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KENYATTAUNIVERSITY

INSTITUTEOFOPENLEARNING

SCH302:CHEMISTRYOFAROMATICCOMPOUNDS

DR.A.W.WANYONYI

DR.A.K.MACHOCHO

CHEMISTRYDEPARTMENT

CHEMISTRYOFAROMATICCOMPOUNDS

PREFACE/GENERALINTRODUCTIONTOSCH302

Thecoursedealswithbenzene,itsderivativesandotherbenzenoidcompounds.Structure

ofbenzene,aromaticityofbenzeneandrelatedcompoundswillformthefirstpartofthe

course.Nomenclatureofbenzeneanditsderivativesaredealtwithindetailssincethis

willformsomeoftheproductsofthereactionsinthelessonsthatfollow.

Mechanisticapproachisemployedtoexplainchemicalreactionsofbenzeneandits

derivatives.Predominantelectrophilicaromaticsubstitutionreactionspatterns,effectof

substituentontheratesofreactionandorientationofincomingsubstituentarediscussed.

Reactionsofbenzenoidsarediscussedinthelightofsimilarityinchemicalbehaviorto

benzene.Itwillbenotedthatmanycompoundswillbesynthesizedinthecourseof

discussingthesereactionsastheendproducts

Keywordsintherelevantlessonsandproblemstoenhanceunderstandingorasset

inductiontolessontobecoverednextandgivepracticetostudentsareprovidedinthe

boxes.Eachchapterhaspracticequestionsattheendandsolutionsareprovidedasan

illustrationonhowtoapproachsimilarproblems.

Practicalsintheappendixaremeanttoillustratespecificreactionsintheactual

conditionsappliedinthepreparationofaromaticcompoundsespeciallythosecoveredin

themodule.

CONTENTS

Page

Chapter1:Structureofbenzeneandaromaticity1

1.0Introduction1

1.1Historicalbackgroundonbenzene1

1.2Structureofbenzene2

1.3Characteristicsofbenzenering3

1.4Aromaticityandnon-aromaticity5

1.5Benzenoidaromaticandnon-benzenoidcompounds6

1.6Someofaromaticcompoundsinnaturalsystems11

1.7Summary13

1.8Questionsandsolutions13

Chapter2:Nomenclatureofsubstitutedbenzenesandother

benzenoidcompounds16

2.0Introduction16

2.1Nomenclatureofmonosubstitutedbenzene16

2.2Nomenclatureofsubstitutedbenzene17

2.3Nomenclatureofotherbenzenederivatives18

2.4Nomenclatureofsubstitutednaphthalenes20

2.5Nomenclatureofanthracenes20

2.6Nomenclatureofphenanthrenes21

2.7Summary23


Chapter3:Reactionsofbenzene25

3.0Introduction25

3.1Reductionofbenzene25

3.2Electrophilicaromaticsubstitution(EAS)ofbenzene:Ageneralmechanism27

3.3Halogenation27

3.4Nitration29

3.5Sulphonation29

3.6Friedel-Craftalkylation30

3.7FriedelCraftacylation31

3.8Nitrosation32

3.9Summary33


Chapter4:Reactionsofbenzenederivatives35

4.0Introduction35

4.1EffectofsubstituentsonEASpatterns/orientation35

4.2Groupsthatdonateelectronstothering39

4.3Effectofgroupsthatdonatebyinductionandrelease

electronsbyresonance41

4.4Effectofathirdsubstituentonthebenzenering42

4.5Summary43


Chapter5:Reactionsofarenesandarylhalides47

5.0Introduction47

5.1Arenes47

5.2ReactionsofArenes48

5.3EASreactionsofarylhalide50

5.4Removalofhalidesubstituentfrominactivatedarylhalide53

5.5Summary54


Chapter6:Phenols57

6.0Introduction57

6.1Physicalandchemicalpropertiesofphenol57

6.2.1Industrialmanufactureofphenol59

6.2.2Laboratorypreparationofphenol60

6.3Reactionsofphenols61

6.3.1Etherandesterformation61

6.3.2Nitrationandsulphonationofphenol62

6.3.3Brominationandacylationofphenol62

6.3.4Kolbeandcouplingreactionsofphenol63

6.4UsesofPhenols64

6.5Summary64


Chapter7:Anilines66

7.0Introduction66

7.1Preparationandpropertiesofaniline66

7.2Reactionsofaniline68

7.3Diazoniumion70

7.4Summary72


Chapter8:Polynucleararomaticcompounds:Naphthalene74

8.0Introduction74

8.1Synthesisofnaphthalene74

8.2Reduction-oxidationreactionsofnaphthalene75

8.3OrientationandEASreactionsofnaphthalene76

8.4Electrophilicaromaticsubstitutionreactionsofnaphthalene77

8.5OrientationEASreactionsnaphthalenesderivatives79

8.6Summary80


Chapter9:AnthraceneandPhenanthrene83

9.0Introductionandpreparations83

9.1OxidationandReductionreactions84

9.2Diels-Alderreactions84

9.3Halogenationofanthraceneandphenanthrene85

9.4Sulphonationofanthraceneandphenanthrene85

9.5Alcoholsofanthracene86

9.6Positionalactivityofanthracene86

9.7Anthraquinones87

9.8Summary88


References90

Practicals91

ElectrophilicAromaticSubstitutionreactions91

Exp.1PreparationofNitrobenzene92

Exp.2ReductionofNitrobenzenetoAniline94

Exp.3NitrationofAnilinetop-Nitroaniline96

Exp.4PreparationofIodobenzene98

Exp.5PreparationofMethylOrange(p-(p-Dimethyl-

Aminophenylazo)BenzeneSulphonicacid,SodiunSalt100

Exp.6Preparationofp-Di-t-Butylbenzene102

CHAPTER1

STRUCTUREOFBENZENEANDAROMATICITY

1.0Introduction

Thischapterwilllookatdetailedscientificargumentsthatledtostructureestablishment

ofbenzene,conceptofsp2orbitalsanddelocalisationofelectronsinporbitalsformingpi

()electroncloudaboveandbelowbenzenemolecule.ApplicationofHuckelsruleto

benzenoidcompoundstoestablisharomaticandnon-aromaticcompoundswillbe

discussed.Someimportantaromaticcompoundsinnaturalsystemswillbementionedas

appreciationoftheimportanceofaromaticcompounds.

Objectives

Bytheendofthislesson,youshouldbeableto:

Discussthebackgroundandargumentsthatledtotheestablishmentofbenzene

structure

Describebondinginbenzenemolecule

Explainusingresonance,thestabilityofbenzenering

Statereasonswhybenzeneissusceptibletoelectrophiles

Statethecriteriaforaromaticity

Distinguisharomatic,benzenoidaromaticandnon-aromaticcompoundsusing

Huckelsrule

Statesomeoftheimportantaromaticcompoundsinnaturalsystems

1.1Historicalbackgroundonbenzene

BenzenewasdiscoveredbyMichaelFaradayin1825fromwhaleoilandreported

empiricalformulaasCH2.However,thecorrectmolecularformulawasestablishedin

1834whenEihartdMitscherlichsynthesizeditandwasgivenasC6H6andthisformula

elicitedmanystructuresuggestions.

Compoundslikebenzenewithrelativelyfewhydrogenatomscomparedtocarbonsare

usuallyobtainedfromnaturaloilsfromeitherplantsoranimals.Duetotheirfragrance,

theywerecalledaromatictodistinguishthemfromaliphatic,whichhavehigher

hydrogentocarbonratioobtainedfromdegradationoffats.Figure1showssomeofthe

mostcommonbenzenoidcompounds.

or

Benzene

Phenanthrene

NaphthaleneAnthracene

Pyrene

Triphenylene

Figure1.Somecommonbenzenoidcompounds

1.2Structureofbenzene

Therewerevarioussuggestionstothestructureofbenzenethatwouldfittheformula

C6H6.SomeofsuchstructuresareillustratedinFig.2below.

1.

Kekule

Structure

2.3.

Dewar

Structure

CH2

4.5.

Figure2.Someoftheproposedstructuresofbenzene

Inthecourseofestablishingtheactualstructureofbenzenethefollowingobservations

weremade:

1.Benzenehasonlyonemono-substitutedisomer,C6H5Y(Y=Cl,Br,I,NO2

etc).Thisargumenteliminatesstructures3and5.

2.Benzeneyieldsonlythree(3)disubstitutedisomers,C6H4XXorC6H4XY.

(e.g.C6H4Br2orC6H4BrNO2).Structure4ofthediyneiseliminatedbecauseit

wouldgivetwoisomers.Structure2(Dewar)waseliminatedbecauseitwould

give6structures!

X

X

Y

X

Y

Y

1,21,31,4

FromtheseargumentsonlytheKekulestructureremained.

Hevisualizedbenzeneasadynamicstructurewherethedoublebondsaredelocalized(

electronsarenon-localized).Useofacircleasinthestructureonright-handbelow

indicatesthattheelectronsaredelocalized.

Thebondlengthsinbenzenearethesameofabout1.39.ThisisestablishedwithX-

raydiffractiontechniques.

EthaneC-C1.54

EtheneC=C1.34

Thusthecarbon-carbonbondinbenzeneisahybridofsingleanddoublebond.

