chap 9
TRANSCRIPT
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ELIMINATION REACTIONS:ELIMINATION REACTIONS:ALKENES, ALKYNESALKENES, ALKYNES
Chapter 9
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Chapter 9 Assigned ProblemsChapter 9 Assigned ProblemsIn-Text Problems1,2, 3 6, 7, 8, 10ab, 12abef 13ac14, 16, 17, 19, 21 23abceh
End of Chapter Problems24- 30 32, 33 36, 3740 - 42 44 49 - 50
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Summer 2008Summer 2008
• Skip the following sections: 9.11, 9.12, 9.13, and 9.14
• Keep Sections 9.15 and 9.16!• Especially study: Volume 2, pp.
855 and 856 in section 9.16
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Section 9.1: NomenclatureSection 9.1: Nomenclature• Review this on your own
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Sect. 9.2 Elimination Sect. 9.2 Elimination ReactionsReactions
C C
X Y
C C + X Y
Dehydrohalogenation (-HX) and Dehydration (-H2O) are the main types of elimination reactions.
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Dehydrohalogenation (-HX)Dehydrohalogenation (-HX)
strongbase
X = Cl, Br, I
+ " "C C
X
H XC C
H
See examples on pp. 770-771
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Sect 9.3: the E2 mechanismSect 9.3: the E2 mechanism
..:..
__
+
+ Br_
..:
concerted mechanism
H O
C C
Br
H
H OH
C C
This reaction is done in strong base at high concentration, such as 1 M NaOH in water.
_
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KineticsKinetics• The reaction in strong base at high
concentration is second order (bimolecular):
• Rate law: rate = k[OH-]1[R-Br]1
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Sect 9.3: the E1 mechanismSect 9.3: the E1 mechanism
1)
++ Br
_slow
+
2)..
:
+fast
O.. +O
C C
Br
C C
H
C C
HC C
HH H
H
H
H
rate determining step
This reaction is done in strong base such as 0.01 M NaOH in water!! Actually, the base solution is weak!
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KineticsKinetics• The reaction in weak base or under
neutral conditions will be first order (unimolecular):
• Rate law: rate = k [R-Br]1
• The first step (slow step) is rate determining!
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Sect 9.4: the E2 mechanismSect 9.4: the E2 mechanism• mechanism• kinetics• isotope effects• stereochemistry of reactants• orientation of elimination (Zaitsev’s
rule)• stereochemistry of products• competing reactions
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E2 mechanismE2 mechanism
..:..
__
+
+ Br_
..:
concerted mechanism
H O
C C
Br
H
H OH
C C
This reaction is done in strong base at high concentration, such as 1 M NaOH in water.
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Kinetics of an E2 reactionKinetics of an E2 reaction• The reactions are second order
(bimolecular reactions).
• Rate = k [R-Br]1[Base]1
second order reaction (1 + 1 = 2)High powered math!!
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energy
Reaction coordinate
C C
H OH
Br-
..:..
__H O
C C
Br
H
..:H O
C C
H
Br
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Isotope EffectsIsotope Effects• Change in rate brought about by
replacing an hydrogen atom by its isotope, deuterium.C-D bond is stronger than a C-H bond!
• Usually expressed as kH/kD
• If kH/kD = about 7.0, this means that the isotopically-labeled bond is being broken in the rate-determining step, indicating that the reaction is E2.
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Stereochemistry of reactantsStereochemistry of reactants• E2 reactions must go by an anti
elimination• This means that the hydrogen atom
and halogen atom must be 180o (coplanar) with respect to each other!!
• Draw a Newman projection formula and place the H and X on opposite sides.
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Stereochemistry of E2 Stereochemistry of E2 ReactionReaction
KOH
AlcoholSolventH
Br
H
HH
CCH3
CH3
CH3
C
H
CH3
CH3
CH3H
H
This is the cis isomer. The trans isomer does not react by an E2 reaction.
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(S,S)-diastereomer(S,S)-diastereomer
KOHethanolheat
(E)-isomer (Z)-isomer
??? ???
