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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
SN2 Reactions of Benzylic Halides
1
SN2 of benzylic halides is faster than allylic halides.How can this be explained if there is no carbocation intermediate?
1
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Allylic SN2 Faster: Two Arguments
2
1. steric hinderance (VWF)2. molecular orbital interactions
• LUMO of C-X bond can adopt a coplanar arrangement with p-orbitals of p-bond =
• electron delocalization over three orbitals =
• lower energy LUMO =
• lower activation energy =
• faster reaction
• Why does lower energy LUMO result in lower activation energy?
2
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Benzylic SN2 Faster: Two Arguments
3
1. steric hinderance (VWF)2. molecular orbital interactions
• LUMO of C-X bond can adopt a coplanar arrangement with p-orbitals of p-bond =
• more p-orbitals in benzene than alkene =
• more electron donation into LUMO of C-Cl bond =
• weaker C-X bond =
• faster reaction
Cl
3
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Preparation of Alkenylbenzenes
4
Same methods used to prepare alkenes can be used to prepare alkenylbenzenes
Dehydrogenation
Dehydration
Dehydrohalogenation
4
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Elimination of Benzylic Hydrogens
5
Addition reactions to alkenes can also be applied to addition reactions to alkenylbenzenes
Hydrogenation
Bromination
5
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Regioselective Addition of Electrophiles to Alkenylbenzenes
6
Addition to alkenylbenzenes favors (faster) formation of the more stable benzylic carbocation
6
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
AWhat is the major regioisomeric halohydr in formed by the reaciton below?
Self Test Question
7
Br2, H2Ono light
0º C
BrBr
BrBr
OH
OHBr
Br
Br
B
C
D
E
7
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
AWhat is the major regioisomeric halohydr in formed by the reaciton below?
Self Test Question
7
Br2, H2Ono light
0º C
BrBrδ+δ+
BrBr
BrBr
OH
OHBr
Br
Br
B
C
D
E
7
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
AWhat is the major regioisomeric halohydr in formed by the reaciton below?
Self Test Question
7
Br2, H2Ono light
0º C
BrBrδ+δ+
BrBr
BrBr
OH
OHBr
Br
Br
B
C
D
E
7
University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Lecture 26Organic Chemistry 1
Professor Duncan Wardrop
April 15, 2010
8
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
With respect to cyclononatetraene, which process below will form an aromatic species?
Self Test Question
9
A. addition of one π-electron to give C9H10–
B. addition of two π-electrons to give C9H102–
C. loss of H+ from the sp3-hybridized carbon to give C9H9–
D. loss of H+ from an sp2-hybridized carbon to give C9H9–
E. loss of :H– from the sp3-hybridized carbon to give C9H9+
9
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
With respect to cyclononatetraene, which process below will form an aromatic species?
Self Test Question
10
A. addition of one π-electron to give C9H10–
B. addition of two π-electrons to give C9H102–
C. loss of H+ from the sp3-hybridized carbon to give C9H9–
D. loss of H+ from an sp2-hybridized carbon to give C9H9–
E. loss of :H– from the sp3-hybridized carbon to give C9H9+
10
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
With respect to cyclononatetraene, which process below will form an aromatic species?
Self Test Question
11
A. addition of one π-electron to give C9H10–
B. addition of two π-electrons to give C9H102–
C. loss of H+ from the sp3-hybridized carbon to give C9H9–
D. loss of H+ from an sp2-hybridized carbon to give C9H9–
E. loss of :H– from the sp3-hybridized carbon to give C9H9+
2–
11
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
With respect to cyclononatetraene, which process below will form an aromatic species?
Self Test Question
12
A. addition of one π-electron to give C9H10–
B. addition of two π-electrons to give C9H102–
C. loss of H+ from the sp3-hybridized carbon to give C9H9–
D. loss of H+ from an sp2-hybridized carbon to give C9H9–
E. loss of :H– from the sp3-hybridized carbon to give C9H9+
12
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
With respect to cyclononatetraene, which process below will form an aromatic species?
Self Test Question
12
A. addition of one π-electron to give C9H10–
B. addition of two π-electrons to give C9H102–
C. loss of H+ from the sp3-hybridized carbon to give C9H9–
D. loss of H+ from an sp2-hybridized carbon to give C9H9–
E. loss of :H– from the sp3-hybridized carbon to give C9H9+
12
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
With respect to cyclononatetraene, which process below will form an aromatic species?
Self Test Question
13
A. addition of one π-electron to give C9H10–
B. addition of two π-electrons to give C9H102–
C. loss of H+ from the sp3-hybridized carbon to give C9H9–
D. loss of H+ from an sp2-hybridized carbon to give C9H9–
E. loss of :H– from the sp3-hybridized carbon to give C9H9+
13
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
With respect to cyclononatetraene, which process below will form an aromatic species?