1.3Characteristicsofbenzene

Ithasbeenestablishedthatbenzenehasverydifferentpropertiesfromcloselyrelated

molecules.Acomparisonofbenzenewithcyclohexeneandcyclohexadieneismade.

CyclohexeneCyclohexadieneCyclohexatriene

(Benzene)

ThealkenesundergoelectrophilicadditionreactionswithH2,Cl2,Br2,I2etc.Benzeneon

theotherhandpreferstoundergoelectrophilicaromaticsubstitution(EAS)reactions.The

Tablebelowillustratessomeotherreactionswherebenzenebehavesdifferentlyfromthe

othertworelatedcompounds.

Thusbenzenehassomespecialstability.

Heatsofhydrogenation

Considerthefollowingheatsofhydrogenationofbenzeneandrelatedcompounds.

Hydrogenationofthethreecompoundsformscyclohexane.

H2,Heat

2H2,Heat

3H2,Heat

3H2,Heat

H(Kcalmol-1)

28.6

55.4

49.8

85.8(calculated)

Theinformationcanberepresentedinformofenergydiagram.

Cyclohexanetriene

(Calculated)

Cyclohexanediene

Energy

(Kcalmol-1)

Cyclohexene

Benzene

85.8

36.0

Resonance

Energy

55.449.8

28.6

ProgressoftheReaction

Figure3.Energydiagramofbenzeneandrelatedcompounds

Itisobservedthattheheatofhydrogenationofbenzeneislowerthanexpectedby36.0

Kcalmol-1.Thisisknownasstabilizationorresonanceenergy.

Thisisbecausethehydrogenationisanadditionreactionthatconvertsbenzeneintoaless

stablestructureasthearomaticityislost.Itis,therefore,easierforbenzenetoundergo

substitutionreactionswherethearomaticpropertiesareretained.

Orbitalpictureofbenzene

Allcarbonatomsofbenzenearesp2hybridized.Therefore,itisatrigonalplanerwith

bondanglesofabout1200.Ithassix(pi)electrons(3bonds),whicharedelocalized

inthep-orbitals.

Itisvisualizedasifthereisacloudoftheelectronsaboveandbelowtheplaneofthe

benzenemolecule.Theelectronsarelooselyheldthanthe(sigma)electronsandare

availableforstartingareaction.Hence,benzeneissusceptibletoelectrophilicreagents.

E

E+E

+

H

2

1

Y-

2

1

E

H

Y

Stableasthearomaticity

isretained

Question:Identifythetypeofreactionindicatedby1and2aboveandcommenton

thestabilityofthecompounds.

1.4Aromaticityandnon-aromaticity

Aromaticcompoundsarethosethatresemblebenzeneinbehavior.Theymusthavethe

followingfeatures:

1.Delocalizedelectrons.

2.ThedelocalizedelectronsthatsatisfyHuckelsrule;4n+2;wheren=0,1,

2,3,4etc.Thatis,thetotalnumberofelectronsmust2,6,8,10,14,18etc.

3.Cyclic.

4.

Planar.

Note:Therearearomaticcompoundsthatdonotresemblebenzeneintermsof

structureandmaynotcontainthephenylring.Theyarementionedin1.5.1.

Thedefinitionofaromaticcompoundscanthereforebemodifiedtoread:Aromatic

compoundsarethosecyclicplanarcompoundsinwhichtheelectronsenergyofthe

cyclicformislowerthanthatoftheopenchain.

1.5Benzenoidaromaticandnon-benzenoidcompounds

Benzenoidsarecompounds,whichresemblebenzeneintermsofstructureandbehavior.

BenzeneNaphthalene

Benzenehassixelectronsthataredelocalized.Itisplanarandcyclic.Accordingto

Huckelsrule,ithassixpielectrons,Thus,6=4n+2n=1.Thereforeitisaromatic.

Naphthalenehas10electrons(5doublebonds)thataredelocalized.Itisplanarand

cyclic.10=4n+2n=2.Itisaromatic.

Anthracene

Naphthacene

Anthracenehasthreebenzeneringsfusedinalinearmanner.Ithas14electrons(7

doublebonds)thataredelocalized.Itisplanarandcyclic.14=4n+2n=3.Itis

aromatic.

Naphthacene(Notnaphthalene!)has18electrons(9doublebonds)thataredelocalized.

Itisplanarandcyclic.18=4n+2n=4.Itisaromatic.

Triphenylene

Triphenylenehas18electrons(9doublebonds)thataredelocalized.Itisplanarand

cyclic.18=4n+2n=4.Itisaromatic.

1.5.1Non-benzenoidaromaticcompounds

Thesearecompoundsthatarearomaticbutarenotbenzenoid,thatis,theydonot

resemblebenzeneanddonotcontainthephenylgroup.

-

-

-

Thestructuresrepresentcyclopentadienylanion.Asshownthenegativechargecanbe

delocalizedtootherpositionsinthestructure.Ithas6electrons(2doublebondsplus

onenegativecharge)andtheyaredelocalisable,itisplanarandcyclic.6=4n+2n=

1.Itisaromatic.Theanionformsstablecomplexeswithtransitionmetalsbecauseof

aromaticproperties.

Considercyclopentadienylcation.Isitaromatic?

+

+

+

Thepositivechargecanbedelocalisable;thecationisplanarandcyclic.Ithas4

electrons;thus4=4n+2n=0.5.Thisisnotanintegerandthereforesystemdoesnot

satisfytheHuckelsrule.Thusthecationisnotaromatic.

Question:Explainwhycyclopentadieneisastrongeracidthanmostother

hydrocarbons.

Answer:Acidityislossofaprotonbyagivenspecies(likeamolecule).

Cyclopentadienethereforedissociatesintoahydrogenionandcyclopentadienylanion,

whichhasbeendescribedasstablebecauseofbeingaromatic.Thereforethe

dissociationisfavored.

Thisiscycloheptatrienecationformedafterlossofahydride(H-).Ithassixelectrons

delocalisableelectron(thepositivechargebedelocalized).Itisplanarandcyclic.6=4n

+2n=1.Itisthereforearomatic.Thecorrespondinganionisnotaromaticbecauseit

doesnotobeyHuckelsrule.

+

Cyclopropenylcationhastwoelectronswhicharedelocalisableelectron.Itisplanar

andcyclic.2=4n+2n=0.Itisthereforearomatic.Itscorrespondinganionisnot

aromaticbecauseitdoesnotobeyHuckelsrule.

Azulene

Azuleneisacompoundcontainingasevenmemberedandfivememberedringsfused

together.Ithas10delocalisableelectrons.Itisplanarandcyclic.10=4n+2n=2.

Itisthereforearomatic.

Note:Sometimesdelocalizedelectronsdonothavetobetheelectronsbutcanbe

non-bondingelectrons(lonepairsofelectrons).

ThisisthecaseinheteroaromaticcompoundswheretheheteroatomslikeO,N,SorP

areintroducedinthering.Theirlonepairsofelectronsareabletocontributetothe

aromaticbehavior.Someexamplesaresitedbelow.

-

H+

N

H+

Pyroleisafive-memberedringwithnitrogenastheheteroatom.Byutilizationofthe

lonepairofelectronsofnitrogen,therearesixdelocalisableelectrons.Itisplanarand

cyclic.6=4n+2n=1.Itisthereforearomatic.

:O:

:S:

Furanandthiophenehaveoxygenandsulphurastheheteroatoms,respectively.By

utilizationofonelonepairofelectronsofoxygenorsulphur,themoleculeshavesix

delocalisableelectronseachandwillobeyHuckelsrule.

..

N

Pyridine

PyridineisplanarcyclicandobeysHuckelsrule.Notethatthelonepairofelectronsin

nitrogenisnotutilizedinthiscase.Ithassimilarpropertiesasthebenzenoidsand

undergoeselectrophilicaromaticsubstitutionreactions.

1.5.2Anti-aromaticcompounds

Thesearecompoundsinwhichthecyclicformshavehigherelectronsenergythanthe

openchainanalogues.Whentheringopensupwhatisformedisstillanti-aromatic.The

bestexamplerepresentingthisgroupofcompoundsiscyclobutadiene.

-H2

LowerEnergy

HigherEnergy

.

LowerEnergy

+

-H2

-H2

.

HigherEnergy

+

LowerEnergy

.

LowerEnergy

HigherEnergy

-H2.

HigherEnergy

AllthespeciesdonotobeyHuckelsruleandasindicatedthecyclicformsareofhigher

energythentheiropenanalogues.

1.5.3BenzenoidcompoundsthatdonotsatisfyHuckelsrule

Therearesomecompoundsthatcontainbenzeneringsfusedindifferentwaysbutdonot

satisfytheHuckelsrule,themaincharacteristicfeatureofaromaticity,yettheyarestill

consideredaromatic.Considerthefollowingexamples:

Pyrene10b,10c-Dihydropyrene

Whichofthesetwocompoundsisaromaticandwhichisnot?Pyreneiscyclic,has

delocalisableelectronsandplanar.Butithas16electrons(8doublebonds)and

applyingHuckelsrule;16=4n+2n=3.5,itdoesnotsatisfyit.Howeverthe

moleculeisconsideredaromatic.10b,10c-Dihydropyreneontheotherhandhas14

electronsandsatisfiesHuckelsruleandallotherconditionsofaromaticity.Itis

consideredasfullyaromaticincomparisonwithpyrene.

