1. Alkenes: Structure and Preparation via Elimination Reactions
Organic Chemistry
Second Edition
David Klein
Chapter 8
Alkenes: Structure and Preparation via Elimination Reactions
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Klein, Organic Chemistry 2e

2. 8.1 Introduction to Elimination
Elimination reactions often compete with substitution reactions.
What are the two main ingredients for a substitution?
A nucleophile and an electrophile with a leaving group
What are the two main ingredients for an elimination?
A base and an electrophile with a leaving group
How is a base both similar and different from a nucleophile?
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Klein, Organic Chemistry 2e

3. 8.1 Introduction to Elimination
Consider –OH, which can act as a base or a nucleophile
Attack at the α Carbon
ALKENE
β or 1,2
Reaction at the β Hydrogen
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Klein, Organic Chemistry 2e

4. 8.3 Alkene Nomenclature
Alkenes are named using the same procedure we used in Chapter 4 to name alkanes with minor modifications
Identify the parent chain, which should include the C=C double bond
Identify and Name the substituents
Assign a locant (and prefix if necessary) to each substituent. Give the C=C double bond the lowest number possible
List the numbered substituents before the parent name in numerical order.
The C=C double bond locant is placed either just before the parent name or just before the -ene suffix
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Klein, Organic Chemistry 2e

5. 8.3 Alkene Nomenclature
Identify the parent chain, which should include the C=C double bond
The name of the parent chain should end in -ene rather than –ane
The parent chain should include the C=C double bond
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Klein, Organic Chemistry 2e

6. 8.3 Alkene Nomenclature Identify and Name the substituents
Assign a locant (and prefix if necessary) to each substituent. Give the C=C double bond the lowest number possible
The locant is ONE number, NOT two. Although the double bond bridges carbons 2 and 3, the locant is the lower of those two numbers
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

7. 8.3 Alkene Nomenclature
List the numbered substituents before the parent name in numerical order.
The C=C double bond locant is placed either just before the parent name or just before the -ene suffix
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Klein, Organic Chemistry 2e

9. 8.3 Alkene Nomenclature Name the following molecule
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10. 8.4 Alkene Isomerism
For the pi bond to remain intact, rotation around a double bond is prohibited
As a result, cis and trans structures are not identical
Is there a difference between cis-butane and trans-butane?
What specific type of isomers are cis and trans butene?
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Klein, Organic Chemistry 2e

11. 8.4 Alkene Isomerism
In cyclic alkenes with less than 8 atoms in the ring, only cis alkenes are stable. WHY?
Draw the structure for trans-cyclooctene
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Klein, Organic Chemistry 2e

12. 8.4 Alkene Isomerism Apply Bredt’s rule to the compounds below
The carbons of the C=C double bond and the atoms that are directly attached to them must be planar to maintain the pi bond overlap.
A handheld model can be used to help visualize the p orbital overlap and resulting geometry
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Klein, Organic Chemistry 2e

13. 8.4 Alkene Isomerism
Cis and trans modifiers are strictly used to describe C=C double bonds with identical groups on each carbon. Where are the identical groups in trans-2-pentene?
For molecules with different groups attached to the C=C double bond, the E/Z notation is used instead of cis/trans notation
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

14. 8.4 Alkene Isomerism Assigning E or Z to a stereoisomers
prioritize the groups attached to the C=C double bond based on atomic number
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Klein, Organic Chemistry 2e

15. 8.4 Alkene Isomerism Assigning E or Z to a stereoisomers
prioritize the groups attached to the C=C double bond based on atomic number
If the top priority groups are on the same side of the C=C double bond, it is Z (for zussamen, which means together)
If the top priority groups are on opposite sides of the C=C double bond, it is E (for entgegen, which means opposite)
Practice with SkillBuilder 8.2
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Klein, Organic Chemistry 2e

17. 8.5 Alkene Stability
Because of steric strain, cis isomers are generally less stable than trans
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Klein, Organic Chemistry 2e

18. 8.5 Alkene Stability Consider the following stability trend
What pattern do you see?
Practice with SkillBuilder 8.3
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Klein, Organic Chemistry 2e

20. 8.5 Alkene Stability
List the following molecules in order of increasing stability
2,3,4-trimethyl-1,3-pentadiene
2-isopropyl-1,4-pentadiene
3,3-dimethyl-1,5-hexadiene
4,5-dimethylcyclohexene
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Klein, Organic Chemistry 2e

