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Chapter 9

Chapter 9. Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination. Substitution versus elimination. E2 reaction. Bimolecular Elimination Reaction. Concerted elimination. b -elimination. Regioselectivities in E2 Reactions. Reaction coordinate diagram.

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Chapter 9

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  1. Chapter 9 Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination

  2. Substitution versus elimination

  3. E2 reaction Bimolecular Elimination Reaction

  4. Concerted elimination b-elimination

  5. Regioselectivities in E2 Reactions

  6. Reaction coordinate diagram less stable more stable

  7. Zaitsev Rule : the more substituted alkene is obtained when a proton is removed from the b-carbon that is bonded to the fewest hydrogens

  8. Effect of the steric properties of the base

  9. Reaction of 2-fluoropentane with methoxide ion Poor leaving tendency of fluoride ion → carbanion-like TS Less substituted carbanion is more stable. Exceptions to Zaitsev

  10. hyperconjugation Inductive effect

  11. The E1 Reaction first-order elimination reaction unimolecular elimination reaction

  12. Follow Zaitsev Rule

  13. Reaction coordinate diagram

  14. Rearrangement may occur in carbocation Intermediates

  15. Reversibility elimination The elimination requires a base to remove a proton from the carbocation to form the alkene

  16. Propose a mechanism

  17. Elimination reactions are regioselective and stereoselective.

  18. Conformations suitable for elimination much slower much faster 대부분의 E2 반응은 anti elimination

  19. An E2 reaction is stereoselective

  20. Reaction coordinate diagram

  21. If only one b-hydrogen is present, then only one conformer is formed.

  22. Elimination of 3-chloro-3,4-dimethylhexane in E1 condition

  23. Elimination reactions of substituted cyclohexanes anti elimination

  24. The reaction is faster if the Keq is large and slower if Keq is small rate = k’ Keq [more stable] [OH-]

  25. E2 condition

  26. E2 condition : strong base The hydrogen that is removed from the b-carbon must be in the axial position.

  27. E1 condition : weak base

  28. E1 condition : weak base

  29. Deuterium kinetic isotope effect The rate kH is 7.1 times greater than kD

  30. Competition Between Substitution and Elimination SN2 and E2 reactions occur under similar conditions. Both use high concentrations of a good nucleophile or strong base.

  31. Competition Between Substitution and Elimination

  32. Primary alkyl halides under SN2 and E2 conditions

  33. Secondary alkyl halide under SN2/E2 conditions.

  34. The relative amounts of substitution and elimination products are affected by temperature A higher temperature favors elimination

  35. Tertiary alkyl halides under SN2/E2 conditions.

  36. SN1 versus E1 conditions Page 416, E2 SN1, E1 conditions SN2, E2 conditions E2

  37. Substitution and Elimination Reaction Reactions in Organic Synthesis Formation of alkoxide ions

  38. Williamson ether synthesis SN2 Preparation of butyl propyl ether

  39. Williamson ether synthesis SN2, E2 E2

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