Benzo[def]chrysene

(12b,12c-Dihydrobenzo[a]pyrene)

Benzo[a]pyrenehas20electronsandwillthereforenotsatisfytheHuckelsrule.Butit

isplanar,hasdelocalisableelectronsandcyclic.Itisalsoconsideredasaromatic.

12b,12c-dihydrobenzo[a]pyrenewith18electronsisfullyaromaticasitsatisfiesthe

Huckelsrule.

Note:Huckelsrulestrictlyappliestomono-benzenoidcompounds.

1.5.4Annulenes

Thesearenon-benzenoidmulti-cycliccompounds.Someofannulenesarenotaromatic

yettheymaybesatisfyingtheHuckelsrule.Consider[10]annuleneforexample.

Naphthalene

Cyclodecapentaene

([10]annulene)

Acloserlookindicatethat[10]annulenecanbeconsideredtobemodifiedfrom

naphthalenewherebythebridgingbondisabsent.However,itremainswith10

electronslikenaphthalene.ItthereforesatisfiesHuckelsrule.Butitisnotplanarandnot

consideredasaromatic.

Anthracene

Cyclotetradecaheptaene

([14]annulene)

Whenanthracenelosesthetwobridgingbonds,itforms[14]annulene,whichretainsthe

sevendoublebonds(14electrons).Like[10]annulene,[14]annuleneiscyclicsatisfies

theHuckelsruleandtheelectronsaredelocalisable.Unlike[10]annulene,however,

[14]annuleneisplanarandthereforearomatic.

Bicyclo[4.2.0]octa-1(8),2,4,6-

tetraene

Cyclooctatetraene

([8]annulene)

Bothoftheabovecompoundsarenotaromatic.Although[8]annulenehastheelectrons

delocalisableandcyclic,isnotplanar,doesnotsatisfyHuckelsrule.Itistermedasanti-

aromaticbecauseitisofhigherelectronsenergythanitsopenanalogue,octa-1,3,5,7-

tetraene.

1.6Someofaromaticcompoundsinnaturalsystems

Aromaticcompoundsareofimportanceinnaturalsystems.Manyredoxreactionsinthe

celloccurduetothepresenceofco-enzymes(complexcompoundsrequiredforany

enzymetofunctioninabiologicalreactions),whicharearomaticinnature.Someofthe

mostimportantaminoacidsarearomatic.Thesearephenylalanineandtyrosine.

O

OH2NCHCOH

H2NCHCOH

CH2

CH2

OH

2-Amino-3-phenyl-propionicacid

(Phenylalanine)

2-Amino-3-(4-hydroxy-phenyl)-propionicacid

(Tyrosine)

Structuresofanumberofhormonescontainaromaticrings.Agoodexampleis

adrenaline,whichactsasatransmitterofnerveimpulses.

O

NH2

HO

CO2H

2-Amino-3-[4-(4-hydroxy-3,5-diiodo-phenoxy)-3,5-diiodo-phenyl]-propionicacid

(Adrenaline)

Estrone(oestrone)isproducedbymammalianovariesandcontrolsdevelopmentof

femalecharacteristicsandmenstruationcycle.Asadrug,itisusedforreplacement

therapyindeficiencystateslikeprimaryamenorrhoea(abnormalabsenceof

menstruation),delayedonsetofpuberty,controlandmanagementofmenopausal

syndromeandmalignantneoplasmoftheprostate.

O

H

H

H

HO

Estrone

Deoxyribonucleicacid(DNA)andRibonucleicacid(RNA)areessentialforstorageof

geneticinformationandsynthesisofenzymesandproteinsneededformetabolisms.They

arecomposedof,amongotherchemicalfeatures,variousaromaticbasesofpurineand

pyrimidine,whicharearrangeddifferentsequence.

NH2

O

N

N

N

N

N

NH

N

H

Purine

NN

HH

AdenineGuanine

NH2

O

NH2

N

N

NH

N

Pyrimidine

NO

H

Cytosine

N

H

Thymine

O

NotethatgeneticinformationflowsfromDNAtoRNAtoproteinsbutincaseof

retroviruseslikeHIVtheinformationflowsisreverse,thatis,RNAtoDNAtoproteins.

Melanine,apolymerofindolederivative,isthedarkpigmentoftheskin.

O

O

HOOC

N

H

N

H

COOH

O

O

Melanine

1.7Summary

Benzenemoleculehasincreasedelectrondensityduetodelocalizedelectronsinsp2

orbitals.

Benzeneisverystableduetohighresonance/delocalisationenergy.

Aromaticnatureofbenzenemakesittohaveuniquechemicalproperties.

Anti-aromaticcompoundshavehigherenergiesincyclicformthaninthecorresponding

openchainanalogues.

Theconditionshavetobesatisfiedbyacompoundbeforeitiscalledaromatic.

Benzenoidsarearomaticcompoundsthatresemblebenzeneintermsofstructure.

TherearebenzenoidcompoundsthatdonotsatisfyHuckelsrulebutarearomatic.

Manyredoxreactionsincellsoccurduetoco-enzymesthatarearomaticinnature.

1.8QuestionsandSolutions

Questions

Q1.(a)DrawallstructuresthatwillsatisfytheformulaC6H6.

(b)Assumethatmonobrominationsubstitutionswerecarriedoutinallthe

structuresin(a)above.Whichofthestructureswillgiveisomersoftheproducts?

Q2.Analyzeandclassifythefollowingchemicalspeciesasaromatic,non-aromaticor

anti-aromatic.

+

(iii)

N

(iv)(v)-(vi)

(vii)

-

Solutions

1(a)

1.

Kekule

Structure

2.3.

Dewar

Structure

CH2

4.5.

(b)(i)Kekulestructure(1)hasonlyoneproduct.

(ii)Dewarstructure(2)hastwoproducts.

(iii)Structure3hasthreeproducts.

(iv)Structure4hasonlyoneproduct

(v)Structure5hastwoproducts

MonobrominationBr

1

Monobromination

+

2

Br

Br

CH2Monobromination

BrBr

CH2+

CH2

+

CHBr

3

Monobromination

4

Monobromination

Br

+

Br

2.

5

Br

(i)Cycloheptatrienylcationhas6electrons,hence,6=4n+2,n=1.Therefore

obeysHuckelsrule.Itisplanar,cyclicandelectronsaredelocalisable.Thusit

isaromatic.

(ii)Cyclopropenylanionhas4electrons,hence,4=4n+2,n=0.5.Therefore

doesnotobeyHuckelsrule.Thusitisnon-aromatic.

(iii)Pyridinehas6electronsalonepaironnitrogenwhichisnotinvolvedin

delocalisationinthiscase,hence,6=4n+2,n=1.Thereforeitobeys

Huckelsrule.Itisplanar,cyclicandelectronsaredelocalisable.Thusitis

aromatic.

(iv)Cyclobutadienehas4electrons,hence,4=4n+2,n=0.5.Itdoesnotobey

Huckelsrule.Itsopenchainanaloguehaslowerelectron.Thusitisanti-

aromatic.

(v)Cyclopentadienylanionhas6electrons,hence,6=4n+2,n=1.Therefore

itobeysHuckelsrule.Itisplanar,cyclicandelectronsaredelocalisable.Thus

itisaromatic.

(vi)9bH-Benzopyreneisapolybenzenoidcompound.Ithas18electrons,hence,

18=4n+2,n=4.ThereforeobeysHuckelsrule.Itisplanar,cyclicand

electronsaredelocalisable.Thusitisaromatic.

(vii)Cyclooctatetraenehas8electrons,hence,8=4n+2,n=1.5.Itdoesnot

obeyHuckelsrule.Itsopenchainanaloguehaslowerelectron.Thusitis

anti-aromatic.

(viii)Cycloheptatrienylanionhas8electrons,hence,8=4n+2,n=1.5.Itdoes

notobeyHuckelsrule.Althoughitselectronsaredelocalisable,itscyclic

andplanar,itisnotaromaticduetoHuckelsrule.Itistermedasnon-

aromatic.

(ix)Cyclopentadienylcationhas4electrons,hence,4=4n+2,n=0.5.Itdoes

notobeyHuckelsrule.Itsopenchainanaloguehaslowerelectron.Thusit

isanti-aromatic.

CHAPTER2

NOMENCLATUREOFSUBSTITUTEDBENZENESANDOTHER

BENZENOIDCOMPOUNDS

2.0Introduction

Inchapterone,welookedatthestructureofbenzeneandotherbenzenoidcompounds.In

thischapterwearegoingtolookatconvectionalrulestofollowwhennaming

monosubstituted;disubstitutedandpolysubstitutedbenzeneandotherbenzenerelated

compounds.IUPACsystemofnomenclaturewillbeemployedinmostcases.However,

specialnamesthatareinternationallyacceptedwillalsobeused.

Objectives


StatebothIUPACnamesandspecialnamesformonosubstitutedbenzene

Drawstructureswhengivennamesofmonosubstitutedbenzene

StatebothIUPACnamesandspecialnamesforsubstitutedbenzene

Drawstructureswhengivennamesofsubstitutedbenzene

StatebothIUPACnamesandspecialnamesofotherbenzenederivatives

Drawstructureswhengivennamesofbenzenederivatives

StatebothIUPACnamesandspecialnamesforsubstitutednaphthalenesand

anthracenes

Drawstructureswhengivennamesofsubstitutednaphthalenesandanthracenes

2.1Nomenclatureofmonosubstitutedbenzene

Likeotherorganiccompoundsbenzeneandotheraromaticcompoundshavevarious

methodofnaming.However,forcontinuityIUPAC(systematic)namingispreferredand

thefollowingruleswillapply.