22. 8.6 Elimination Reactions in Detail
In general, a H atom and a leaving group are eliminated
To understand the mechanism of elimination, first recall the 4 mechanistic steps we learned in chapter 7
Nucleophilic attack
Loss of a leaving group
Proton transfer
Rearrangement
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Klein, Organic Chemistry 2e

23. 8.6 Elimination Reactions in Detail
The 4 mechanistic steps we learned in chapter 7
Which of the 4 steps MUST take place in every elimination mechanism?
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Klein, Organic Chemistry 2e

24. 8.6 Elimination Reactions in Detail
All elimination reactions involve both loss of a leaving group and proton transfer
The mechanism may be a concerted (one step) process or a step-wise process. Which process is shown below?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

25. 8.6 Elimination Reactions in Detail
All elimination reactions involve both Loss of a leaving group and proton transfer
The mechanism of the step-wise process:
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

28. 8.7 Elimination by E2
The E2 mechanism below matches the observed rate law. Write a reasonable rate law for the mechanism
How will a change in [base] or [substrate] affect the reaction rate?
What do the E and the 2 of the E2 notation represent?
Practice with conceptual checkpoint 8.13
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Klein, Organic Chemistry 2e

29. 8.7 The Effect of Substrate on E2
The kinetics of E2 and SN2 are quite similar. WHY?
However, tertiary substrates are unreactive toward SN2 while they react readily by E2. WHY?
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Klein, Organic Chemistry 2e

30. 8.7 The Effect of Substrate on E2
3° substrates are more reactive toward E2 than are 1° substrates even though 1° substrates are less hindered
The 3° substrate should proceed through a more stable transition state (kinetically favored) and a more stable product (thermodynamically favored). Let’s take a look at the energy diagram – see next slide
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Klein, Organic Chemistry 2e

31. 8.7 Regioselectivity of E2
If there are multiple reactive sites or regions on a molecule, multiple products are possible
In elimination reactions, there are often different β sites that could be deprotonated to yield different alkenes
What is the relationship between the alkene products?
Regioselectivity occurs when one constitutional product is formed predominantly over the other
Zaitsev product
Hofmann product
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Klein, Organic Chemistry 2e

32. 8.7 Regioselectivity of E2
The identity of the base can affect the regioselesctivity
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Klein, Organic Chemistry 2e

33. 8.7 Regioselectivity of E2
Why does the Zaitsev product predominate when a base that is NOT sterically hindered is used?
Is the Zaitsev product kinetically favored, thermodynamically favored, or both?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

34. 8.7 Regioselectivity of E2
Why does a sterically hindered base favor the Hofmann product?
Sterically hindered bases (sometimes called non-nucleophilic) are useful in many reactions
Practice with SkillBuilder 8.5
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Klein, Organic Chemistry 2e

36. 8.7 Stereoselectivity of E2
Consider the dehydrohalogenation (elimination of a hydrogen and a halogen) of 3-bromopentane
Why are both the transition state and product more stable for the trans product?
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Klein, Organic Chemistry 2e

37. 8.7 Stereospecificity of E2
What is the difference between stereoselective and stereospecific?
Consider dehydrohalogenation for the molecule below
There is only one available β Hydrogen to be eliminated
You might imagine that it would be possible to form both the E and Z alkene products from this reaction. Draw both the E and the Z products
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Klein, Organic Chemistry 2e

38. 8.7 Stereospecificity of E2
When the reaction is actually performed, only the E product is observed
Is the E formed exclusively because it is formed through a slightly lower transition state?
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Klein, Organic Chemistry 2e

39. 8.7 Stereospecificity of E2
To rationalize the stereospecificty of the reaction, consider the transition state for the reaction
In the transition state, the C-H and C-Br bonds that are breaking must be rotated into the same plane as the pi bond that is forming
Draw the transition state structure illustrating the coplanar geometry
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Klein, Organic Chemistry 2e

40. 8.7 Stereospecificity of E2
There are two coplanar options for the molecule
Why does the reaction proceed exclusively through the Anti coplanar structure?
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Klein, Organic Chemistry 2e

41. 8.7 Stereospecificity of E2
To see the difference between Anti and Syn, Newman projections and hand held models can be helpful
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Klein, Organic Chemistry 2e