C C
Br
HCH3
CH3
H
C C
CH3 CH3
H t-butylC C
H CH3
CH3 t-butyl
t-butyl
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(E)-isomer
C C
CH3 CH3
H T-butyl
This one is formed!
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(R,S)(R,S)-diastereomer-diastereomer
KOHethanolheat
(E)-isomer (Z)-isomer
??? ???
C C
Br
HH
CH3
CH3
t-butyl
C C
CH3 CH3
H T-butylC C
H CH3
CH3 t-butyl
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(Z)-isomer
C C
H CH3
CH3 t-butyl
This one is formed!
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Orientation of elimination: Orientation of elimination: regiochemistry/ Zaitsev’s regiochemistry/ Zaitsev’s
RuleRule• In reactions of removal of hydrogen halides from alkyl halides or the removal of water from alcohols, the hydrogen which is lost will come from the more highly-branched -carbon.
A. N. Zaitsev -- 1875 C C C C
H
H
H H
X
H
H
HHCH3
Less branchedMore branched
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Product formed from previous Product formed from previous slideslide
CC C
C
H
HH
H
HCH3
HH
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Typical bases used in E2 Typical bases used in E2 reactionsreactions
High concentration of the following >1MIf the concentration isn’t given, assumethat it is high concentration!• Na+ -OH• K+ -OH• Na+ -OR• Na+ -NH2
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Orientation of elimination: Orientation of elimination: regiochemistry/ Zaitsev’s regiochemistry/ Zaitsev’s
RuleRuleExplaination of Zaitsev’s rule:
When you remove a hydrogen atom from the more branched position, you are forming a more highly substituted alkene.
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Stereochemistry of productsStereochemistry of products• The H and X must be anti with
respect to each other in an E2 reaction!
• You take what you get, especially with diastereomers! See the previous slides of the reaction of diastereomers.
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Competing reactionsCompeting reactions• The substitution reaction (SN2)
competes with the elimination reaction (E2).
• Both reactions follow second order kinetics!
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Sect 9.5: the E1 mechanismSect 9.5: the E1 mechanism• mechanism• kinetics• isotope effects• stereochemistry of reactants• orientation of elimination (Zaitsev’s
rule)• stereochemistry of products• competing reactions
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E1 mechanismE1 mechanism
1)
++ Br
_slow
+
2)..
:
+fast
O..+O
C C
Br
C C
H
C C
HC C
H
H H
H
H
H
water helpsto stabilizecarbocation
This reaction is done in strong base at low concentration, such as 0.01 M NaOH in water)
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E1 Reactions E1 Reactions • These reactions proceed under neutral
conditions where a polar solvent helps to stabilize the carbocation intermediate.
• This solvent also acts as a weak base and removes a proton in the fast step.
• These types of reactions are referred to as solvolysis reactions.
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• tertiary substrates go by E1 in polar solvents, with little or no base present!
• typical polar solvents are water, ethanol, methanol and acetic acid
• These polar solvents help stabilize carbocations
• E1 reactions also occur in a low concentration of base (i.e. 0.01M NaOH).
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•With strong base (i.e. >1M), goes by E2•Example reactions
However!!!!However!!!!
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Structure of the Carbocation Structure of the Carbocation IntermediateIntermediate
C CH3
CH3
CH3
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Carbocation stability orderCarbocation stability order
Tertiary (3o) > secondary (2o) > primary (1o)
It is hard (but not impossible) to get primary compounds to go by E1. The reason for this is that primary carbocations are not stable!
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Kinetics of an E1 reactionKinetics of an E1 reaction• E1 reactions follow first order
(unimolecular) kinetics:
Rate = k [R-X]1
The solvent helps to stabilize the carbocation, but it doesn’t appear in the rate law!!
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energy
Reaction coordinate
CH
CBr
CH
C
Br-
C C
H
C C
H
C C + H+
intermediate
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Isotope effectsIsotope effects• E1 reactions do not show an isotope
effect: kH/kD = 1
• This tells us that the C-D or C-H bonds are not broken in the rate determining step (step 1). They are broken in the fast step (step 2) in the mechanism).