Rule1.Formonosubstitutedcompoundsthenameshouldreadinsuchawaythatthe

prefixofthesubstituentsappearbeforethewordbenzene.

FCl

Br

I

NO2

FluorobenzeneChlorobenzeneBromobenzeneIodobenzene

Nitrobenzene

Somemono-substitutedaromaticcompoundshavespecialnamesthatareusedinsteadof

thesystematicnames.

NH2OH

CH3OCH3

Aniline

(Hydroxybenzene)

Toluene

(Methylbenzene)

Anisole

(Methoxybenzene)

COOH

Benzoicacid

SO3H

Benzenesulphonicacid

(benzenesulfonicacid)

COCH3

Acetophenone

CHO

Benzaldehyde

2.2.Nomenclatureofsubstitutedbenzene

Rule2.Ifthereareseveralsubstituentsontheringthentherelativepositionsareshown

bynumberingthecarbonatomstowhichtheyareattached.Numberingstartsfromthe

mostelectronegativegroupandthesumofthenumbersmustbetheminimum.

NO2

oo

mm

Br

NO2

Cl

Note:

1-2ortho(o)

1-3meta(m)

1-4para(p)

3-Bromonitrobenzene

(m-Bromonitrobenzene)

2-Chloronitrobenzene

(o-Chloronitrobenzene)

Rule3.Whenagroupwithspecialnameispresenttheparentspecialnameisused.

NO2

NH2

OH

NO2

OCH3

Br

HO2C

I

CH3

SO3H

3-Nitroaniline

(m-Nitroaniline)

2-Nitrophenol

(p-bromoanisole)

2-Iodobenzoicacid4-Toluenesulfonicacid

Rule4.Whentwoormoresimilargroupsarepresentthentheprefixdi,tri,tetra,penta,

hexaetcisusedtodenotethenumberofgroupspresent.

Cl

Cl

Br

Br

O2N

NO2

NO2

Cl

Cl

Cl

Cl

1,3-Dichlorobenzene

(m-Dichlorobenzene)

1,2-Dibromobenzene

2.3Nomenclatureofotherbenzenederivatives

Rule5.Ifmorethantwogroupsthataredifferentareattachedtothebenzene,the

numberingisdonefromthemostelectronegativetogivetheminimumsumpossibleand

prefixmustbeinalphabeticalorder.

Br

Cl

NO2

I

Cl

Br

NO2

Cl

Br

3-Bromo-1-Chloro-5-Iodobenzene5-Bromo-3-chloronitrobenzene

2-Bromo-3-chloronitrobenzene

Rule6.Ifthereisaspecialgroup,thenthederivativeisgiventhespecialnameand

numberingstartedfromthere.

OH

NH2

Cl

Br

Br

NO2

Br

O2N

CH3

NO2

2-Chloro-4-nitrophenol

2,4,6-tribromoaniline

2,6-Dinitrotoluene

Rule7.Ifthereisanalkylgroupthatisnotbranched,thebenzenenucleusistakenasthe

parent.

CH3

CH2CH3

CH2CH2CH3

Methylbenzene

Propylbenzene

Rule8.Iftherearetwoormorealkylgroupsthelongeronetakespreference.

CH2CH3

CH2CH2CH3

3-Ethylpropylbenzene

Rule9.Ifthealkylgroupisbranched,thealkylgroupistakenastheparentandbenzene

nucleusasasubstituent.

2-Methyl-1-phenylpropane2-Methyl-2-phenylpropane3-Phenylheptane

Rule10.Ifthechainisbranchedandbenzeneringhasfurthersubstituentsonit,thenthe

branchedchainstilltakentheparent.

NO2

CH3

2-(4'-Methyl-3'-nitrophenyl)-2,5-dimethylhexane

Note:

=C6H5=Phenylgroup=Ph

CH2=

O

C6H5CH2=Benzylgroup

=

C6H5CO=

Benzoylgroup=Bz

2.4Nomenclatureofderivativesofnaphthalene

Unlikebenzenethecarbonatomsofnaphthalenearenotequivalent.Ithasthreetypesof

carboncentersasindicatedbelow.Thebridgingcarbonatomslackthehydrogenatoms

andarenotnumbered,astheycannotbeinvolvedinanyformofsubstitution.Asthe

carbonatomsarenotequivalentinnaphthalene,introductionofasubstituentgivestwo

isomers.Beforeanysubstituentisintroducedbothringsareequivalent.Forthatthereare

twoalcoholsofnaphthaleneandlikephenol,theyhavespecialnames.Ifsubstituentsare

morethanone,thenumberingfollowstheordershownabove.Examplesareshown

below.

7

6

8

5

1

4

2

3

b

b

a

a

a

a

b

b

Fusedcarbonatoms

OH

OH

NH2

NO2

NO2

1-Naphthol

(1-Hydroxynaphthalene)

(a-Naphthol)

SO3H

H2N

2-Naphthol

(b-naphthol)

Cl

NO2

2,4-Dinitro-1-naphthalamine1,5-Dinitronaphthalene

CH3Br

OH

6-Amino-2-naphthalene

sulfonicacid

7-Chloro-1-naphthol1-Bromo-8-methyl

naphthalene

2.5Nomenclatureofanthracene

Thenumberinginanthraceneisdifferentfromthatofbenzeneandnaphthaleneinthatthe

middleringisnumberedlast.Thatis,thetwocarbonatomsofmiddleringarenumbered

9and10.Duetopresenceofmorepositionsanthracenehasverymanyvariedderivatives.

Afewselectedexamplesaregivenbelow.

7

6

89

1

2

3

5

4

Anthracen-9-ol

(9-Anthrol)

OH

OH

Anthracene-9,10-diol

O

O

9,10-Anthraquinone

OH

9,10-Dihydro-anthracen-9-ol

SO3H

Anthracene-2-sulfonicacid

OH

Anthracen-1-ol

(1-Anthrol)

O

9-Anthrone

NO2

NO2

9,10-Dinitroanthracene

SO3H

HO3S

Anthracene-2,6-disulfonicacid

2.7Nomenclatureofphenanthrene

Numberinginphenanthreneissimilartothatofanthracene.Itisimportanttonotethe

differencebetweenthetwothree-ringedaromaticclassesofcompounds.Inanthracene

thethreeringarefusedlinearlywhileinphenanthrenethefusionisnotlinearbutinsuch

awaythatthetwomiddlecarbonatomsbeonthesameside.Inanthracenethereare

manypositionsthusmanyderivatives.

3

4

2

6

5

1

7

89

10

OH

Phenanthrene

Phenanthren-9-ol

OH

OH

O

O

Phenanthrene-9,10-diol

O

10-Hydrophenanthren-9-one

Phenanthrene-9,10-dione

9,10-Dihydrophenanthrene

HO

OH

Phenanthren-1-olPhenanthren-4-ol

Cl

NO2

NO2

1,10-Dinitrophenanthrene

Cl

2,7-Dichloro-phenanthrene

2.8Summary

InIUPACnomenclatureofbenzenes,thenamebenzeneappearsasasuffixifthenameis

basedonbenzene.

Relativepositions;1:2,1:3and1:4onbenzeneringarereferredtoasortho(o),meta(m)

andpara(p)respectivelyandsometimesusedinnomenclature.

Numberingstartsatthespecialgroupifpresentonbenzeneringandhencespecialname

isusedasparent.

Whenbranchedalkylgroupisattachedtobenzenering,itistakenastheparentand

benzenebecomesasubstituent.

Naphthalenehastwodifferentpositionswhileanthraceneandphenanthrenehavethree

each.

Inanthraceneandphenanthrenethetwocarbonatomsofthemiddleringisnumberedlast

aspositions9and10.

2.9QuestionsandSolutions

Questions

Q1.Givethesystematic(IUPAC)namesforthefollowingcompounds

NH2

Br

Br

(i)(ii)

Cl

NO2

COCl

(iii)

H3C

CCCH3

H

OCH3

(iv)(v)CH2CHCH3

BrCO2H

F

(vi)

O3N

OH

NO2

NO2

BrSO3H

NO2

(vii)

OH

(viii)

(ix)Cl

Cl

NO2

OH

OH

Q2.Drawstructuresofthefollowingcompounds.

