  30. Carbocation Rearrangement Ring Expansion a more stable carbocation

  31. Carbocation does not always rearrange …

  32. Addition of Halogens to Alkene

  33. Addition of Water to Alkene (alcohols)

  34. Acid-Catalyzed Addition of Alcohol (ethers)

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

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

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

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

  39. 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.)

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

  41. Carbene: another reactive intermediate Reaction with an Alkene

  42. Synthesis of Bromobutane Isomers

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

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

  45. Relative Stabilities of Alkyl Radicals

  46. Addition of Hydrogen to Alkenes

  47. Catalytic Hydrogenation of an Alkene

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