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Organic Chemistry
6th EditionPaula Yurkanis Bruice
Chapter 16
Reactionsof
Substituted
Benenes
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Examples of Substituted Benzenes
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'e t"o substituents are listed in alpabetical order$
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) Common names are preferred in naming certain
substituted benenes! e"g"! toluene! aniline! phenol"
) #o not deconstruct the common name$ e"g"! do notchange %toluene& to %methylbenene"&
) 'he substituent that is part of the common name is
position 1! but do not label as such in the chemical name"
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Some disubstituted benzenes a!e common names tat
incorporate bot substituents$
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%amin Pol#substituted Benzenes
'e substituents are numbered in te direction tatresults in te lo"est possible number$
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'e substituent incorporated into te common name is
te 1position$
(l)ays gi*e substituents the lo)est possible numbers+
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Substituted benzenes undero te fi!e electropilic
aromatic substitution reactions discussed in apter 1($
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'e slo" step of an electropilic aromatic substitution
reaction is te formation of te carbocation intermediate$
) Electron,donating substituents increase the rate of
substitution reactions by stabiliing the carbocation
intermediate"
) Electron,)ithdra)ing substituents decrease the rate ofsubstitution reactions by destabiliing the carbocation
intermediate"
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Inducti!e Electron itdra"al
Electron onation b# #percon4uation
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5esonance Electron onation and itdra"al
Substituents suc as %2, 6, 65, and l donate
electrons b# resonance, but te# also "itdra" electronsinducti!el#$
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Substituents suc as 76, 8%, S63, and %62
"itdra" electrons b# resonance$
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Electron,donating substituents
increase the reacti*ity of
the benene ring to)ard
electrophilic aromatic substitution
Electron,)ithdra)ing substituents
decrease the reacti*ity of the
benene ring to)ard electrophilic
aromatic substitution
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Electron donation into te benzene rin b# resonance ismore sinificant tan inducti!e electron "itdra"al from
te rin$
Electrononatin Substituents
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5esonance donation into te benzene rin competes
"it resonance donation into te carbon#l
6!erall, tese substituents "ea9l# release electrons
Inducti!e "itdra"al into te benzene rin also occurs
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'ese substituents are less effecti!e in donatin
electrons into te rin because:
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;l9#l, ar#l, and 75 roups are "ea9l# acti!atin
substituents because te# are slitl# more electron
donatin tan te# are electron "itdra"in$
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'ese substituents donate into te rin b# resonance
and "itdra" electrons from te rin inducti!el#$
'e# "itdra" electrons inducti!el# more stronl# tan
te# donate electrons b# resonance
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'ese substituents "itdra" electrons bot inducti!el#
and b# resonance$
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'ese substituents are po"erful electron"itdra"in
roups$
Except for te ammonium ions, tese substituents
"itdra" electrons bot inducti!el# and b# resonance
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'e substituent alread# attaced to te benzene rin
determines te location of te ne" substituent$
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;ll acti!atin substituents are orto
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'e "ea9l# deacti!atin aloens are orto
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;ll substituents tat are more deacti!atin tan aloens
are meta directors$
; t di ti b tit t
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;n orto,paradirectin substituent$
; t di ti b tit t
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;n orto,paradirectin substituent$
; t di ti b tit t
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;n metadirectin substituent$
'e Effect of Substituents on pK
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Electron"itdra"in roups stabilize a base and
terefore increase te strent of its con4uate acid
Electrondonatin roups destabilize a base and tus
decrease te strent of its con4uate acid
'e Effect of Substituents on pK a
'e more electronic deficient a s bstit ent on penol
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'e more electronic deficient a substituent on penol,
te stroner te acid$
'o understand the relati*e pK a *alues! consider the delocaliation of
the phenolate anion -stars sho) anion distribution./
0nstable
Stable/ %through resonance&
of anion into nitro
ore stable anion 2
lo)er pK a
'e more electronic deficient a substituent on benzoic
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'e more electronic deficient a substituent on benzoic
acid, te stroner te acid$
Substituent effect on pK a is minimal in benoic acids
because only inducti*e electronic effects are present/
3hy4Because the benene ring is
cross con5ugated )ith the
carboylate anion
'e more electronic deficient a substituent on a
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'e more electronic deficient a substituent on a