(i)4-Cyclohexylanisole

(ii)3-methylphenylbenzoate

(iii)3-Bromo-5-nitrobenzenesulphonicacid

(iv)3-Chloro-7-fluoro-1-naphthol

(v)2-amino-4-butoxybiphenyl

(vi)2,7-anthracenedisulphonicacid

Solutions

1.(i)2-Bromo-4-chloroaniline

(ii)2-Bromo-5-nitrobenzoylchloride

(iii)(cis)-3-Phenyl-2-buteneor3-phenylbut-2(Z)-ene

(iv)3-Bromo-5-methoxybenzoicacid

(v)1-Phenyl-2-(3-fluorocyclopentyl)propane

(vi)2,4,6-Trinitrophenol

(vii)5-Nitro-2-naphthol

(viii)8-Bromo-3-chloro-5-hydroxy-1-naphthalenesulphonicacid

(ix)6-Chloro-9-nitro-1-anthrol

2.Structures

O

(i)H3CO(ii)H3COC

(iii)

O2N

SO3H

Br

F

Cl

H2N

O

HO3S

SO3H

CHAPTER3

REACTIONSOFBENZENE

3.0.Introduction

Inchapterone,welookedatwhataromaticityisandchaptertwo,welookedathow

variousbenzenederivativesandbenzenoidsarenamedusingIUPACnamesandsome

timesspecialnames.Inthischapter,wewilllookatinfluenceofaromaticitytoreactions

shownbybenzene.Detailedmethodforwritingtheelectrophilicaromaticsubstitution

(EAS)reactionmechanismswillbediscussed.Halogenation,nitration,nitrosationand

Friedel-CraftsreactionwillbeusedtoillustrateEASreactionsshownbybenzene.

Objectives


Explainusingmechanism,thereductionofbenzene.

Describeusingcurlyarrows,thegeneralmechanismforelectrophilicaromatic

substitution(EAS)reaction.

WriteequationstoshowthegenerationofspecificnucleophilesforEAS

reactions.

Writedownreactionmechanismsforhalogenation,nitration,sulphonation,

nitrosationandFriedel-Craftsreactions.

ApplyEASpatternstosynthesizesubstitutedbenzenes.

3.1Reductionofbenzene

Ashadbeenstatedearlierbenzenepreferstoundergootherreactionswherearomaticity

isretained.However,underpressureandinpresenceofametalcatalystlikenickel,

platinumorpalladium,benzenereactswiththreemolesofhydrogentoformcyclohexane.

H2/Ni

Highpressure

slow

+H2/Ni

fast

H2/Ni

fast

BenzeneCyclohexadienesCyclohexeneCyclohexane

Firststepisslowduetoaromaticcharacterofbenzeneascomparedtocyclohexadienes.

Theintermediatesinthisreaction,cyclohexadienesandcyclohexene,cannotbeisolated

becausetheyundergoreductionveryfastundersameconditions.

BirchReduction

ThisisaformoflimitedreductiondiscoveredbyBirch,anAustralianchemist.The

methodisforproducing1,3-cyclohexadienefrombenzene.

Na,NH3,t-BuOH

+

1,4-Cyclohexadiene1,3-Cyclohexadiene

Majorproduct(Minorproduct)

Thereactioninvolvesthetreatmentofbenzenederivativewithsodium(orlithium)metal

inliquidammoniaorethanol.Themetaldissolvesintheammoniaandsothesetypesof

reactionsarealsoreferredtoasadissolvingmetalreduction.

Mechanism:

-

..

AB

NotethatintermediateBismorestablethanintermediateAbecauseofelectrons

(negativechargeandtheradical)arefurthestfromeachotherhenceleastrepulsion.Each

oftheseintermediatesreactsfurtherwithammoniaandsodiumtoformtheirrespective

products.

-

A

-

.

HNH2

HNH2

.

Na.

-HNH2

1,3

B

.

.

Na.

-HNH2

1,4

Although1,3productisstableduetoconjugation,1,4productispreferredandformed

duetoitsformationfromastableintermediate.

3.2Electrophilicaromaticsubstitution(EAS)reactionsofbenzene:

Ageneralmechanism

Thesearethemostcommonreactionsthatbenzeneandotheraromaticcompounds

undergo.Duetotheelectronscloudthearomaticringactsasanucleophilethatis

attackedbyanelectrophile.Generallythemechanisminvolvesformationofsigma

complexesfollowedbyelimination,whichrestoresaromaticitytothering.[E+=

Electrophile;B-=Base]

E+

H

+

E

+

H

E

+H

E

SigmaComplexes

Then,

H

+

E

B-E

3.3Halogenation

Bromination:BrominemoleculeitselfisnotsufficientlyreactivetoparticipateinEAS.

ALewisacidcatalystmustbeadded.

Br2

FeBr3

Br

+HBr+FeBr3

InitialstepisformationofaBr2-FeBr3complexthatservesastheE+ingeneration

mechanism.

2Fe+3Br22FeBr3

FeBr3+Br-BrBr-Br-FeBr3

Br-Br-FeBr3

HBr

+

+

Br-FeBr3

FeBr4-

Br

Resonance-stabilized

Sigmacomplex

+HBr+FeBr3

Formationofthesigmacomplexistherate-determiningstep.Thisstepishighly

endothermic,whilethereactionasawholeisexothermic.

Br2

FeBr3

Slow

RDS

HBr

+

Resonance-stabilized

Fast

Br

Sigmacomplex

Figure4.Energydiagramofbrominationofbenzene

Chlorination:Chlorinationhasananalogousmechanismtothatofbromination.AlCl3

canbeusedastheLewisacidcatalyst.

Cl2

AlCl3

Cl

+HCl+AlCl3

Chlorobenzene

Iodination:Requiresmorespecializedconditions.Nitricacid(HNO3)isusedasa

promoter(notcatalyst).Iodoniumionistheelectrophilegeneratedwiththehelpofnitric

acidasshownbelow.

I2

I

HNO3+

NO2

+

H2O

Iodobenzene

2H++2HNO3+I22I++2NO2+2H2O

3.4Nitration

Directreactionofnitricacidwithbenzeneisslow.Thereactionisspedupbytheaddition

ofsulfuricacid.

HNO3

H2SO4

NO2

+H2O

Nitricacidandsulfuricacidreacttogivethenitroniumion,whichistheactive

electrophile(E+).

-O

O

N+

OH

O

HOSOH

O

-O

O

N+

OH2

+

O

-

OSOH

O

ONO+

+H2O

Nitroniumion

Othernitratingagentsarenitroniumperchlorate(NO2+ClO4-)andnitroniumfluoroborate

(NO2+BF4-).

3.5Sulphonation

ReactionofbenzenewithSO3inH2SO4(fumingsulfuricacid)givesbenzenesulphonic

(sulphonic)acid.

SO3

H2SO4

SO3H

Benzenesulfonicacid

SO3isastrongelectrophileandunlikenitrationandhalogenation,sulphonationis

reversible.

O

OS

-

O

OOSO

SO3H

H2SO4Heat

+SO3

3.6Friedel-Craftalkylation

ReactionofaromaticcompoundswithalkylhalidesinpresenceofLewisacid,affords

alkylatedproducts.

+RX

AlX3

R

+HX

X=Cl,Br,IR=Alkylgroup

+Cl

AlCl3

+HCl

Inthefirststepthereactionisgenerationofthecarbocation,whichistheelectrophile.

Thenextstepisattackoftheelectrophiletothearomaticsystemtogivethesigma

complexes,followedbyelimination.

ClAlCl3++AlCl4-

H

AlCl4-

+

Resonancestabilized

Sigmacomplexes

For20and30alkylhalides,thenakedcarbocationislikelyinvolved,astheyare

relativelystable.Thecarbocationsformedfrom10alkylhalidesaremuchlessstableand

soacomplexbetweenthealkylhalideandLewisacidisprobablythespeciesattackedby

thearomaticring.

H2CClAlCl3

CH3

H

+

CH2CH3

AlCl4-CH2CH3

Note:Therearethreelimitationstoalkylationreactions.

1).Onlyworkswellforbenzene,halobenzenesandactivatedringsystems.

2).Electrophilicspeciesarecarbocationsthatarepronetorearrangement.

3).Multiplealkylationsfrequentlyoccurbecausethealkylatedproductismore

activatedthenbenzeneitself.

+AlCl3

H

H

C

+CH3

CH3

CH3

NotFormed

HCH3

H3CCCCl

CH3H

Productformedvia

0

0

HCH3

H3CCCCl

CH3H

AlCl3

H

H3CCCCH3

CH3H

Rearrangement

H

H3CCCCH3

CH3H

20carbocation30carbocation

3.7Friedel-Craftsacylation

Anacylgrouphasacarbonylgroupattachedtohydrogenoranalkylgroup.Thereaction

isofacylhalideandaLewisacid.

+

RCl

AlCl3

R+

HCl

Lewisacidassistsingenerationofanacyliumionastheelectrophile

R

O

ClAlCl3R

O+

R

+AlCl4-

Acyliumion

UnliketheFriedel-Craftsalkylationtheacylationreactiondoesnotsufferfrom

rearrangementoftheelectrophilenoristheproductsusceptibletofurtherreaction.The

acylationreactioncanbeusedtosynthesizealkylbenzenesindirectly.Forexample,

Clemmensenreductioncanbeusedwherecarbonylgroupisconvertedtomethylene

(CH2)groupsuponreactionwithazinc/mercuryamalgam.

O

CH3Zn(Hg)

CH3

HCl

3.8.Nitrosation

Thereactioninvolvingnitrousacidisknownasnitrosation.Theelectrophileinvolvedis

nitrosocationgeneratedbynitrousacid.

NO

+HNO2

-

Nitrosobenzene

+NO+

+H2O

+

H

+

NO

OH-NO

3.9Summary

BenzeneundergoesmainlyEASreactionstogivevarioussubstitutedproductsdueits

aromaticity.

ProductsofEASofbenzeneareusedaseitherintermediatesorfinalproductsoras

precursorsonindustrialscale.

