ºÝºÝߣ

ºÝºÝߣShare a Scribd company logo

Elimination

•Download as PPT, PDF•
•
M
mondalakash2012

1) Alkyl halides can undergo elimination reactions (E1 and E2) in addition to substitution reactions. E2 reactions involve a concerted removal of the halide leaving group and a proton, while E1 reactions proceed through a carbocation intermediate. 2) The rate and mechanism of elimination reactions depend on factors like the structure of the substrate, the base used, and solvent. E2 favors less substituted alkene products following Zaitsev's rule, while E1 can form either stereoisomer and favors more substituted alkenes. 3) Conditions like strong base, aprotic solvent, and high temperature promote elimination over substitution. Tertiary alkyl halides undergo only elimination

1 of 65
Downloaded 534 times
Organic Chemistry 6th Edition Chapter 9 Paula Yurkanis Bruice Elimination Reactions of Alkyl Halides Competition Between Substitution and Elimination 1 © 2011 Pearson Education, Inc.
In addition to substitution, an alkyl halide can undergo an elimination reaction 2 © 2011 Pearson Education, Inc.
The E2 Reaction • reaction and mechanism • kinetics • Stereochemistry • regioselectivity • substrate structure • Base • leaving groups • solvents © 2011 Pearson Education, Inc.
The E2 Reaction Hydroxide cannot act as a nucleophile in this reaction because of the bulky tertiary halide. Rather, hydroxide acts as a base and abstracts a proton. 4 © 2011 Pearson Education, Inc.
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.
The removal of a proton and a halide ion is called dehydrohalogenation: 6 © 2011 Pearson Education, Inc.
kinetics © 2011 Pearson Education, Inc.
stereochemistry © 2011 Pearson Education, Inc.
Stereochemistry of the E2 Reaction The bonds to the eliminated groups (H and X) must be in the same plane The anti elimination is favored over the syn elimination 9 © 2011 Pearson Education, Inc.
Another view of the E2 reaction mechanism The best overlap of the interacting orbitals is achieved through back-side attack Anti elimination avoids repulsion of the electron-rich base 10 © 2011 Pearson Education, Inc.
Elimination from Cyclic Compounds In an E2 reaction, groups to be eliminated must be in axial positions 11 © 2011 Pearson Education, Inc.
12 © 2011 Pearson Education, Inc.
13 © 2011 Pearson Education, Inc.
14 © 2011 Pearson Education, Inc.
The hydrogen that is removed and the leaving group both have to be in the axial position, therefore… 15 © 2011 Pearson Education, Inc.
16 © 2011 Pearson Education, Inc.
Elimination of 2-bromopentane © 2011 Pearson Education, Inc.
Consider the stereoselectivity of the E2 reaction The alkene with the bulkiest groups on opposite sides of the double bond will be formed in greater yield, because it is the more stable alkene 18 © 2011 Pearson Education, Inc.
19 © 2011 Pearson Education, Inc.
Regioselectivity of the E2 Reaction © 2011 Pearson Education, Inc.
The Regioselectivity of the E2 Reaction The major product of an E2 reaction is the most stable alkene: 21 © 2011 Pearson Education, Inc.
The Zaitsev Rule The more substituted alkene product is obtained when a proton is removed from the β-carbon that is bonded to the fewest hydrogens The most stable alkene is generally (but not always) the most substituted alkene 22 © 2011 Pearson Education, Inc.
23 © 2011 Pearson Education, Inc.
24 © 2011 Pearson Education, Inc.
Exception to Zaitsev’s rule © 2011 Pearson Education, Inc.
Based on Substrate © 2011 Pearson Education, Inc.
Conjugated alkene products are preferred over the more substituted alkene product: Do not use Zaitsev’s rule to predict the major product in these cases 27 © 2011 Pearson Education, Inc.
