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. Organic Chemistry
6th Edition Chapter 9
Paula Yurkanis Bruice
Elimination Reactions
of Alkyl Halides
Competition Between
Substitution and
Elimination
1
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2. In addition to substitution, an alkyl halide can undergo an
elimination reaction
2
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3. The E2 Reaction
• reaction and mechanism
• kinetics
• Stereochemistry
• regioselectivity
• substrate structure
• Base
• leaving groups
• solvents
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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
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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
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6. The removal of a proton and a halide ion is called
dehydrohalogenation:
6
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7. kinetics
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8. stereochemistry
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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
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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
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11. Elimination from Cyclic Compounds
In an E2 reaction, groups to be eliminated must be in
axial positions
11
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12. 12
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13. 13
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14. 14
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15. The hydrogen that is removed and the leaving group
both have to be in the axial position, therefore…
15
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16. 16
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17. Elimination of 2-bromopentane
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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
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19. 19
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20. Regioselectivity of the E2 Reaction
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21. The Regioselectivity of the E2 Reaction
The major product of an E2 reaction is the most stable
alkene:
21
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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
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23. 23
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24. 24
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25. Exception to Zaitsev’s rule
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26. Based on Substrate
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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
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28. Based on Base
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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
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30. 30
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31. Based on leaving group
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32. Another exception to Zaitsev’s rule
32
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33. Consider the elimination of 2-fluoropentane…
A carbanion-like transition state
33
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34. E2 Reactions
• Substrate : primary halides
• Base: high concentration of strong base
• Solvent : aprotic polar solvent
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35. E1 Reactions
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36. The E1 Reaction
• reaction and mechanism
• kinetics
• Stereochemistry
• Regioselectivity
• Carbocation rearrangement
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37. Reaction and mechanism
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38. The E1 Reaction
38
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39. The E1 reaction, like the SN1 reaction, involves a
carbocation intermediate:
39
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40. kinetics
A unimolecular reaction:
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41. stereochemistry
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42. E1 Elimination from Cyclic Compounds
No axial orientation required for the E1 reaction.
42
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43. Regioselectivity of the E1 Reaction
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44. The major product in an E1 reaction is generally the
more substituted alkene
44
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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
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46. Reaction coordinate diagram for the E1 reaction of
2-chloro-2-methylbutane
46
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47. Carbocation rearrangement
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48. Because the E1 reaction forms a carbocation
intermediate, we need to consider carbocation
rearrangement
48
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49. • 1-bromo 2,3 dimethyl cyclohexane
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50. 50
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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
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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
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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
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54. 54
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55. 55
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56. A bulky alkyl halide or a sterically hindered nucleophile
encourages elimination over substitution
56
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57. A strong or a bulky base encourages elimination over
substitution
57
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58. Tertiary alkyl halides undergo only elimination under
SN2/E2 conditions:
58
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59. High temperature favors elimination over substitution:
Why? Because elimination is entropically favorable.
59
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60. Synthesis Example
60
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61. Synthesis Example
Racemic
61
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62. Synthesis Example
62
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63. Synthesis Example
63
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64. The E2 reaction should be used if you want to
synthesize an alkene:
Hydroxide
Water
64
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65. Consecutive E2 Elimination Reactions
Requires a strong base NaNH2. Reaction carried out in liquid ammonia.
65
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