Fiedel-Craftsalkylationgivesmixedproductsduetorearrangementoftheresulting

carbocation.Therefore,itisnotagoodmethodtosynthesisalkylatedbenzene

derivatives.

Fiedel-Craftsacylationdoesnothaverearrangedproductsthuspreferred.

3.10Questionsandsolutions

Questions

Q1.(a)Byuseofreactionmechanismsshowhowthefollowingtransformationsare

carried:

(i)Benzenetonitrobenzene

(ii)Benzenetoethylbenzene

(iii)Benzenetonitrosobenzene

(iv)Benzenetobenzenesulphonicacid

(v)BenzenetoIodobenzene

Q2.Explainthefollowingbyuseofappropriatestructuresand/orequations.

(i)Friedel-Craftalkylationreactionsleadsinformationofamixtureof

alkylatedbenzenecompounds.

(ii)

Innitrationofbenzeneconcentratedsulphuricacidisrequired.

Solutions

1.Refertothenoteswithinthischapter.

2.(i)ThecarbocationformedwhenthealkylhalidereactwithaLewisacidisproneto

rearrangementleadingtomorealkylatedproducts.Consider1-chloropropane.

CH3CH2CH2Cl

AlCl3

CH3CH2CH2++AlCl4-

Rearrangement

CH3CHCH3+amorestable

carbocation

Thesetwocarbocationswillreactwithbenzenetogivetwodifferentproducts

althoughthesecondarycarbocationwillformthemajorproduct,isopropylbenzene.

Thatwasnottheinitiallyexpectedproduct.

CH2CH2CH3

Propylbenzene

MinorProduct

H3CCCH3H

2-Phenylpropane

(Isoprylbenzene)

MajorProduct

Itisalsonotedthatthealkylatedproductformedisactivatedtoelectrophilic

substitutionreactionsthanbenzenethusformsmulti-alkylatedbenzeneproductsat

orthoandparapositions.

R

R

R

R

R

R

R

RR

R

R

CHAPTER4

REACTIONSOFBENZENEDERIVATIVES

4.0Introduction

InchapterthreewelookedatEASreactionsofbenzenewithvariousnucleophilic

reagents.Inthischapterwearegoingtostudytheeffectofgroupsattachedtobenzene

ringintermsofhowtheyaffectrateofelectrophilicaromaticsubstitution(EAS)

reactionsandpositionatwhichtheincomingsubstituentgetsattachedtothering.These

reactionswillalsobeusedtoshowthesynthesisofvarioussubstitutedbenzenes.

Industriallyusefulsubstitutedbenzeneswillalsobediscussed.

Objectives


ExplaintheeffectofelectrondonatinggrouponbenzeneontherateofEASand

orientationofanincomingnucleophile

ExplaintheeffectofelectronwithdrawinggrouponbenzeneontherateofEAS

andorientationofincomingnucleophile

DescribehowtherateofEASreactionofbenzeneandsubstitutedbenzeneis

determined

Statecharacteristicsofcommonortho-paraandmetadirectors

Explainusingresonancestructures,theeffectofagrouponbenzenethatdonates

electronsbyinductionandthatreleaseselectronsbyresonanceonEAS

Explainwhyhalogensaredeactivatorsyettheyareortho-paradirectorstoan

incomingelectrophile

Explaintheeffectoftwosubstituentsonbenzenetotheratereactionand

orientationofincomingsubstituent

Statesomeofindustriallyusefulsubstitutedbenzenes

4.1EffectofsubstituentsonEASpatternsandorientation

4.1.1NitrationofToluene

Likebenzene,tolueneundergoeselectrophilicaromaticsubstitutionreactions.The

reactionisfasterthanthatofbenzene.Nitrationreaction,forexample,isabout25fold

faster.Thereactiongivesthreeproducts,twomajorandoneminorandalmostnot

realized.

CH3

CH3HNO3CH3

H2SO4++

NO2

O2N

CH3

p-Nitrotoluene

40%3%57%

Ifnitrationwererandom,a20:40:40para:meta:orthoratiowouldbeproduced.Thisis

duetothefactthattherearetwoorthopositions,twometapositionsandonepara

position.Howeverthesituationisnotobservedexperimentally.Thesubstituentshave

effectsonEASofbenzenederivatives.Intheabovecaseofnitrationoftoluenetheortho

andparaproductsarefavored.

Fromtheaboveobservationsonnitrationoftoluenetwoconclusionscanbemadeabout

themethylgroupattachedtobenzenering:

1.Itisanactivatinggroupbecausethereactionisfasterthanthatof

benzenetowardselectrophilicsubstitution.

2.Itisanorthoparadirector.

Recallthattherate-determiningstepistheformationofthesigmacomplexes.The

complexesleadingtotheorthoandparaproductsaremorestablethanthatleadingtothe

metaproduct.Accordingly,thetransitionstatesleadingtotheorthoandparaproducts

areoflowerenergythanthoseleadingtothemetaproduct.

CH3

Ortho

+

Twowith20carbocations,onewitha30carbocation.

+H

HNO3

H2SO4

Meta

+

CH3

+

CH3

+

CH3

HNO2

HNO2HNO2

Allthreewith20carbocations

Para

H

O2N+

CH3

H

O2N

+

CH3

+

H

O2N

CH3

Twowith20carbocations,onewitha30carbocation.

Figure5.Energydiagramofnitrationoftoluene

4.1.2Nitrationofnitrobenzene

Unlikethecaseofnitrationoftoluene,nitrationofnitrobenzeneisaveryslowreaction.It

isabout105timeslessreactivetoEASreactionsthanbenzene.Thereactiontakesplaceat

highertemperaturewhencomparedwiththenitrationofbenzene.Thereactionleadsto

formationofthreeproducts,withm-dinitrobenzene,themetaproductpredominatesand

paraproductalmostnotrealized.

NO2HNO3

H2SO41000C

NO2

NO2

NO2

++

NO2

O2N

NO2

p-Dinitrobenzene

6%90%0.7%

ThenitrogroupiselectronwithdrawingwhichdeactivatestheringtowardsEAS.The

groupremoveselectrondensityfromtheringandslowsdownthereaction.

OO-

N

N

+O

Fromtheaboveobservationsonnitrationofnitrobenzenetwoconclusionscanbedrawn

abouttheanitrogroupattachedtobenzenering:

1.Itisadeactivatinggroupbecausethereactionisslowerthanthatof

benzenetowardselectrophilicsubstitution.

2.Itisametadirector

Thesigmacomplexleadingtothemetaproductislessdestabilizedthanthoseleadingto

theorthoandparaproducts.

O

O

+

Ortho

HNO2

Especiallydestabilized

NO2

HNO2+HNO2

NO2

(Adjacent+charges)

HNO3

H2SO4

Meta

+

NO2

+

NO2

NO2

+

HNO2

HNO2HNO2

Para

H

O2N+

NO2

H

O2N

+

O

NO

+

+

H

O2N

NO2

Especiallydestabilized

(Adjacent+charges)

Generallygroupsthatdeactivatetheringareusuallymetadirectors.Otherdeactivating

metadirectinggroupsinclude,ketones,esters,nitriles,sulphonicacidsandammonium

salts.Allareeitherpositivelycharged,orhavearesonanceforminwhichapositive

chargeisimmediatelyadjacenttothering.

4.1.3Determinationofrelativereactivityofreaction

1.Timerequiredforareactiontooccurunderidenticalconditions

2.Theseverityofconditions(temperature,pressureandconcentration)requiredfora

comparablereactiontooccurunderotheridenticalconditions.Forexample,

nitrationofbenzeneandnitrobenzeneforonehourisat600Cand900C,

respectively.Benzeneisthereforemorereactivethannitrobenzene.

3.Competitivereactionswherebyamixtureofequalmolesofcompoundsismadeto

competeforalimitedamountofreagentandquantitativeanalysisoftheproduct

andthereactantsdone.

4.1.4.Orientationofthenitrogroup

Thetablebelowpresentssomeinformationonthenitrationofmonosubstitutedbenzene.

Fromthetablebelowonecanbeabletocomeupwithaclassificationofthegroupsas

ortho-paradirectorsormetadirectors.

Relativepercentageofproduct

Groupattached

OrthoMetaPara

tobenzene

-OH55045

-CH340357

-OCH345155

-NH250050

-CH2CH335163

-Cl35164

-Br40060

-NHCOCH319279

-NO26901

-CO2H20800

-CN-81-

-SO3H21727

-CHO-72-

-+N(CH3)308911

Strongactivatingagents(ortho-paradirecting):-NH2,-NHR,-NR2,-OH,OMe

Moderatelyactivating(ortho-paradirecting):-NHCOCH3

Weaklyactivating(ortho-paradirecting):-C6H5,-CH3,-CH2CH3

Deactivatingagents(ortho-paradirecting):Halogens(-F,-Cl,-Br,I)

Deactivatingagents(metadirecting):-NO2,-CO2H,-CN,-SO3H,-CHOetc

Note:

a)Anactivatinggroupactivatesallthepositionsonthebenzeneringincludingthe

metapositionbutactivatesorthoandparapositionsmorethanmeta.

b)Adeactivatinggroupdeactivatesallthepositionsonthebenzeneringincluding

themetapositionsbutlessthantheorthoandpara.

