Elimination

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  • 09_02
  • Elimination

    1. 1. Organic Chemistry 6th Edition Chapter 9 Paula Yurkanis Bruice Elimination Reactions of Alkyl Halides Competition Between Substitution and Elimination 1 © 2011 Pearson Education, Inc.
    2. 2. In addition to substitution, an alkyl halide can undergo anelimination reaction 2 © 2011 Pearson Education, Inc.
    3. 3. The E2 Reaction• reaction and mechanism• kinetics• Stereochemistry• regioselectivity• substrate structure• Base• leaving groups• solvents © 2011 Pearson Education, Inc.
    4. 4. The E2 ReactionHydroxide cannot act as a nucleophile in this reactionbecause of the bulky tertiary halide. Rather, hydroxideacts as a base and abstracts a proton. 4 © 2011 Pearson Education, Inc.
    5. 5. An E2 reaction is also called a β-elimination or a 1,2-elimination reaction:The weaker the base, the better it is as a leaving group 5 © 2011 Pearson Education, Inc.
    6. 6. The removal of a proton and a halide ion is calleddehydrohalogenation: 6 © 2011 Pearson Education, Inc.
    7. 7. kinetics© 2011 Pearson Education, Inc.
    8. 8. stereochemistry © 2011 Pearson Education, Inc.
    9. 9. Stereochemistry of the E2 ReactionThe bonds to the eliminated groups (H and X) must bein the same planeThe anti elimination is favored over the syn elimination 9 © 2011 Pearson Education, Inc.
    10. 10. Another view of the E2 reaction mechanismThe best overlap of the interacting orbitals is achievedthrough back-side attackAnti elimination avoids repulsion of the electron-richbase 10 © 2011 Pearson Education, Inc.
    11. 11. Elimination from Cyclic CompoundsIn an E2 reaction, groups to be eliminated must be inaxial positions 11 © 2011 Pearson Education, Inc.
    12. 12. 12© 2011 Pearson Education, Inc.
    13. 13. 13© 2011 Pearson Education, Inc.
    14. 14. 14© 2011 Pearson Education, Inc.
    15. 15. The hydrogen that is removed and the leaving groupboth have to be in the axial position, therefore… 15 © 2011 Pearson Education, Inc.
    16. 16. 16© 2011 Pearson Education, Inc.
    17. 17. Elimination of 2-bromopentane © 2011 Pearson Education, Inc.
    18. 18. Consider the stereoselectivity of the E2 reactionThe alkene with the bulkiest groups on opposite sides of thedouble bond will be formed in greater yield, because it is the morestable alkene 18 © 2011 Pearson Education, Inc.
    19. 19. 19© 2011 Pearson Education, Inc.
    20. 20. Regioselectivity of the E2 Reaction © 2011 Pearson Education, Inc.
    21. 21. The Regioselectivity of the E2 ReactionThe major product of an E2 reaction is the most stablealkene: 21 © 2011 Pearson Education, Inc.
    22. 22. The Zaitsev RuleThe more substituted alkene product is obtained when aproton is removed from the β-carbon that is bondedto the fewest hydrogensThe most stable alkene is generally (but not always) themost substituted alkene 22 © 2011 Pearson Education, Inc.
    23. 23. 23© 2011 Pearson Education, Inc.
    24. 24. 24© 2011 Pearson Education, Inc.
    25. 25. Exception to Zaitsev’s rule © 2011 Pearson Education, Inc.
    26. 26. Based on Substrate © 2011 Pearson Education, Inc.
    27. 27. Conjugated alkene products are preferred over themore substituted alkene product: Do not use Zaitsev’s rule to predict the major product in these cases 27 © 2011 Pearson Education, Inc.
    28. 28. Based on Base © 2011 Pearson Education, Inc.
    29. 29. Bulky bases affect the product distribution resulting in the Hofmann product, the least substituted alkene: Hofmann product Zaitsev productHowever, it takes a lot of steric hindrance for the lessstable product to be the major product: 29 © 2011 Pearson Education, Inc.
    30. 30. 30© 2011 Pearson Education, Inc.
    31. 31. Based on leaving group © 2011 Pearson Education, Inc.
    32. 32. Another exception to Zaitsev’s rule 32 © 2011 Pearson Education, Inc.
    33. 33. Consider the elimination of 2-fluoropentane…A carbanion-like transition state 33 © 2011 Pearson Education, Inc.
