Transcript Chapter 8 slides (Klein, 2e)

Organic Chemistry
Second Edition
David Klein
Chapter 8
Alkenes: Structure and Preparation via
Elimination Reactions
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Klein, Organic Chemistry 2e
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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8-2
Klein, Organic Chemistry 2e
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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8-3
Klein, Organic Chemistry 2e
8.2 Alkenes
• Important alkenes
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8-4
Klein, Organic Chemistry 2e
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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8-5
Klein, Organic Chemistry 2e
8.2 Alkenes
• Why might it be helpful to know the chemical structure
of pheromones such as those below?
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8-6
Klein, Organic Chemistry 2e
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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8-7
Klein, Organic Chemistry 2e
8.2 Alkenes
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8-8
Klein, Organic Chemistry 2e
8.3 Alkene Nomenclature
•
Alkenes are named using the same procedure we used
in Chapter 4 to name alkanes with minor modifications
1. Identify the parent chain, which should include the C=C
double bond
2. Identify and Name the substituents
3. Assign a locant (and prefix if necessary) to each substituent.
Give the C=C double bond the lowest number possible
4. List the numbered substituents before the parent name in
alphabetical order. Ignore prefixes (except iso) when ordering
alphabetically
5. The C=C double bond locant is placed either just before the
parent name or just before the -ene suffix
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8-9
Klein, Organic Chemistry 2e
8.3 Alkene Nomenclature
1. 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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8-10
Klein, Organic Chemistry 2e
8.3 Alkene Nomenclature
2. Identify and Name the substituents
3. 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
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8-11
Klein, Organic Chemistry 2e
8.3 Alkene Nomenclature
4. List the numbered substituents before the parent name
in alphabetical order. Ignore prefixes (except iso) when
ordering alphabetically
5. 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
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8-12
Klein, Organic Chemistry 2e
8.3 Alkene Nomenclature
•
Name the following molecule
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8-13
Klein, Organic Chemistry 2e
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 transbutane?
•
What specific type of isomers are cis and trans butene?
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8-14
Klein, Organic Chemistry 2e
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
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8-15
Klein, Organic Chemistry 2e
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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8-16
Klein, Organic Chemistry 2e
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
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8-17
Klein, Organic Chemistry 2e
8.4 Alkene Isomerism
•
Assigning E or Z to a stereoisomers
1. prioritize the groups attached to the C=C double bond based
on atomic number
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8-18
Klein, Organic Chemistry 2e
8.4 Alkene Isomerism
•
Assigning E or Z to a stereoisomers
1. prioritize the groups attached to the C=C double bond based
on atomic number
2. If the top priority groups are on the same side of the C=C
double bond, it is Z (for zussamen, which means together)
3. 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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8-19
Klein, Organic Chemistry 2e
8.5 Alkene Stability
•
Because of steric strain, cis isomers are generally less
stable than trans
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8-20
Klein, Organic Chemistry 2e
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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8-21
Klein, Organic Chemistry 2e
8.5 Alkene Stability
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8-22
Klein, Organic Chemistry 2e
8.5 Alkene Stability
•
Consider the following stability trend
•
•
What pattern do you see?
Practice with SkillBuilder 8.3
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8-23
Klein, Organic Chemistry 2e
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
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8-24
Klein, Organic Chemistry 2e
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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8-25
Klein, Organic Chemistry 2e
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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8-26
Klein, Organic Chemistry 2e
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?
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8-27
Klein, Organic Chemistry 2e
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
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8-28
Klein, Organic Chemistry 2e
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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8-29
Klein, Organic Chemistry 2e
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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8-30
Klein, Organic Chemistry 2e
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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8-31
Klein, Organic Chemistry 2e
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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8-32
Klein, Organic Chemistry 2e
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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8-33
Klein, Organic Chemistry 2e
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
Zaitsev product
What is the relationship
between the alkene
products?
Regioselectivity occurs
when one constitutional
product is formed
predominantly over the
Hofmann product
other
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8-34
Klein, Organic Chemistry 2e
8.7 Regioselectivity of E2
•
The identity of the base can affect the regioselesctivity
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8-35
Klein, Organic Chemistry 2e
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?
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8-36
Klein, Organic Chemistry 2e
8.7 Regioselectivity of E2
•
•
•
Why does a sterically hindered base favor the Hofmann
product?
Sterically hindered bases (sometimes called nonnucleophilic) are useful in many reactions
Practice with SkillBuilder 8.5
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8-37
Klein, Organic Chemistry 2e
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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8-38
Klein, Organic Chemistry 2e
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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8-39
Klein, Organic Chemistry 2e
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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8-40
Klein, Organic Chemistry 2e
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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8-41
Klein, Organic Chemistry 2e
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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8-42
Klein, Organic Chemistry 2e
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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8-43
Klein, Organic Chemistry 2e
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 antiperiplanar transition state can often lead to the less
stable Z isomer
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8-44
Klein, Organic Chemistry 2e
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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8-45
Klein, Organic Chemistry 2e
8.7 Stereospecificity of E2
•
•
•
Assuming they proceed through an anti-periplanar
transition state, predict all of the products for the
following reaction
Will the reaction be stereospecific or stereoselective,
and what factors most affect the product distribution?