4.2Groupsthatdonateelectronstothebenzene

Therearetwotypesofgroupsunderthiscategory.Therearethosegroupsthatdonate

electronsbyinductiveeffectandthosethatdonatebyresonance.

Groupsthatdonateelectronsbyinductiveeffectaregroupsthatdonothavelonepairof

electronstodonateforresonancestabilization.Buttheyareelectronreleasingby

inductiveeffect.Thustheyincreasetheelectrondensitytothebenzeneringandhencethe

ringbecomesmoreattractivetowardsanincomingelectrophilethanbenzeneitself.They

arehowevertermedweakactivators.Alkygroup,methyl,ethyl,propyletc,representthis

class(refertonitrationoftoluene).

Groupsthatreleaseelectronsbyresonancehavelonepairsofelectrons,andarepowerful

activatorsoforthoandparapositions.Theyincludeamino(-NH2),andhydroxyl(-OH)

groups.Theyreleaseelectronsbymeansofresonance.Consideracaseofnitrationof

aniline.

NH2+

NH2

Ortho

..

NH2

+

HNO2

HNO2

NH2

HNO2+HNO2

HNO3

H2SO4

Meta

+

NH2

+

NH2

NH2

+

HNO2

HNO2HNO2

Para

H

O2N+

NH2

H

O2N

+

..

NH2

+

H

O2N

NH2

+

NH2

H

O2N

Thelonepairofelectronsofnitrogenprovidesanadditionalstabilizationofthesigma

complexbyresonancedonationoftheelectrons.Thereisanadditionalresonanceform

forthecomplexesleadingtotheorthoandparaproducts.Hencethesesigmacomplexes

aremorestable.

UndersomeconditionsanilineissoreactivethatitundergoesEASwithoutacatalyst.

BrominationoccurswithouttheLewisacidandsubstitutioncantakesplaceinthree

positions.

NH2

3Br2

Br

NH2

Br

H2O,NaHCO3

Br

SodiumbicarbonateisrequiredtoreactwiththeHBrthatforms.Otherwise,thebasic

aminogroupprotonatesandEASslowdown.Protonatedaminogroupisanelectron-

withdrawinggroupandremoveselectronsdensityfromtheringandslowsdownthe

reaction.

NH2

+

HBr

4.3Groupsthatwithdrawelectronsfromthebenzeneringbyinduction

andreleaseelectronsbyresonance

Theydisplaytwoopposingeffects:

1.Theyareelectronegativeandhencedeactivatetheringthrough

inductiveeffects.

2.Theycandonateelectrondensitythroughresonanceandhenceactivate

theringthroughresonancethroughformationofahaloniumion

(C=X+).

NeteffectisthattherateofEASisslowerforhalobenzenethanbenzene,butthese

groupsgiveorthoandparaproducts,asthemajorones.

Br

HNO3/H2SO4

Br

NO2

Br

++

O2N

Br

NO2

ortho~35%

meta~1%

para~64%

Itisobservedthatlikethesigmacomplexesofaniline,chlorineatomcancontributea

lonepairofelectronsandthesigmacomplexesfororthoandparaincreasedbyoneeach

throughformationofthehaloniumion.

+

Cl

Cl

Ortho

..

Cl:..

+

HNO2

HNO2

Cl

HNO2+HNO2

HNO3

H2SO4

Meta

+

Cl

+

Cl

+

Cl

HNO2

HNO2HNO2

Para

H

O2N+

Cl

H

O2N

+

..

Cl:..

+

H

O2N

Cl

+

Cl

H

O2N

4.4Effectofathirdsubstituentonthebenzenering

Presenceofalreadytwosubstituentsonthebenzeneringcomplicatetherulesmentioned

earlierifthethirdsubstituentistobeintroduced.However,afewgeneralrulesforsome

casescanbepredictedbycombiningtheeffectsofeachofthetwogroups.

Ifthetwosubstituentsdirectanincominggrouptothesameposition(s),thenthatwillbe

theprincipalpositionofthethirdsubstituent.Thatis,thetwosubstituentsmaybelocated

insuchawaythattheirdirectiveinfluencereinforceseachother.Afewexamplesare

citedbelow.

SO3H

NHCOCH3

CH3

H3C

NO2

NO2

Highlydeactivated

CN

CH3Hindered

PredictedsiteifEAS

worksatall

Reactionbetweentwosubstituentsgenerallynotfavoredduetosterichindrance.

Althoughthetwomethylgroupsinm-xylenewouldreinforceeachothernoreaction

takesplacefortheorthopositionbetweenthetwogroups.

Iftwodeactivatinggroupsarepresent,regardlessoftheirposition,itmaybedifficultto

effectathirdsubstitution(caseofm-nitrobenzenesulphonicacidabove).

Iftwogroupsconflictintheirdirectiveeffects,themorepowerfulactivatorwillexertthe

predominanteffect(NH2>OH>OCH3>NHCOCH3>C6H5>CH3>halogens>metadirectors.

HO

Cl

X2

HO

X

Cl

H3C

NHCOCH3

Major

H3C

NHCOCH3

Br2/FeBr3Br

H3C

NHCOCH3

Minor

Br

4.5Summary

RateofEASreactionofmonosubstitutedbenzenewithanincomingnucleophileandthe

orientationisaffectedbynatureofthesubstituentalreadyonthering.

Electronwithdrawinggroups(activators)aremetadirectors.

Electrondonatinggroups(deactivators)areorthoandparadirectors.

Halogensareorthoandparadirectoralthoughtheyaredeactivators.

Thepositionofthethirdincomingsubstituentonbenzeneringdoesnotfollowtheusual

EASpatterns.


Questions

Q1.Explainthefollowingbyuseofreactionmechanismsandresonancestructures.

(a)Aketogroupattachedtobenzeneringisdeactivatinganddirectstheincoming

electrophiletothemeta.

(b)Birchreductionofbenzoicacid(A)givescompound(B)andnotcompound

(C).

CO2HCO2HCO2H

Li,NH3(l)

EtOH

ABC

Q2.Suggestthemajororganiccompounds(D-H)inthefollowingreactions.

NHCOCH3

(i)Br2,CH3CO2HD(ii)

H3C

OH

HNO3

E

NH2

(iii)

NO2

Cl2/H2OF(iv)NO2

CH3

HNO3/H2SO4

G

O

OCH3

O

H

O

Solutions

1.(a)Thecarbonofthecarbonylgroupispolarizedinsuchawaythatthecarbonatomof

thecarbonylcarriesapartialpositivechargewhiletheoxygencarriesthepartial

negativecharge.Thatis,carbonisinneedofelectrons.Theseelectronsaresupplied

bythebenzenering.Astheelectrondensityofthebenzeneisreduced,thecarbonyl

groupisthereforetermedasadeactivatinggroupbecausethebenzenering

reactivitytowardselectrophilicattackhasbeenlowered.

O

R

Anyelectrophilicattack,whichwouldleadtoresonancestructureswherebythebenzene

carbonattachedtothecarbonylgroupcarboncarriesapositivecharge,isunfavorable.

Thisisnotedintheorthoandparaelectrophilicattacks.Althoughthewholeringis

deactivated,orthoandparapositionaremoredeactivatedthanthemetawherethe

positivechargedoesnotfallonthecarbonatomofthebenzeneattacheddirectlytothe

carbonylcarbonasobservedintheresonancestructuresbelow.

COCH3

HNO3/H2SO4

metaattack

COCH3

H

NO2

+

+

COCH3

H

NO2

COCH3

+

H

NO2

COCH3

HNO3/H2SO4

orthoattack

+NO2NO2

COCH3COCH3

++

COCH3

HNO3/H2SO4

paraattack

COCH3

+

HNO2

COCH3

+

HNO2

COCH3

+

HNO2

1(b).Acarboxylicgroupisanelectron-withdrawinggroupasthecarboxylcarbonhighly

deprivedelectronsbythetwooxygensattached.Themechanismbelowwillhelpto

explainwhytheproductisformed.

CO2H

HOCO

HOO-

Li

-

.

.

+etc

EtOH

Negativechargeresonancestabilized

bytheelectrondeficientCO2Hgroup

HOCO

.

HOO

.Li,EtOH

+etc

CO2H

Favoredproduct

Lowerenergypathway

CO2H

CO2H

CO2H

CO2H

CO2H

Li

.

-

-

.

-EtOHLi

.EtOH

Negativechargenotstabilizedbythe

CO2Hgroup.(Higherenergypathway)

-Notfovorable

2.

NHCOCH3

NH2

(i)D=

(ii)E=

OH

(iii)F=

Cl

H3CNO2

Br

NO2

NO2

(iv)G=

NO2

(v)

H=

OCH3

CH3

OC

CH2CH2CO2H

CHAPTER5

ARENESANDARYLHALIDES

5.0Introduction

Inchapterfour,welookedatEASreactionsofbenzeneanditsderivativesandeffectof

substituentsonratesofEASreactionsaswellastheirorientationpatterns.Inthischapter,

preparationofarenesandreactionsshownbyareneswillbestudied.DiscussionofEAS

reactionofarylhalideswillbedoneinadditiontothosereactionsthatwillleadto

removalofthehalogenfromthering.Reactionsinvolvingarylhalidesusefulinthe

synthesisofimportantcompoundswillalsobementioned

Objectives


Describeusingequationspreparationofsomecommonarenes.