protonated aniline, te stroner te acid$
'o understand the relati*e pK a *alues! consider the delocaliation of the
aniline lone pair of the con5ugate base -stars sho) anion distribution./
0nstable
Stable/ %through resonance&
of lone pair into nitro
ore stable lone pair
2 lo)er pK a
' t d t ti d it i
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'e orto
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=etox# and #drox# substituents are so stronl#
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=etox# and #drox# substituents are so stronl#
acti!atin tat aloenation is carried out "itout a
>e"is acid$
'e presence of >e"is acid and excess bromine
enerates te tribromo deri!ati!e$
; benzene rin "it a meta director cannot undero a
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; benzene rin "it a meta director cannot undero a
?riedel
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;niline and N substituted anilines do not undero
?riedel
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In desinin a disubstituted benzene, consider
te order of substitution$
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'e ?riedel
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In te s#ntesis of para clorobenzoic acid from toluene,
te met#l roup is oxidized after clorination$
In te s#ntesis of metaclorobenzoic acid, te met#l
roup is oxidized before clorination$
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'o s#ntesize pprop#lbenzenesulfonic acid$) Introduce te prop#l roup b# ?riedel
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S#ntesis of 'risubstituted Benzenes
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Steric indrance ma9es te position bet"een te
substituents less accessible
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; stronl# acti!atin substituent "ill "in out o!er a
"ea9l# acti!atin substituent or a deacti!atin
substituent
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If te t"o substituents a!e similar acti!atin properties,
neiter "ill dominate$
S#ntesis of Substituted Benzenes
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S#ntesis of Substituted Benzenes
Asin ;renediazonium Salts
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Preparation of te iazonium Salt
=ecanism$
%itrosonium ion formation
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iazonium ion formation$
Caution/ #iaonium salts are eplosi*e+
'e reaction stops because a secondar# amine lac9s a
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'e reaction stops because a secondar# amine lac9s a
second proton$
'e bul9# dial9#l amino roup bloc9s te approac of te
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# # p pp
nitrosonium ion to te orto position$
unnumbered fi, p */0C
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onsider te s#ntesis of paracloroet#lbenzene$
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onsider te s#ntesis of para cloroet#lbenzene$
?luorination and Iodination of Benzene
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?luorination and Iodination of Benzene
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#drox#lation of Benzene
S f i i 5 ti
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Summar# of iazonium 5eactions
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S#ntesis Example
S#ntetic taret$Propose a
synthesis from a
monosubstituted
benene
;ns"er$
'e ;renediazonium Ion as an
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'e ;renediazonium Ion as an
Electropile
6nl# il# acti!ated benzene rins can undero tisreaction
Substitution ta9es place preferentiall# at te para
position
o"e!er if te para position is bloc9ed
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o"e!er, if te para position is bloc9ed :
=ecanism$
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i
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iazo #es#iaonium coupling affords synthetic dyes/
7arge dipole results in deep color -high etinction
coefficient./
Electronic %push,pull&
produces a dipole
i d S lf
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iazo #es and Sulfa rus8erhard #omagk studied the antibiotic properties of diao
dyes" 9e )as a)arded the :obel Prie for medicine in 1;
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%ucleopilic ;romatic Substitution
5eactions
%ucleopilic aromatic substitution reactions reDuire at least onestronl# electron"itdra"in substituent to occur$
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Electron"itdra"in substituents increase te reacti!it#of te benzene rin to"ard nucleopilic substitution and
decrease te reacti!it# of te benzene rin to"ard
electropilic substitution
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'e electron"itdra"in substituents must be orto or
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para to te site of nucleopile attac9,
so tat electrons of te attac9in nucleopile can be
delocalized into tese substituents
'e incomin roup as to be a stroner base tan te
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roup tat is bein replaced$
;ent 6rane and %ucleopilic
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;ent 6rane and %ucleopilic
;romatic SubstitutionS#ntesis of ;ent 6rane$
Side reaction$
#ioin is carcinogenic and causes birth defects
?ormation of Benz#ne
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?ormation of Benz#ne
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Benz#ne Is an Extremel# 5eacti!e
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Benz#ne Is an Extremel# 5eacti!e
Species
Pol#c#clic Benzoid #drocarbons
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Pol#c#clic Benzoid #drocarbons