1. 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?
Copyright 2012 John Wiley & Sons, Inc.

2. 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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3. 8.2 Alkenes
Important alkenes
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4. 8.2 Alkenes
C=C double bonds are found in a variety of compounds including pheromones and many other classes of compounds
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5. 8.2 Alkenes
Why might it be helpful to know the chemical structure of pheromones such as those below?
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6. 8.2 Alkenes
Alkenes are also important compounds in the chemical industry
70 billion pounds of propylene (propene) and 200 billion pounds of ethylene (ethene) are both made from cracking petroleum each year
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7. 8.2 Alkenes
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8. 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 alphabetical order. Ignore prefixes (except iso) when ordering alphabetically
The C=C double bond locant is placed either just before the parent name or just before the -ene suffix
Copyright 2012 John Wiley & Sons, Inc.

9. 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
Copyright 2012 John Wiley & Sons, Inc.

10. 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 2012 John Wiley & Sons, Inc.

11. 8.3 Alkene Nomenclature
List the numbered substituents before the parent name in alphabetical order. Ignore prefixes (except iso) when ordering alphabetically
The C=C double bond locant is placed either just before the parent name or just before the -ene suffix
Practice with SkillBuilder 8.1
Copyright 2012 John Wiley & Sons, Inc.

12. 8.3 Alkene Nomenclature Name the following molecule
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13. 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?
Copyright 2012 John Wiley & Sons, Inc.

14. 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
When applied to bicycloakenes, this rule is called Bredt’s rule
Copyright 2012 John Wiley & Sons, Inc.

15. 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
Copyright 2012 John Wiley & Sons, Inc.

16. 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 2012 John Wiley & Sons, Inc.

17. 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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18. 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
Copyright 2012 John Wiley & Sons, Inc.

19. 8.5 Alkene Stability
Because of steric strain, cis isomers are generally less stable than trans
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20. 8.5 Alkene Stability
The difference in stability can be quantified by comparing the heats of combustion
How does heat of combustion relate to stability?
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21. 8.5 Alkene Stability
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22. 8.5 Alkene Stability Consider the following stability trend
What pattern do you see?
Practice with SkillBuilder 8.3
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23. 8.5 Alkene Stability
List the following molecules in order of increasing heat of combustion
2,3,4-trimethyl-1,3-pentadiene
2-isopropyl-1,4-pentadiene
3,3-dimethyl-1,5-hexadiene
4,5-dimethylcyclohexene
Copyright 2012 John Wiley & Sons, Inc.

24. 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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25. 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?
Copyright 2012 John Wiley & Sons, Inc.

26. 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 2012 John Wiley & Sons, Inc.

27. 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:
Could the steps happen in the reverse order?
Practice with SkillBuilder 8.4
Copyright 2012 John Wiley & Sons, Inc.

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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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?
Copyright 2012 John Wiley & Sons, Inc.

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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31. 8.7 The Effect of Substrate on E2
How would both the transition state energy and the product energy be different if the substrate were 1°?
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32. 8.7 The Effect of Substrate on E2
Notice the differences in transition state and in product energies
Practice with conceptual checkpoint 8.14
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33. 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 product is formed predominantly over the other
Copyright 2012 John Wiley & Sons, Inc.

34. 8.7 Regioselectivity of E2
The identity of the base can affect the regioselesctivity
Copyright 2012 John Wiley & Sons, Inc.

35. 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 2012 John Wiley & Sons, Inc.

36. 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
Copyright 2012 John Wiley & Sons, Inc.

37. Study Guide for sections 8.1-8.7regio
DAY 17, Terms to know:
Sections regio Elimination reaction, alkene, Bredt’s rule, E/Z notation, E1, E2, Zaitsev product, Hofmann product, regioselectivity
DAY 17, Specific outcomes and skills that may be tested on exam 3:
Sections regio
Be able to compare and contrast bases and nucleophiles.
Be able to predict products given reactants and reagents for an elimination reaction including regioselectivity concerns.
Be able to name a given alkene or draw an alkene given its name.
Be able to differentiate between cis and trans isomers in alkenes.
Be able to apply and explain Bredt’s rule for syclic and bicyclic alkenes.
Be able to use proper E/Z notation with alkenes, and be able to determine where cis and trans notation is not appropriate and E/Z is.
Be able to assess the stability of alkenes based on degree of substitution, sterics, and number of pi vs sigma bonds.
Be able to relate stability and heat of combuistion.
Be able to draw both E1 and E2 mechanisms with proper arrow pushing.
Be able to explain how the identity of the base/nucleophile used either promotes E1 or E2.
Be able to explain how the identity of the electrophile (sterics and quality of leaving group) used either promotes E1 or E2.
Be able to explain how the identity of the base/nucleophile used either promotes elimination or substitution.
Be able to explain how the identity of the electrophile (sterics and quality of leaving group) used either promotes elimination or subsitution.
Given the mechanism, be able to give the correct rate law for an elimination and explain why.
Given information about the rate law for a reaction, be able to distinguish between E1 and E2 and explain why.
Be able to draw transition states for each step in an E1 or E2 reaction and explain how the properties of the reactant and substrate affect the stability of the transition state and thus the rate of one mechanism vs. another.
Be able to choose an appropriate base that will give desired regioselectivity for an elimination reaction.
Be able to make a kinetic vs. thermodynamic argument to explain regioselectivity in elimination reactions.
Klein, Organic Chemistry 2e

38. Practice Problems for sections 8.1-8.7regio
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period
Klein, Organic Chemistry 2e

39. Prep for Day 18 Must Watch videos: Other helpful videos:
(E2 stereoselectivity)
(E2 stereospecificity)
(E1 reactions)
(E1 regioselectivity)
(substitution competing with elimination)
Other helpful videos:
(E2 stereospecificity with cyclohexane)
(E1 regioselectivity with rearrangement)
(lecture 26)
Read sections 8.7stereochemistry-8.12
Klein, Organic Chemistry 2e