15. Chemistry of Benzene: Electrophilic Aromatic Substitution
Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution
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Transcript of Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution
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Chapter 12Chapter 12Reactions of Arenes:Reactions of Arenes:
Electrophilic Aromatic SubstitutionElectrophilic Aromatic Substitution HH
EE
++ EE YY ++ HH YY++ ––
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12.1Representative Electrophilic Aromatic
Substitution Reactions of Benzene HH
EE
++ EE YY ++ HH YY++ ––
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HH
EE
++ EE YY ++ HH YY++ ––
Electrophilic aromatic substitutions include:Electrophilic aromatic substitutions include:
NitrationNitrationSulfonationSulfonation
HalogenationHalogenationFriedel-Crafts AlkylationFriedel-Crafts Alkylation
Friedel-Crafts AcylationFriedel-Crafts Acylation
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HH
Table 12.1: Table 12.1: Nitration of BenzeneNitration of Benzene
++ ++ HH22OO
HH22SOSO44
HOHONONO22
NONO22
NitrobenzeneNitrobenzene(95%)(95%)
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HH
Table 12.1: Sulfonation of BenzeneTable 12.1: Sulfonation of Benzene
++ ++ HH22OOheatheat
HOHOSOSO22OHOH
SOSO22OHOH
Benzenesulfonic acidBenzenesulfonic acid(100%)(100%)
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HH
Table 12.1: Halogenation of BenzeneTable 12.1: Halogenation of Benzene
++ ++ HHBrBr
FeBrFeBr33
BrBr22
BrBr
BromobenzeneBromobenzene(65-75%)(65-75%)
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HH
Table 12.1: Friedel-Crafts Alkylation of BenzeneTable 12.1: Friedel-Crafts Alkylation of Benzene
++ ++ HHClCl
AlClAlCl33
C(CHC(CH33))33
terttert-Butylbenzene-Butylbenzene(60%)(60%)
(CH(CH33))33CCClCl
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HH
Table 12.1: Friedel-Crafts Acylation of BenzeneTable 12.1: Friedel-Crafts Acylation of Benzene
++ ++ HHClCl
AlClAlCl33
1-Phenyl-1-propanone1-Phenyl-1-propanone(88%)(88%)
OO
CHCH33CHCH22CCClCl
CCHCCH22CHCH33
OO
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12.212.2Mechanistic PrinciplesMechanistic Principles
ofofElectrophilic Aromatic SubstitutionElectrophilic Aromatic Substitution
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Step 1: attack of electrophileStep 1: attack of electrophileon on -electron system of aromatic ring-electron system of aromatic ring HH HH
HH HHHH HH
EE++ HH HH
HHHH
HH HH EE
++
highly endothermichighly endothermiccarbocation is allylic, but not aromaticcarbocation is allylic, but not aromatic
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Step 2: loss of a proton from the carbocationStep 2: loss of a proton from the carbocationintermediateintermediate
HH HH
HHHH
HH HH EE
++
highly exothermichighly exothermicthis step restores aromaticity of ringthis step restores aromaticity of ring
HH HH
HH EEHH HH
HH++
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HH
H H
H + E+
H
EH
H H
H + H+
H
H H
HH
H H E+
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Based on this general mechanism:Based on this general mechanism:
what remains is to identify the electrophile in what remains is to identify the electrophile in nitration, sulfonation, halogenation, Friedel-nitration, sulfonation, halogenation, Friedel-Crafts alkylation, and Friedel-Crafts acylation Crafts alkylation, and Friedel-Crafts acylation to establish the mechanism of specific to establish the mechanism of specific electrophilic aromatic substitutionselectrophilic aromatic substitutions
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12.3Nitration of Benzene
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HH
Nitration of BenzeneNitration of Benzene
++ ++ HH22OO
HH22SOSO44
HOHONONO22
NONO22
Electrophile isElectrophile isnitronium ion nitronium ion
OO NN OO••••
++••••
•••• ••••
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Step 1: attack of nitronium cationStep 1: attack of nitronium cationon on -electron system of aromatic ring-electron system of aromatic ring
HH HH
HH HHHH HH
NONO22++
HH HH
HHHH
HH HH NONO22
++
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Step 2: loss of a proton from the carbocationStep 2: loss of a proton from the carbocationintermediateintermediate
HH HH
HHHH
HH HH NONO22
++
HH HH
HH NONO22
HH HH
HH++
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Where does nitronium ion come from?Where does nitronium ion come from?