    34. 34. E2 Reactions• Substrate : primary halides• Base: high concentration of strong base• Solvent : aprotic polar solvent © 2011 Pearson Education, Inc.
    35. 35. E1 Reactions© 2011 Pearson Education, Inc.
    36. 36. The E1 Reaction• reaction and mechanism• kinetics• Stereochemistry• Regioselectivity• Carbocation rearrangement © 2011 Pearson Education, Inc.
    37. 37. Reaction and mechanism © 2011 Pearson Education, Inc.
    38. 38. The E1 Reaction 38 © 2011 Pearson Education, Inc.
    39. 39. The E1 reaction, like the SN1 reaction, involves acarbocation intermediate: 39 © 2011 Pearson Education, Inc.
    40. 40. kineticsA unimolecular reaction: © 2011 Pearson Education, Inc.
    41. 41. stereochemistry © 2011 Pearson Education, Inc.
    42. 42. E1 Elimination from Cyclic Compounds No axial orientation required for the E1 reaction. 42 © 2011 Pearson Education, Inc.
    43. 43. Regioselectivity of the E1 Reaction © 2011 Pearson Education, Inc.
    44. 44. The major product in an E1 reaction is generally themore substituted alkene 44 © 2011 Pearson Education, Inc.
    45. 45. Stereochemistry of the E1 ReactionBoth syn and anti elimination can occur in an E1 reaction The major stereoisomer obtained from an E1 reaction is the alkene in which the bulkiest substituents are on opposite sides of the double bond 45 © 2011 Pearson Education, Inc.
    46. 46. Reaction coordinate diagram for the E1 reaction of2-chloro-2-methylbutane 46 © 2011 Pearson Education, Inc.
    47. 47. Carbocation rearrangement © 2011 Pearson Education, Inc.
    48. 48. Because the E1 reaction forms a carbocationintermediate, we need to consider carbocationrearrangement 48 © 2011 Pearson Education, Inc.
    49. 49. • 1-bromo 2,3 dimethyl cyclohexane © 2011 Pearson Education, Inc.
    50. 50. 50© 2011 Pearson Education, Inc.
    51. 51. Because the first step is the rate-determining step, the rate of anE1 reaction depends both on the ease with which the carbocationis formed and how readily the leaving group leaves 51 © 2011 Pearson Education, Inc.
    52. 52. Competition Between E2 and E1 ReactionsAn E2 is favored by a high concentration of strong base and anaprotic polar solventAn E1 is favored by a weak base and a protic polar solvent 52 © 2011 Pearson Education, Inc.
    53. 53. Competition Between Substitution and EliminationAlkyl halides can undergo SN2, SN1, E2, and E11) decide whether the reaction conditions favor SN2/E2 or SN1/E1 •SN2/E2 reactions are favored by a high concentration of a good nucleophile/strong base •SN1/E1 reactions are favored by a poor nucleophile/weak base2) decide how much of the product will be the substitution product and how much of the product will be the elimination product 53 © 2011 Pearson Education, Inc.
    54. 54. 54© 2011 Pearson Education, Inc.
    55. 55. 55© 2011 Pearson Education, Inc.
    56. 56. A bulky alkyl halide or a sterically hindered nucleophileencourages elimination over substitution 56 © 2011 Pearson Education, Inc.
    57. 57. A strong or a bulky base encourages elimination oversubstitution 57 © 2011 Pearson Education, Inc.
    58. 58. Tertiary alkyl halides undergo only elimination underSN2/E2 conditions: 58 © 2011 Pearson Education, Inc.
    59. 59. High temperature favors elimination over substitution:Why? Because elimination is entropically favorable. 59 © 2011 Pearson Education, Inc.
    60. 60. Synthesis Example 60 © 2011 Pearson Education, Inc.
    61. 61. Synthesis Example Racemic 61 © 2011 Pearson Education, Inc.
    62. 62. Synthesis Example 62 © 2011 Pearson Education, Inc.
    63. 63. Synthesis Example 63 © 2011 Pearson Education, Inc.
    64. 64. The E2 reaction should be used if you want tosynthesize an alkene: Hydroxide Water 64 © 2011 Pearson Education, Inc.
    65. 65. Consecutive E2 Elimination ReactionsRequires a strong base NaNH2. Reaction carried out in liquid ammonia. 65 © 2011 Pearson Education, Inc.

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