42. 8.7 Stereospecificity of E2
Evidence suggests that a strict 180° angle is not necessary for E2 mechanisms.
Similar angles (175–179°) are sufficient
The term, anti-periplanar is generally used instead of anti-coplanar to account for slight deviations from coplanarity
Although the E isomer is usually more stable because it is less sterically hindered, the requirement for an anti-periplanar transition state can often lead to the less stable Z isomer
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Klein, Organic Chemistry 2e

43. 8.7 Stereospecificity of E2
Assuming they proceed through an anti-periplanar transition state, predict the products for the following reactions, and label them as cis or trans
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Klein, Organic Chemistry 2e

45. 8.7 Stereospecificity of E2
Consider the dehydrohalogenation of a cyclohexane
Given the anti-periplanar requirement, which of the two possible chair conformations will allow for the elimination to occur?
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Klein, Organic Chemistry 2e

46. 8.7 Stereospecificity of E2
Which of the two molecules below will NOT be able to undergo an E2 elimination reaction? WHY?
It might be helpful to draw their chair structures and make a handheld model
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

47. 8.7 Stereospecificity of E2
Draw all of the possible products if each of the molecules below were to undergo dehydrohalogenation
Practice with conceptual checkpoint 8.20
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Klein, Organic Chemistry 2e

48. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

49. Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

50. Klein, Organic Chemistry 2e
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

51. 8.8 Predicting Products for E2
Consider both regioselestivity and stereoselectivity to predict the products for the eliminations below, and draw complete mechanisms
Practice with SkillBuilder 8.7
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Klein, Organic Chemistry 2e

52. 8.9 The E1 Mechanism The E1 mechanism is a 2-step process
Similar to SN1 (chapter 7), the reaction rate for E1 is not affected by the concentration of the base
What does the E and the 1 stand for in the E1 notation?
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Klein, Organic Chemistry 2e

53. 8.9 The E1 Mechanism
Given the rate law for E1, which step in the mechanism is the rate-determining slow step?
If the second step were the slow step, how would you write the rate law?
Practice with conceptual checkpoint 8.26
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Klein, Organic Chemistry 2e

54. 8.9 The Effect of Substrate on E1
How does the substrate reactivity trend for E1 compare to the trend we discussed in chapter 7 for SN1? WHY?
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Klein, Organic Chemistry 2e

55. 8.9 The Effect of Substrate on E1
Because E1 and SN1 proceed by the same first step, their competition will generally result in a mixture of products
How might you promote one reaction
over the other?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

56. 8.9 The Effect of Substrate on E1
Alcohols can also undergo elimination or dehydration by E1, but the –OH group is not a stable leaving group
In the E1 reaction below, once the water leaving group leaves the carbocation, what base should be used to complete the elimination?
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Klein, Organic Chemistry 2e

57. 8.9 Regioselectivity for E1
The final step of the E1 mechanism determines the regioselectivity
E1 reactions generally produce the Zaitsev product predominantly. WHY?
Why can’t we control the regioselectivity in this reaction like we can in and E2 reaction?
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Klein, Organic Chemistry 2e

58. 8.9 Stereoselectivity for E1
In the last step of the mechanism, a proton is removed from a β carbon adjacent to the sp2 hybridized carbocation
Draw the appropriate carbocation that forms in the reaction below, and rationalize the product distribution
Practice with conceptual checkpoint 8.31
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Klein, Organic Chemistry 2e

59. Klein, Organic Chemistry 2e
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Klein, Organic Chemistry 2e

60. 8.10 Complete E1 Mechanisms Recall the similarities between SN1 and E1
After the carbocation is formed and possibly rearranged, the E1 proton transfer neutralizes the charge
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Klein, Organic Chemistry 2e

61. 8.10 Complete E1 Mechanisms
The mechanism shows the formation of the major products
Predict the minor elimination products as well
Practice with SkillBuilder 8.9
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

63. 8.11 Complete E2 Mechanisms
In E2, the base removes the β proton as the LG leaves
Will such a reaction require a relatively strong base?
Will E2 dehydrations likely involve a proton transfer prior to the elimination?
Practice with conceptual checkpoint 8.37
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

64. 8.12 Substitution vs. Elimination
Substitution and Elimination are always in competition
Sometimes products are only observed from S or E
Sometimes a mixture of products is observed
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Klein, Organic Chemistry 2e

65. 8.12 Substitution vs. Elimination
To predict whether substitution or elimination will predominate, consider the factors below
Determine the function of the reagent. Is it more likely to act as a base, a nucleophile, or both?
Analyze the substrate and predict the expected mechanism (SN1, SN2, E1, or E2)
Consider relevant regiochemical and stereochemical requirements
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