HH22SOSO44
OONN
HH
OO
OO
++•••• ••••
••••••••
•••• ••••••••
––OO
NN
HH
OO
OO
++•••• ••••
••••
•••• ••••••••
––
HH++
OO NN OO••••
++••••
•••• •••• ++ HHOO••••HH
••••
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12.4Sulfonation of Benzene
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HH
Sulfonation of BenzeneSulfonation of Benzene
++ ++ HH22OOheatheat
HOHOSOSO22OHOH
SOSO22OHOH
Several electrophiles present:Several electrophiles present:a major one is sulfur trioxidea major one is sulfur trioxide
OOSS
OO
OO
++•••• ••••
••••••••
•••• ••••••••
––
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Step 1: attack of sulfur trioxideStep 1: attack of sulfur trioxideon on -electron system of aromatic ring-electron system of aromatic ring
HH HH
HH HHHH HH
SOSO33 HH HH
HHHH
HH HH SOSO33––
++
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Step 2: loss of a proton from the carbocationStep 2: loss of a proton from the carbocationintermediateintermediate
HH HH
HHHH
HH HH SOSO33––
++
HH HH
HH SOSO33––
HH HH
HH++
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Step 3: protonation of benzenesulfonate ionStep 3: protonation of benzenesulfonate ion
HH HH
HH SOSO33––
HH HH HH22SOSO44
HH HH
HH SOSO33HHHH HH
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12.5Halogenation of Benzene
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HH
Halogenation of BenzeneHalogenation of Benzene
++ ++ HHBrBr
FeBrFeBr33
BrBr22
BrBr
Electrophile is a Lewis acid-Lewis baseElectrophile is a Lewis acid-Lewis basecomplex between FeBrcomplex between FeBr33 and Br and Br22..
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The BrThe Br22-FeBr-FeBr33 Complex Complex
++••••BrBr BrBr•••••••• ••••
•••• ••••
Lewis baseLewis base Lewis acidLewis acid
FeBrFeBr33
BrBr BrBr•••••••• ••••
•••• ••••FeBrFeBr33
––++
ComplexComplex
The BrThe Br22-FeBr-FeBr33 complex is more electrophilic complex is more electrophilic than Brthan Br22 alone. alone.
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Step 1: attack of BrStep 1: attack of Br22-FeBr-FeBr33 complex complex on on -electron system of aromatic ring-electron system of aromatic ring HH HH
HH HHHH HH
BrBr BrBr FeBrFeBr33
––++ HH HH
HHHH
HH HH BrBr
++
+ FeBr+ FeBr44––
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Step 2: loss of a proton from the carbocationStep 2: loss of a proton from the carbocationintermediateintermediate
HH HH
HHHH
HH HH BrBr
++
HH HH
HH BrBrHH HH
HH++
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12.6Friedel-Crafts Alkylation of Benzene
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HH
Friedel-Crafts Alkylation of BenzeneFriedel-Crafts Alkylation of Benzene
++ ++ HHClCl
AlClAlCl33
C(CHC(CH33))33
(CH(CH33))33CClCCl
Electrophile is Electrophile is terttert-butyl cation-butyl cation CC CHCH33
HH33CC
HH33CC++
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acts as a Lewis acid to promote ionizationacts as a Lewis acid to promote ionizationof the alkyl halideof the alkyl halide
Role of AlClRole of AlCl33
(CH(CH33))33CC ClCl ••••••••
••••++ AlClAlCl33
++(CH(CH33))33CC ClCl
••••
••••AlClAlCl33––
(CH(CH33))33CC++
ClCl••••
••••AlClAlCl33––
••••++
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Step 1: attack of tert-butyl cationStep 1: attack of tert-butyl cationon on -electron system of aromatic ring-electron system of aromatic ring
HH HH
HH HHHH HH
HH HH
HHHH
HH HH C(CHC(CH33))33
++
C(CHC(CH33))33++
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Step 2: loss of a proton from the carbocationStep 2: loss of a proton from the carbocationintermediateintermediate
HH HH
HHHH
HH HH C(CHC(CH33))33
++
HH HH
HH C(CHC(CH33))33
HH HH
HH++
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Rearrangements in Friedel-Crafts AlkylationRearrangements in Friedel-Crafts Alkylation
Carbocations are intermediates.Carbocations are intermediates.Therefore, rearrangements can occurTherefore, rearrangements can occur
HH(CH(CH33))22CHCHCHCH22ClCl
AlClAlCl33
Isobutyl chlorideIsobutyl chloride terttert-Butylbenzene-Butylbenzene(66%)(66%)
C(CHC(CH33))33
++
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HH
Rearrangements in Friedel-Crafts AlkylationRearrangements in Friedel-Crafts Alkylation
Isobutyl chloride is the alkyl halide.Isobutyl chloride is the alkyl halide.But But terttert-butyl cation is the electrophile.-butyl cation is the electrophile.