Based on Base © 2011 Pearson Education, Inc.
Bulky bases affect the product distribution resulting in the Hofmann product, the least substituted alkene: Hofmann product Zaitsev product However, it takes a lot of steric hindrance for the less stable product to be the major product: 29 © 2011 Pearson Education, Inc.
30 © 2011 Pearson Education, Inc.
Based on leaving group © 2011 Pearson Education, Inc.
Another exception to Zaitsev’s rule 32 © 2011 Pearson Education, Inc.
Consider the elimination of 2-fluoropentane… A carbanion-like transition state 33 © 2011 Pearson Education, Inc.
E2 Reactions • Substrate : primary halides • Base: high concentration of strong base • Solvent : aprotic polar solvent © 2011 Pearson Education, Inc.
E1 Reactions © 2011 Pearson Education, Inc.
The E1 Reaction • reaction and mechanism • kinetics • Stereochemistry • Regioselectivity • Carbocation rearrangement © 2011 Pearson Education, Inc.
Reaction and mechanism © 2011 Pearson Education, Inc.
The E1 Reaction 38 © 2011 Pearson Education, Inc.
The E1 reaction, like the SN1 reaction, involves a carbocation intermediate: 39 © 2011 Pearson Education, Inc.
kinetics A unimolecular reaction: © 2011 Pearson Education, Inc.
stereochemistry © 2011 Pearson Education, Inc.
E1 Elimination from Cyclic Compounds No axial orientation required for the E1 reaction. 42 © 2011 Pearson Education, Inc.
Regioselectivity of the E1 Reaction © 2011 Pearson Education, Inc.
The major product in an E1 reaction is generally the more substituted alkene 44 © 2011 Pearson Education, Inc.
Stereochemistry of the E1 Reaction Both 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.
Reaction coordinate diagram for the E1 reaction of 2-chloro-2-methylbutane 46 © 2011 Pearson Education, Inc.
Carbocation rearrangement © 2011 Pearson Education, Inc.
Because the E1 reaction forms a carbocation intermediate, we need to consider carbocation rearrangement 48 © 2011 Pearson Education, Inc.
• 1-bromo 2,3 dimethyl cyclohexane © 2011 Pearson Education, Inc.
50 © 2011 Pearson Education, Inc.
Because the first step is the rate-determining step, the rate of an E1 reaction depends both on the ease with which the carbocation is formed and how readily the leaving group leaves 51 © 2011 Pearson Education, Inc.
Competition Between E2 and E1 Reactions An E2 is favored by a high concentration of strong base and an aprotic polar solvent An E1 is favored by a weak base and a protic polar solvent 52 © 2011 Pearson Education, Inc.
Competition Between Substitution and Elimination Alkyl halides can undergo SN2, SN1, E2, and E1 1) 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 base 2) 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 © 2011 Pearson Education, Inc.
55 © 2011 Pearson Education, Inc.
A bulky alkyl halide or a sterically hindered nucleophile encourages elimination over substitution 56 © 2011 Pearson Education, Inc.
A strong or a bulky base encourages elimination over substitution 57 © 2011 Pearson Education, Inc.
Tertiary alkyl halides undergo only elimination under SN2/E2 conditions: 58 © 2011 Pearson Education, Inc.
High temperature favors elimination over substitution: Why? Because elimination is entropically favorable. 59 © 2011 Pearson Education, Inc.
Synthesis Example 60 © 2011 Pearson Education, Inc.
Synthesis Example Racemic 61 © 2011 Pearson Education, Inc.
Synthesis Example 62 © 2011 Pearson Education, Inc.
Synthesis Example 63 © 2011 Pearson Education, Inc.
The E2 reaction should be used if you want to synthesize an alkene: Hydroxide Water 64 © 2011 Pearson Education, Inc.
Consecutive E2 Elimination Reactions Requires a strong base NaNH2. Reaction carried out in liquid ammonia. 65 © 2011 Pearson Education, Inc.