66. 8.12 Reagent Function: Nucleophilicy
Assessing the strength of a nucleophile
The greater the negative charge, the more nucleophilic it is likely to be
The more polarizable it is, the more nucleophilic it should be
The less sterically hindered it is, the more
nucleophilic it should be. WHY?
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Klein, Organic Chemistry 2e

67. 8.12 Reagent Function: Basicity
Assessing the strength of a base
Assess the strength of its conjugate acid quantitatively using the pKa of its conjugate acid
Which is a stronger base Cl- or HSO4-?
As bases, are Cl- and HSO4- relatively strong or weak?
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Klein, Organic Chemistry 2e

68. 8.12 Reagent Function: Basicity
Assessing the strength of a base
If the base is neutral, assess the strength of its conjugate acid qualitatively using ARIO (atom, resonance, induction, orbital)
Compare CH3OH and CH3NH2
If the base carries a negative formal charge, qualitatively assess the strength of the base using ARIO
Compare Cl- and HSO4-
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Klein, Organic Chemistry 2e

69. 8.12 Basicity vs. Nucleophilicity
Consider each of the reagent categories
Reagents that act as nucleophiles only are either highly polarizable and/or they have very strong conjugate acids
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Klein, Organic Chemistry 2e

70. 8.12 Basicity vs. Nucleophilicity
Reagents that act as bases only have either very low polarizability and/or they are sterically hindered
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Klein, Organic Chemistry 2e

71. 8.12 Basicity vs. Nucleophilicity
The stronger the reagent, the more likely it is to promote SN2 or E2. WHY?
The more sterically hindered reagents are more likely to promote elimination than substitution. WHY?
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

72. 8.12 Basicity vs. Nucleophilicity
The weaker the reagent, the more likely it is to promote SN1 or E1. WHY?
Practice with SkillBuilder 8.10
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Klein, Organic Chemistry 2e

73. 8.13 Predicting Subst. vs. Elim.
Analyze the function of the reagent (nucleophile and/ or base)
Analyze the substrate (1°, 2°, or 3°)
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Klein, Organic Chemistry 2e

74. 8.13 Predicting Subst. vs. Elim.
Analyze the function of the reagent (nucleophile and/ or base)
Analyze the substrate (1°, 2°, or 3°)
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Klein, Organic Chemistry 2e

75. 8.13 Predicting Subst. vs. Elim.
Analyze the function of the reagent (nucleophile and/ or base)
Analyze the substrate (1°, 2°, or 3°)
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e

76. 8.13 Predicting Subst. vs. Elim.
Analyze the function of the reagent (nucleophile and/ or base)
Analyze the substrate (1°, 2°, or 3°)
Practice with SkillBuilder 8.11
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Klein, Organic Chemistry 2e

77. 8.14 Predicting Products
Analyze the function of the reagent (nucleophile and/ or base)
Analyze the substrate (1°, 2°, or 3°)
Consider regiochemistry and stereochemistry
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Klein, Organic Chemistry 2e

78. 8.14 Predicting Products Consider regiochemistry and stereochemistry
Practice with SkillBuilder 8.12
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Klein, Organic Chemistry 2e

79. Additional Practice Problems
Name the following molecules
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Klein, Organic Chemistry 2e

80. Additional Practice Problems
Label the molecules below as either cis or trans and either E or Z where appropriate
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Klein, Organic Chemistry 2e

81. Additional Practice Problems
For the substrate, give both the kinetically favored E2 product and the thermodynamically favored E2 product. Explain what conditions can be used to favor each.
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Klein, Organic Chemistry 2e

82. Additional Practice Problems
Since tertiary substrates react more readily than secondary or primary in both E1 and E2 mechanisms, what factor(s) usually controls which mechanism will dominate and why?
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Klein, Organic Chemistry 2e

83. Additional Practice Problems
Consider both regioselestivity and stereoselectivity to predict the major product for the elimination below
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Klein, Organic Chemistry 2e

84. Additional Practice Problems
Predict the major product if the alcohol below were treated with concentrated sulfuric acid. Be aware of the possible rearrangements.
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Klein, Organic Chemistry 2e

85. Additional Practice Problems
Predict the major product for the following reactions considering competing substitution and elimination pathways.
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Klein, Organic Chemistry 2e