(CH(CH33))22CHCHCHCH22ClClAlClAlCl33
Isobutyl chlorideIsobutyl chloride terttert-Butylbenzene-Butylbenzene(66%)(66%)
C(CHC(CH33))33
++
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Rearrangements in Friedel-Crafts AlkylationRearrangements in Friedel-Crafts Alkylation
CC CHCH22HH33CC
CHCH33
HH
ClCl••••
••••AlClAlCl33
++ ––
CC CHCH22HH33CC
CHCH33
HH++
++ ClCl••••
••••AlClAlCl33––
••••
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HH
Reactions Related to Friedel-Crafts AlkylationReactions Related to Friedel-Crafts Alkylation
HH22SOSO44++
CyclohexylbenzeneCyclohexylbenzene(65-68%)(65-68%)
Cyclohexene is protonated by sulfuric acid, Cyclohexene is protonated by sulfuric acid, giving cyclohexyl cation which attacks the giving cyclohexyl cation which attacks the benzene ringbenzene ring
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12.7Friedel-Crafts Acylation of Benzene
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HH
Friedel-Crafts Acylation of BenzeneFriedel-Crafts Acylation of Benzene
++ ++ HHClCl
AlClAlCl33OO
CHCH33CHCH22CClCCl
CCHCCH22CHCH33
OO
Electrophile is an acyl cationElectrophile is an acyl cation
••••CHCH33CHCH22CC OO ••••++
CHCH33CHCH22CC OO ••••++
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Step 1: attack of the acyl cationStep 1: attack of the acyl cationon on -electron system of aromatic ring-electron system of aromatic ring
HH HH
HH HHHH HH
OO
CCHCCH22CHCH33++ HH HH
HHHH
HH HH
++
OO
CCHCCH22CHCH33
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Step 2: loss of a proton from the carbocationStep 2: loss of a proton from the carbocationintermediateintermediate
HH HH
HHHH HH
HH++
OO
CCHCCH22CHCH33
HH HH
HHHH
HH HH
++
OO
CCHCCH22CHCH33
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can be used instead of acyl chloridescan be used instead of acyl chlorides HH
Acid AnhydridesAcid Anhydrides
AcetophenoneAcetophenone(76-83%)(76-83%)
AlClAlCl33 OO
CCHCCH33
OO
CHCH33COCCHCOCCH33
OO
++
OO
CHCH33COHCOH++
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12.8Acylation-Reduction
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Reduction of aldehyde and ketoneReduction of aldehyde and ketonecarbonyl groups using Zn(Hg) and HCl is carbonyl groups using Zn(Hg) and HCl is called the called the Clemmensen reductionClemmensen reduction..
Acylation-ReductionAcylation-Reduction HH OO
CRCR
AlClAlCl33
RCClRCCl
OO
Zn(Hg), HClZn(Hg), HCl CHCH22RR
permits primary alkyl groups to be attachedpermits primary alkyl groups to be attachedto an aromatic ringto an aromatic ring
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Reduction of aldehyde and ketoneReduction of aldehyde and ketonecarbonyl groups by heating with Hcarbonyl groups by heating with H22NNHNNH22
and KOH is called theand KOH is called the
Wolff-Kishner reductionWolff-Kishner reduction..
Acylation-ReductionAcylation-Reduction HH OO
CRCRHH22NNHNNH22, KOH,, KOH,
triethylene glycol,triethylene glycol,heatheat
AlClAlCl33
RCClRCCl
OO CHCH22RR
permits primary alkyl groups to be attachedpermits primary alkyl groups to be attachedto an aromatic ringto an aromatic ring
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Example: Prepare isobutylbenzeneExample: Prepare isobutylbenzene
No! Friedel-Crafts alkylation of benzene No! Friedel-Crafts alkylation of benzene using isobutyl chloride fails because of using isobutyl chloride fails because of rearrangement.rearrangement.
(CH(CH33))22CHCHCHCH22ClCl
AlClAlCl33
CHCH22CH(CHCH(CH33))33
HH
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RecallRecall
(CH(CH33))22CHCHCHCH22ClClAlClAlCl33
Isobutyl chlorideIsobutyl chloride terttert-Butylbenzene-Butylbenzene(66%)(66%)
C(CHC(CH33))33
++
HH
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Use Acylation-Reduction InsteadUse Acylation-Reduction Instead
++ (CH(CH33))22CHCClCHCCl
OO
AlClAlCl33 OO
CCH(CHCCH(CH33))22
Zn(Hg)Zn(Hg)HClHCl
CHCH22CH(CHCH(CH33))33
HH