Self Test Question
14
A. addition of one π-electron to give C9H10–
B. addition of two π-electrons to give C9H102–
C. loss of H+ from the sp3-hybridized carbon to give C9H9–
D. loss of H+ from an sp2-hybridized carbon to give C9H9–
E. loss of :H– from the sp3-hybridized carbon to give C9H9+
14
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
E
Predict the product.
Self Test Question
15
1. Br2, CH2Cl22. NaNH2 (excess)3. H2O
4. B2H6, diglyme5. H2O2, NaOH
BrNH2
O
O
H
H
BrOH
15
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
E
Predict the product.
Self Test Question
15
1. Br2, CH2Cl22. NaNH2 (excess)3. H2O
4. B2H6, diglyme5. H2O2, NaOH1.
BrBr
2.
3.H
4.
HBH2
5.
OH
H
BrNH2
O
O
H
H
BrOH
15
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
E
Predict the product.
Self Test Question
15
1. Br2, CH2Cl22. NaNH2 (excess)3. H2O
4. B2H6, diglyme5. H2O2, NaOH1.
BrBr
2.
3.H
4.
HBH2
5.
OH
H
BrNH2
O
O
H
H
BrOH
15
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Review: Addition Reactions to Alkenes
16
Electrophilic Addition (AdE)• In all addition reactions, the alkene is the nucleophile (Lewis base: weakly
held π-electrons are used/donated to form stronger s-bonds). • Reagent being added to the alkene is an electrophile (Lewis acid: accepting
electrons to form s-bonds).
C C E Yδ+ δ−
+ E Y
16
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Review: Addition Reactions to Alkenes
16
Electrophilic Addition (AdE)• In all addition reactions, the alkene is the nucleophile (Lewis base: weakly
held π-electrons are used/donated to form stronger s-bonds). • Reagent being added to the alkene is an electrophile (Lewis acid: accepting
electrons to form s-bonds).
C C E Yδ+ δ−
+ E Y
C CE Y
16
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Review: Addition Reactions to Alkenes
16
Electrophilic Addition (AdE)• In all addition reactions, the alkene is the nucleophile (Lewis base: weakly
held π-electrons are used/donated to form stronger s-bonds). • Reagent being added to the alkene is an electrophile (Lewis acid: accepting
electrons to form s-bonds).
C C E Yδ+ δ−
+ E Y
C CE Y
variations of this mechanism apply to different electrophiles
16
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Review: Addition Reactions to Alkenes
17
Electrophilic Addition (AdE)• In all addition reactions, the alkene is the nucleophile (Lewis base: weakly
held π-electrons are used/donated to form stronger s-bonds). • Reagent being added to the alkene is an electrophile (Lewis acid: accepting
electrons to form s-bonds).
C C H Clδ+ δ−
+ H Cl
C CH Cl
17
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Review: Addition Reactions to Alkenes
18
Electrophilic Addition (AdE)• In all addition reactions, the alkene is the nucleophile (Lewis base: weakly
held π-electrons are used/donated to form stronger s-bonds). • Reagent being added to the alkene is an electrophile (Lewis acid: accepting
electrons to form s-bonds).
C C Br Br+ Br Br
C C BrBr
18
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Review: Addition Reactions to Alkenes
19
Electrophilic Addition (AdE)• In all addition reactions, the alkene is the nucleophile (Lewis base: weakly
held π-electrons are used/donated to form stronger s-bonds). • Reagent being added to the alkene is an electrophile (Lewis acid: accepting
electrons to form s-bonds).
C C H BH2+ H2B H
H2B H
19
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Review: Addition Reactions to Alkenes
20
Electrophilic Addition (AdE)• In all addition reactions, the alkene is the nucleophile (Lewis base: weakly
held π-electrons are used/donated to form stronger s-bonds). • Reagent being added to the alkene is an electrophile (Lewis acid: accepting
electrons to form s-bonds).
C C H H+ H H
H
H Pd
20
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
Assuming that benzene reacts similarly to alkenes, which structure could not be an intermediate for AdE with the general electrophile depicted?
Self Test Question
21
E Yδ+ δ−
E E E EE
B C D E
21
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
Assuming that benzene reacts similarly to alkenes, which structure could not be an intermediate for AdE with the general electrophile depicted?
Self Test Question
21
E Yδ+ δ−
E E E EE
B C D E
21
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
Assuming that benzene reacts similarly to alkenes, which structure could not be an intermediate for AdE with the general electrophile depicted?
