Eliminación E1

8/19/2019 Eliminación E1

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© 2011 Pearson Education, Inc.1

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$



-

'e substituent incorporated into te common name is

te 1position$

(l)ays gi*e substituents the lo)est possible numbers+



/

Substituted benzenes undero te fi!e electropilic

aromatic substitution reactions discussed in apter 1($



10

'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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(0

# # p pp

nitrosonium ion to te orto position$

unnumbered fi, p */0C


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(1

onsider te s#ntesis of paracloroet#lbenzene$


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(2

onsider te s#ntesis of para cloroet#lbenzene$

?luorination and Iodination of Benzene


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(3

?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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*0

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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*1

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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*2

%ucleopilic ;romatic Substitution

5eactions

%ucleopilic aromatic substitution reactions reDuire at least onestronl# electron"itdra"in substituent to occur$


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*3

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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*/


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+0

Benz#ne Is an Extremel# 5eacti!e


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+1

Benz#ne Is an Extremel# 5eacti!e

Species

Pol#c#clic Benzoid #drocarbons


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Pol#c#clic Benzoid #drocarbons

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