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Stereochemistry of the Stereochemistry of the reactantsreactants
• E1 reactions do not require an anti coplanar orientation of H and X.
• Diastereomers give the same products with E1 reactions, including cis- and trans products.
• Remember, E2 reactions usually give different products with diastereomers.
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Orientation of eliminationOrientation of elimination• E1 reactions faithfully follow
Zaitsev’s rule!• This means that the major product
should be the product that is the most highly substituted.
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Stereochemistry of productsStereochemistry of productsE1 reactions usually give the thermodynamically most stable product as the major product. This usually means that the largest groups should be on opposite sides of the double bond. Usually this means that the trans product is obtained.
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Some examples of E1 and E2 Some examples of E1 and E2 reactionsreactions
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Competing reactionsCompeting reactionsSkip for Summer 07Skip for Summer 07
• The substitution reaction (SN1) competes with the elimination reaction (E1).
• Both reactions follow first order kinetics!
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Whenever there are Whenever there are carbocations…carbocations…
• They can undergo elimination (E1)• They can undergo substitution
(SN1)• They can rearrange
– and then undergo elimination– or substituion
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Sect 9.6: Dehydration of Sect 9.6: Dehydration of Alcohols (acid assisted E1)Alcohols (acid assisted E1)
strong acid+ H2OR C C R
R
OH
H
R
C C
R
R R
R
Acid assisted reactions are always E1
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Which strong acids are used?Which strong acids are used?• H2SO4
• H3PO4
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Mechanism of DehydrationMechanism of Dehydration1)
2)
3)
+ H+
+
+
slow
++ H2O
++ H
+
CH3 C CH3
CH3
OH
CH3 C CH3
CH3
OH2
CH3 C CH3
CH3
OH2
CH3 C CH3
CH3
CH3 C CH3
CH3
C CH2
CH3
CH3
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85% H3PO4
80 °C
H H+
C
:
H
CH+
_
::H
secondary carbocation
CH3 C CH CH3
CH3
CH3
O
CH3 C
CH3
CH3
O
CH3
CH3 C
CH3
CH3
H2O
CH3
Sect 9.7: rearrangements in Sect 9.7: rearrangements in dehydration reactions dehydration reactions
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+
+
major
minor
trace
CH3 C C CH3
CH3
CH3
H
CH3 C CH CH2
CH3
CH3
CH3 C C CH3
CH3
HCH3
CH2 C CH
CH3
CH3
CH3
C C
CH3
CH3 CH3
CH3
tertiary carbocationsecondary carbocation
Sect 9.7: rearrangements in Sect 9.7: rearrangements in dehydration reactionsdehydration reactions
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RearrangementsRearrangements• Alkyl groups and hydrogen can
migrate in rearrangement reactions to give more stable intermediate carbocations.
• You shouldn’t assume that rearrangements always occur in all E1 reactions, otherwise paranoia will set in!!
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Sect 9.8: comparison of E2 / Sect 9.8: comparison of E2 / E1E1
• E1 reactions occur under essentially neutral conditions with polar solvents, such as water, ethyl alcohol or acetic acid.
• E1 reactions can also occur with strong bases, but only at low concentration, about 0.01 to 0.1 M or below.
• E2 reactions require strong base in high concentration, about 1 M or above.
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Sect 9.8: comparison of E2 / Sect 9.8: comparison of E2 / E1E1
• E1 is a stepwise mechanism (two or more);Carbocation intermediate!
• E2 is a concerted mechanism (one step)No intermediate!
• E1 reactions may give rearranged products
• E2 reactions don’t give rearrangement• Alcohol dehydration reactions are E1
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Sect 9.9: bulky leaving Sect 9.9: bulky leaving groups -- Hofmann groups -- Hofmann
EliminationElimination
+OH
_
heat
+
6%
94%
CH3 CH2 CH2 CH CH3
N
CH3
CH3CH3
CH3 CH2 CH CH CH3
CH3 CH2 CH2 CH CH2
This give the anti-Zaitsev product (least substituted product is formed)!