Practice with SkillBuilder 8.6
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8-46
Klein, Organic Chemistry 2e
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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8-47
Klein, Organic Chemistry 2e
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
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8-48
Klein, Organic Chemistry 2e
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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8-49
Klein, Organic Chemistry 2e
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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8-50
Klein, Organic Chemistry 2e
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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8-51
Klein, Organic Chemistry 2e
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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8-52
Klein, Organic Chemistry 2e
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?
•
Just like we did for SN1 in chapter 7, to explain the
reactivity trend above, we must compare the energy
diagrams for each substrate
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8-53
Klein, Organic Chemistry 2e
8.9 The Effect of Substrate on E1
•
To compare their energies, draw the structures for each
transition state, intermediate, and product below
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8-54
Klein, Organic Chemistry 2e
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?
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8-55
Klein, Organic Chemistry 2e
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?
Practice with conceptual checkpoint 8.27
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8-56
Klein, Organic Chemistry 2e
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?
Practice with SkillBuilder 8.8
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8-57
Klein, Organic Chemistry 2e
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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8-58
Klein, Organic Chemistry 2e
8.9 Stereoselectivity for E1
•
Considering stereochemistry and regiochemistry,
predict the products if the molecule below was treated
with concentrated sulfuric acid
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8-59
Klein, Organic Chemistry 2e
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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8-60
Klein, Organic Chemistry 2e
8.10 Complete E1 Mechanisms
•
•
Why is the first proton transfer necessary?
Practice with conceptual checkpoint 8.32
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8-61
Klein, Organic Chemistry 2e
8.10 Complete E1 Mechanisms
•
•
Explain why the carbocation rearranges
Practice with conceptual checkpoint 8.33
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8-62
Klein, Organic Chemistry 2e
8.10 Complete E1 Mechanisms
•
The maximum number of steps in an E1 mechanism is
generally four
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8-63
Klein, Organic Chemistry 2e
8.10 Complete E1 Mechanisms
•
•
Consider the
energy
diagram for
the
mechanism
on the
previous
slide
Assess each
free energy
change
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8-64
Klein, Organic Chemistry 2e
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
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8-65
Klein, Organic Chemistry 2e
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
•
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8-66
Klein, Organic Chemistry 2e
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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8-67
Klein, Organic Chemistry 2e
8.12 Substitution vs. Elimination
•
To predict whether substitution or elimination will
predominate, consider the factors below
1. Determine the function of the reagent. Is it more likely
to act as a base, a nucleophile, or both?
–
–
Kinetics control nucleophilicity. WHY? HOW?
Thermodynamics control basicity. WHY? HOW?
2. Analyze the substrate and predict the expected
mechanism (SN1, SN2, E1, or E2)
3. Consider relevant regiochemical and stereochemical
requirements
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8-68
Klein, Organic Chemistry 2e
8.12 Reagent Function: Nucleophilicy
1. 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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8-69
Klein, Organic Chemistry 2e
8.12 Reagent Function: Basicity
1. 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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8-70
Klein, Organic Chemistry 2e
8.12 Reagent Function: Basicity
1. 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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8-71
Klein, Organic Chemistry 2e
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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8-72
Klein, Organic Chemistry 2e
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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8-73
Klein, Organic Chemistry 2e
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?
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8-74
Klein, Organic Chemistry 2e
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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8-75
Klein, Organic Chemistry 2e
8.13 Predicting Subst. vs. Elim.
1. Analyze the function of the reagent (nucleophile and/
or base)
2. Analyze the substrate (1°, 2°, or 3°)
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8-76
Klein, Organic Chemistry 2e
8.13 Predicting Subst. vs. Elim.
1. Analyze the function of the reagent (nucleophile and/
or base)
2. Analyze the substrate (1°, 2°, or 3°)
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8-77
Klein, Organic Chemistry 2e
8.13 Predicting Subst. vs. Elim.
1. Analyze the function of the reagent (nucleophile and/
or base)
2. Analyze the substrate (1°, 2°, or 3°)
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8-78
Klein, Organic Chemistry 2e
8.13 Predicting Subst. vs. Elim.
1. Analyze the function of the reagent (nucleophile and/
or base)
2. Analyze the substrate (1°, 2°, or 3°)
•
Practice with SkillBuilder 8.11
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8-79
Klein, Organic Chemistry 2e
8.14 Predicting Products
1. Analyze the function of the reagent (nucleophile and/
or base)
2. Analyze the substrate (1°, 2°, or 3°)
3. Consider regiochemistry and stereochemistry
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8-80
Klein, Organic Chemistry 2e
8.14 Predicting Products
3. Consider regiochemistry and stereochemistry
•
Practice with
SkillBuilder 8.12
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8-81
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Name the following molecules
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8-82
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Label the molecules below as either cis or trans and
either E or Z where appropriate
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8-83
Klein, Organic Chemistry 2e
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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8-84
Klein, Organic Chemistry 2e
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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8-85
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Consider both regioselestivity and stereoselectivity to
predict the major product for the elimination below
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
8-86
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Predict the major product if the alcohol below were
treated with concentrated sulfuric acid. Be aware of the
possible rearrangements.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
8-87
Klein, Organic Chemistry 2e
Additional Practice Problems
•
Predict the major
product for the
following reactions
considering
competing
substitution and
elimination pathways.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
8-88
Klein, Organic Chemistry 2e