Self Test Question
21
E Yδ+ δ−
E E E EE
B C D E
21
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Formation of Arenium Ion
22
• arenium ion: carbocation formed from an aromatic ring
• formation of arenium ions is slow since the ground state is so stable (loss of aromaticity)
• requires very reactive (high energy) electrophiles
22
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Arenium Ions
23
• arenium ion: also known as a s-complex• arenium ions are allylic carbocations• stabilized by resonance• no aromatic
23
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Fate of Arenium Ion
24
Electrophilic addition (AdE2Ar)
Electrophilic aromatic substitution (SE2Ar)
24
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Fate of Arenium Ion
24
Electrophilic addition (AdE2Ar)
Electrophilic aromatic substitution (SE2Ar)
rearomatization
24
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Potential Energy Diagram
25
25
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Nitronium Ion
O N O
Preparation of Electrophiles
26
HNO3 + H2SO4 O N O H2O HSO4–+ +
HON
O
O
nitric acid
H OH2+ ON
O
OH
H
O N Onitronium ion
H
26
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Nitration
27
27
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Sulfur trioxide
Preparation of Electrophiles
28
OSO
O
HO S OO
nitric acid
+
sulfur trioxide
H
OO S OH
O
OH O S O
OH
O H
H
OSO
O
H2SO4 SO3 H2O HSO4–+ +
28
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Sulfonation
29
29
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Brominecomplex
Preparation of Electrophiles
30
30
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Bromination
31
31
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Carbocations
Preparation of Electrophiles
32
R X AlCl3+
(R = 3º or 2º)
R+ AlCl4–+
32
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Friedel-Crafts Alkylation
33
33
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Acylium Ions
Preparation of Electrophiles
34
R C O R C O R Cl
O+ AlCl3 R C O + AlCl4–
34
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Friedel-Crafts Acylation
35
35
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Summary of Some Representative Electrophilic Aromatic Substitutions
36
HNO3, H2SO4H NO2
H SO2H
Br2, FeBr3H Br
RCl, AlCl3H R
H2SO4
HR Cl
O
AlCl3 R
O
nitration
sulfonation
bromination
alkylation
acylation
36
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
E
If anhydrides also react with AlCl3 to give acylium ions, what is the product for the acylation reaction below?
Self Test Question
37
H3C O
O
CH3
O+ AlCl3
acetic anhydride
CH3
O CH3
O
OH
O
CH3
O
O
O
CH3
O
37
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
E
If anhydrides also react with AlCl3 to give acylium ions, what is the product for the acylation reaction below?
Self Test Question
37
H3C O
O
CH3
O+ AlCl3
acetic anhydride
CH3
O CH3
O
OH
O
CH3
O
O
O
CH3
OH3C O
O
CH3
O AlCl3H3C C O +
O
O
CH3
AlCl3
acylium ion
37
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
E
If anhydrides also react with AlCl3 to give acylium ions, what is the product for the acylation reaction below?
Self Test Question
37
H3C O
O
CH3
O+ AlCl3
acetic anhydride
CH3
O CH3
O
OH
O
CH3
O
O
O
CH3
OH3C O
O
CH3
O AlCl3H3C C O +
O
O
CH3
AlCl3
acylium ion
37
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Complication of Friedel-Crafts Alkylations
38
• rearrangement to more stable carbocations always predominates
• 1º alkyl chlorides (except ethyl chloride) cannot be employed in FC alkylations
38
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Reduction of Aryl Ketones
39
OZn(Hg)
HCl
H2NNH2, KOH
O
Clemmensen Reduction
Wolf-Kishner Reduction
39
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Alternative Strategy to Alkybenzenes
40
R R
O
R
O
Cl+
40
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 26: April 15
Alternative Strategy to Alkybenzenes
40
R R
O
R
O
Cl+
methylene group (CH2)
40
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
Predict the product.
Self Test Question
41
N
CN
N
O
CN
CN
N
Br
Br
N
CN
N
O
Cl
1. benzene, AlCl32. Zn(Hg), HCl
3. NBS, CHCl34. NaCN
41
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
Predict the product.
Self Test Question
41
1.
2.N
O
N 3. N
Br4.
N
CN
N
O
CN
CN
N
Br
Br
N
CN
N
O
Cl
1. benzene, AlCl32. Zn(Hg), HCl
3. NBS, CHCl34. NaCN
41
UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 25: April 15
A
B
C
D
Predict the product.
Self Test Question
41
1.
2.N
O
N 3. N
Br4.
N
CN
N
O
CN
CN
N
Br
Br
N
CN
N
O
Cl
1. benzene, AlCl32. Zn(Hg), HCl
3. NBS, CHCl34. NaCN
41
University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Chapter 12: Sections 12.9-12.13
Next Lecture...
Quiz Next Week. . .
Synthesis Problem
Chapters 11 & 12
42