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Orientation of elimination: Orientation of elimination: regiochemistry/ Hofmann’s regiochemistry/ Hofmann’s
Rule Rule • In bimolecular elimination reactions in the presence of either a bulky leaving group or a bulky base, the hydrogen that is lost will come from the LEAST LEAST highly-branched -carbon.
C C C C
H
H
H H
X
H
H
HHCH3
Less branchedMore branched
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Product from previous slideProduct from previous slide
CC
C
H
H
H
HCH3
HH
C
H
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Sect 9.10 Elimination with Sect 9.10 Elimination with bulky basesbulky bases
• Non-bulky bases, such as hydroxide and ethoxide, give Zaitsev products.
• Bulky bases, such as potassium tert-butoxide, give larger amounts of the least substituted alkene (Hoffmann) than with simple bases.
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Comparing Ordinary and Comparing Ordinary and Bulky BasesBulky Bases
CH3 C CH CH3
Br
NaOC2H5
C2H5OHheat
C CHCH3 CH3
CH3 C CH CH3
Br
KOC(CH3)3
(CH3)3COHheat
C CHCH3 CH2
Major
H
CH3 CH3
CH3
H
CH3
Major
H
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1-butene: watch out for 1-butene: watch out for competing reactions!competing reactions!
H3C CH2 CH2 CH2 Br
KOCH3
Non-bulky
SN2
H3C CH2 CH2 CH2 O-CH3
H3C CH2 CH CH2
bulky baseKO-t-butyl
E2
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Sect 9.11 the E1cb Sect 9.11 the E1cb mechanism: skip Summer mechanism: skip Summer
20062006.. _
.. :....
fast .._
1)
2)
.._
slow+ Br
_
C
O
C
O
C
O
C
O
C C
Br
C C
Br
H
H O
C C
Br
H O H
C C
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Sect 9.13 alpha-Elimination Sect 9.13 alpha-Elimination Reactions: skip Summer Reactions: skip Summer
20062006• These unusual reactions occur with
one carbon compounds, only.• Examples include chloroform and
methylene chloride.• Cyclopropane compounds are
formed.
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Sect 9.14: Dehalogenation: Sect 9.14: Dehalogenation: skip Summer 2006skip Summer 2006
+ ZnCH3COOH
+ ZnBr2CH3 C CH CH3
CH3
Br
Br
C C
CH3
CH3 H
CH3
This reaction requires the two Br’s to be anti.
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Sect 9.15: Preparation of Sect 9.15: Preparation of Alkynes -- double Alkynes -- double
dehydrohalogenation dehydrohalogenation
KOH
ethanol
NaNH2
NaNH2
R C C R
Br
H
Br
H
C C
R
H R
Br
C CR R
R C C R
Br
H
Br
H
C CR R
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Sect. 9.16: Multistep Sect. 9.16: Multistep reactions and Synthesis -- reactions and Synthesis --
Example 1 Synthesis: Example 1 Synthesis: Example 1Example 1
CH3 CH CH3
OH
CH3 CH2 CH2 Br
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Multistep reactions and Multistep reactions and Synthesis Example 2Synthesis Example 2
Cl CH2 CH2 CH2 CH3 CH3 CH CH2 CH3
Cl
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Multistep reactions and Multistep reactions and Synthesis Example 3Synthesis Example 3
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Multistep reactions and Multistep reactions and Synthesis Example 4Synthesis Example 4
Br CH2 CH2 CH2 CH2 CH3 CH3 C CH2 CH2 CH3
Br
Br
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Synthesis: Example 5Synthesis: Example 5
CH2 CH2 CH2 CH2 CH3 CH3 C CH2 CH2 CH2 CH3
O
CH2Br
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Highlights of Chapter NineHighlights of Chapter Nine• Dehydrohalogenation -- E2 Mechanism• Zaitsev’s Rule• Isotope Effects• Dehydrohalogenation -- E1 Mechanism• Dehydration of Alcohols -- E1• Carbocation Rearrangements -- E1• Elimination with Bulky Leaving Groups
and Bulky Bases -- Hofmann Rule -- E2• Multistep Reactions and Synthesis