More Related Content

Elimination

  • 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. In addition to substitution, an alkyl halide can undergo an elimination reaction 2 © 2011 Pearson Education, Inc.
  • 3. The E2 Reaction • reaction and mechanism • kinetics • Stereochemistry • regioselectivity • substrate structure • Base • leaving groups • solvents © 2011 Pearson Education, Inc.
  • 4. The E2 Reaction Hydroxide cannot act as a nucleophile in this reaction because of the bulky tertiary halide. Rather, hydroxide acts as a base and abstracts a proton. 4 © 2011 Pearson Education, Inc.
  • 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. The removal of a proton and a halide ion is called dehydrohalogenation: 6 © 2011 Pearson Education, Inc.
  • 7. kinetics © 2011 Pearson Education, Inc.
  • 8. stereochemistry © 2011 Pearson Education, Inc.
  • 9. Stereochemistry of the E2 Reaction The bonds to the eliminated groups (H and X) must be in the same plane The anti elimination is favored over the syn elimination 9 © 2011 Pearson Education, Inc.
  • 10. Another view of the E2 reaction mechanism The best overlap of the interacting orbitals is achieved through back-side attack Anti elimination avoids repulsion of the electron-rich base 10 © 2011 Pearson Education, Inc.
  • 11. Elimination from Cyclic Compounds In an E2 reaction, groups to be eliminated must be in axial positions 11 © 2011 Pearson Education, Inc.
  • 12. 12 © 2011 Pearson Education, Inc.
  • 13. 13 © 2011 Pearson Education, Inc.
  • 14. 14 © 2011 Pearson Education, Inc.
  • 15. The hydrogen that is removed and the leaving group both have to be in the axial position, therefore… 15 © 2011 Pearson Education, Inc.
  • 16. 16 © 2011 Pearson Education, Inc.
  • 17. Elimination of 2-bromopentane © 2011 Pearson Education, Inc.
  • 18. Consider the stereoselectivity of the E2 reaction The alkene with the bulkiest groups on opposite sides of the double bond will be formed in greater yield, because it is the more stable alkene 18 © 2011 Pearson Education, Inc.
  • 19. 19 © 2011 Pearson Education, Inc.
  • 20. Regioselectivity of the E2 Reaction © 2011 Pearson Education, Inc.
  • 21. The Regioselectivity of the E2 Reaction The major product of an E2 reaction is the most stable alkene: 21 © 2011 Pearson Education, Inc.
  • 22. The Zaitsev Rule The more substituted alkene product is obtained when a proton is removed from the β-carbon that is bonded to the fewest hydrogens The most stable alkene is generally (but not always) the most substituted alkene 22 © 2011 Pearson Education, Inc.
  • 23. 23 © 2011 Pearson Education, Inc.
  • 24. 24 © 2011 Pearson Education, Inc.
  • 25. Exception to Zaitsev’s rule © 2011 Pearson Education, Inc.
  • 26. Based on Substrate © 2011 Pearson Education, Inc.
  • 27. Conjugated alkene products are preferred over the more substituted alkene product: Do not use Zaitsev’s rule to predict the major product in these cases 27 © 2011 Pearson Education, Inc.
  • 28. Based on Base © 2011 Pearson Education, Inc.
  • 29. Bulky bases affect the product distribution resulting in the Hofmann product, the least substituted alkene: Hofmann product Zaitsev product However, it takes a lot of steric hindrance for the less stable product to be the major product: 29 © 2011 Pearson Education, Inc.
  • 30. 30 © 2011 Pearson Education, Inc.
  • 31. Based on leaving group © 2011 Pearson Education, Inc.
  • 32. Another exception to Zaitsev’s rule 32 © 2011 Pearson Education, Inc.
  • 33. Consider the elimination of 2-fluoropentane… A carbanion-like transition state 33 © 2011 Pearson Education, Inc.
  • 34. E2 Reactions • Substrate : primary halides • Base: high concentration of strong base • Solvent : aprotic polar solvent © 2011 Pearson Education, Inc.
  • 35. E1 Reactions © 2011 Pearson Education, Inc.
  • 36. The E1 Reaction • reaction and mechanism • kinetics • Stereochemistry • Regioselectivity • Carbocation rearrangement © 2011 Pearson Education, Inc.
  • 37. Reaction and mechanism © 2011 Pearson Education, Inc.
  • 38. The E1 Reaction 38 © 2011 Pearson Education, Inc.
  • 39. The E1 reaction, like the SN1 reaction, involves a carbocation intermediate: 39 © 2011 Pearson Education, Inc.
  • 40. kinetics A unimolecular reaction: © 2011 Pearson Education, Inc.
  • 41. stereochemistry © 2011 Pearson Education, Inc.
  • 42. E1 Elimination from Cyclic Compounds No axial orientation required for the E1 reaction. 42 © 2011 Pearson Education, Inc.
  • 43. Regioselectivity of the E1 Reaction © 2011 Pearson Education, Inc.
  • 44. The major product in an E1 reaction is generally the more substituted alkene 44 © 2011 Pearson Education, Inc.
  • 45. Stereochemistry of the E1 Reaction Both 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. Reaction coordinate diagram for the E1 reaction of 2-chloro-2-methylbutane 46 © 2011 Pearson Education, Inc.
  • 47. Carbocation rearrangement © 2011 Pearson Education, Inc.
  • 48. Because the E1 reaction forms a carbocation intermediate, we need to consider carbocation rearrangement 48 © 2011 Pearson Education, Inc.
  • 49. • 1-bromo 2,3 dimethyl cyclohexane © 2011 Pearson Education, Inc.
  • 50. 50 © 2011 Pearson Education, Inc.
  • 51. Because the first step is the rate-determining step, the rate of an E1 reaction depends both on the ease with which the carbocation is formed and how readily the leaving group leaves 51 © 2011 Pearson Education, Inc.
  • 52. Competition Between E2 and E1 Reactions An E2 is favored by a high concentration of strong base and an aprotic polar solvent An E1 is favored by a weak base and a protic polar solvent 52 © 2011 Pearson Education, Inc.
  • 53. Competition Between Substitution and Elimination Alkyl halides can undergo SN2, SN1, E2, and E1 1) 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 base 2) 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 © 2011 Pearson Education, Inc.
  • 55. 55 © 2011 Pearson Education, Inc.
  • 56. A bulky alkyl halide or a sterically hindered nucleophile encourages elimination over substitution 56 © 2011 Pearson Education, Inc.
  • 57. A strong or a bulky base encourages elimination over substitution 57 © 2011 Pearson Education, Inc.
  • 58. Tertiary alkyl halides undergo only elimination under SN2/E2 conditions: 58 © 2011 Pearson Education, Inc.
  • 59. High temperature favors elimination over substitution: Why? Because elimination is entropically favorable. 59 © 2011 Pearson Education, Inc.
  • 60. Synthesis Example 60 © 2011 Pearson Education, Inc.
  • 61. Synthesis Example Racemic 61 © 2011 Pearson Education, Inc.
  • 62. Synthesis Example 62 © 2011 Pearson Education, Inc.
  • 63. Synthesis Example 63 © 2011 Pearson Education, Inc.
  • 64. The E2 reaction should be used if you want to synthesize an alkene: Hydroxide Water 64 © 2011 Pearson Education, Inc.
  • 65. Consecutive E2 Elimination Reactions Requires a strong base NaNH2. Reaction carried out in liquid ammonia. 65 © 2011 Pearson Education, Inc.

Editor's Notes