Explainwiththehelpofmechanismswhydifferentproductsareobtainedwhen

arenesarehalogenatedindarkandinpresenceoflight.

Predictreactionproductswhenunsaturatedarenesundergohalogenationinthe

presenceofLewisacids.

Writedownoxidationandreductionproductsofstyrene.

Explaindeactivatingeffectofhalogensusingresonancestructures.

DescribeEASreactionsofarylhalides.

Describehowahalogensubstituentisremovedfromactivatedandinactivated

arylhalide.

Statesomeoftheimportantarenes,arylhalidesandcompoundsobtainedfrom

them.

5.1Arenes

Arenesarealiphaticaromatichydrocarbons.Recallthemethodsofnamingarenes

(chapter2)andmethodsofpreparations.Notethatifadoubleispresent,dependingon

theposition,geometricalisomerismmaybeexhibited.

H

CH2

H

CH3

H3C

H

HH

H

H

3-Phenyl-1-propene

(Trans)-1-Phenylpropene(Cis)-1-Phenylpropene

Arenesareoflowpolarityandinsolubleinwaterbutsolubleinorganicsolventslike

CCl4,ether,hexaneetc.Disubstitutedparaisomershavehighermeltingpointdueto

betterpacking.

5.2ReactionsofArenes

Arenesundergomostofthereactionsofbenzenering,thatis,EASreactions.Theycan

alsoundergoreactionsinvolvingthesidechain.Recallthattheyareortho-paradirectors

andactivatestheringbyinductiveeffect.Theparaproductusuallypredominatesover

theorthoproductduetostericeffects.

NotethatthehalogenationwithLewisacidiscarriedoutindarkness.Inpresenceoflight

thesidechainwillreactwiththehalogen.

CH2CH3

CH2CH3

Br2/Light

Br2/FeCl3

Dark

Minor

CH3

BrH

CH2CH3CH2CH3

+

BrBr

Major

5.2.1.HalogenationofArenes

Halogenationofthesidechainisaradicalmediatedreaction.1-Bromo-1-phenylethane,

shownaboveistheonlyproductduetostabilityoftheintermediateradicalinvolved

beingstabilizedbythephenylgroupbyresonance.Thisisareactivityselectivity

principle.

Br2hvBr

.

+Br.

CH2CH3

CHCH3.

+Br.

+Br2

HBr

+

CH3

CH

Br

CHCH3.

+Br.

Onlyproduct

Chlorinationofethylbenzene,however,givestwoproductswith1-chloro-1-phenylethane

(91%)predominatingover1-chloro-2-phenylethane(9%).

Question:Whataretheterminatingproductsofthehalogenationofethylbenzenein

presenceoflight?

UnsaturatedarenesundergohalogenationinabsenceoftheLewisacidjustlikealkenes.

Br2/FeCl3

Br

Br

1,2-Dibromo-1-phenylethane

Br

major

+

Br

Bromostyrenes

Styrene

HBrH

Br

CH3

1-Bromo-1-phenylethane

(Markovnikov'sproduct)

HBr

PhCOOOCOPh

H

H

CH2Br

1-Bromo-2-phenylethane

(Anti-Markovnikov'sproduct)

(Dibenzoylperoxide)

5.2.2Reductionreactionsofarenes

Thereactionofstyrenecanoccurbothonthesidechainandalsoonthering.Consider

reductionwithhydrogeninpresenceofnickel.

H2/Ni

200C,2-3Atm

CH2CH3

Ethylbenzene

Styrene

H2/Ni

1150C,110Atm

CH2CH3

Ethylcyclohexane

Inpresenceofhydrogenperoxideinaceticacid,styreneisconvertedtoaglycol,whichis

oxidizedfurtherbyoxidativecleavagetobenzoicacid.Anysidechainwiththecarbon

atomthatisdirectlyattachedtothering(benzyliccarbon),andhasatleasttwo

hydrogens,isoxidativelycleavedbystrongoxidizingagenttobenzoicacid.

Styrene

H2O2/HCO2H

HOH

CH2OH

Phenyl-1,2-ethanediol

KMnO4

CO2H

Benzoicacid

(aGlycol)

KMnO4,OH,Heat-

H+

CO2H

CO2H

CO2H

KMnO4,OH,Heat-

H+Noreaction

Question:Howare(i)saturatedand(ii)unsaturatedarenesprepared?

ForthesaturatedarenesrefertoFriedel-Craftsalkylation.Recallthatthemethodhasits

limitations.ItwasalsomentionedthatthelimitationscouldbeavoidedbyuseofFriedel-

CraftsacylationlaterreducingtheacylatedproductbyClemmensenreduction(HClin

presenceofZn/Hg)orWolf-Krisherreduction(N2H4inpresenceofabase).Unsaturated

arenesarepreparedbynormalreductionofacylatedproducts.

Notethatifthealkylhalideshavemorethanonehalogenatom,morethanonephenyl

groupswillbepresentintheproducts.

2AlCl3

Diphenylmethane

3AlCl3

3AlCl3

H

Cl

Triphenylmethane

Chlorotriphenylmethane

Triphenylmethaneisacidicduetoresonancestabilizationofcarboanion.Useofcarbon

tetrachloridedoesnotleadtoformationoftetraphenylmethaneduetostericfactors.

5.3Arylhalidesandnucleophilicsubstitutionreactions

Arylhalides,ashadbeenmentionedearlier,arecompoundsinwhichhalogenatomis

directlyattachedtothearomaticring.Theirpreparationmethodswerementionedin

chapter3.Arylhalidesbehavedifferentlyfromalkylhalides.Theydonotundergo

nucleophilicsubstitutionthateasilyasdothealkylhalides.Theydonotgiveapositive

silvernitratetestasalkylhalidesdo.Insomecasetheybehavelikevinylhalides(a

halogenattachedtoacarbonatomwithadoublebond),whichdonotreactwith

nucleophiles.

XCH2X

orArX

ArylhalideNotanarylhalide

R-X+Nu-SN1orSN2R-Nu+X-

ArX+Nu-Noreaction

Thisbehaviorofarylhalideisduetothecontributionofoneofthelonepairsofthe

halogentowardsextensionofdelocalisation.

:X:

-

..

..

-

-

Thustheringasawholeisdeactivatedtowardsnucleophilicattack.However,inpresence

ofanelectronwithdrawinggroupatorthoorpara(orboth)withrespecttothehalide

activatesuchareaction.Thisisanaddition-eliminationreaction.Presenceofthe

deactivatinggroupatmetahasnosucheffectonthearylhalide.

ClO-

N+

O

ortho

+

O-

N+

O-

Nu

O-

O-

Nu

Cl

Cl

+Nu-

ClNu

ON+O-

para

-ON+O-

-ON+O-

NO2

Themoretheelectronwithdrawinggroupsthereareatorthoandparatheeasieritisto

replacethehalide.Onlyeffectedatorthoandparapositions.Ringactivatorscannot

effectthechange.

Theorderofreactivityofarylhalidestonucleophilicsubstitutionis:

I>Br>Cl>F

Cl

OH

NO2

NaOH,1300C

Reflux

NO2

Cl

NO2

aqNaHCO3

1000C

OH

NO2

NO2

NO2

O2N

Cl

NO2

aqNaHCO3

O2N

OH

NO2

350C

NO2

NO2

Avarietyofproductscanbeprepareddependingonthetypeofsubstitutedarylhalide

andthebaseused.

CH3ONa

O2N

1000C

O2N

Cl

NO2

I

NH2CH3

1600C

C6H5NH2

NHCH3

NO2

NHC6H5

NC

CN

EtOH,950C

NC

CN

Otherimportantreactionsinvolvingarylhalidesareshownbelow.

Cl

CN

CO2H

CO2R

NO2

KCN

CO2R

NO2

aqH+

CO2R

NO2

F

+

Li

H2O

Li

Biphenyl

Br

OCH3

NaNH2/NH3

OCH3

H2N

2-Amino-4-methoxybiphenyl

(4-Methoxy-2-phenylaniline)

Br

+Mg(s)

THForEt2O

Dry

MgBr

GrignardReagent

5.4Removalofhalidesubstituentfrominactivatedarylhalide

Forinactivatedarylhalides,thesubstitutioniscarriedoutathightemperaturesin

presenceofastrongbase.Thenucleophilicsubstitutionoccursbutunderadifferent

mechanism.Inthemechanismabenzyne(abenzeneatriplebond)isimplicated(See

exampleandthemechanisminthepagethatfollows).

Thisreactioncannottakeplaceifthereisnoorthohydrogentobelostintheprocessof

formationoftheextrabond.

Cl

+NaOHH2O

Highpressure

DowProcess

ONa

H+

OH

Br

-330C

NH2

+KBr

Cl

NH2

H3C

H3C

H3CNH2

50%50%

Mechanism:

Br

CH3

H

NH2-

CH3

NH2-

H3C

-HNH2

NH2

CH3

NH2

+NH2-

Cl

Benzyne

intermediate

AdditionofNH2totheotherendoftriplebondprovidesthe

otherregioisomer

H3CO

CH3

NaNH2/NH3

Noreaction

CF3

Cl

NaNH2/NH3

CF3

NH2

Thelasttransformationcanonlybeexplainedbyconsideringabenzyneintermediate.

ThebenzynegeneratedastheintermediatecanbetrappedinaDielsAlderreaction.

+OO

BenzyneFuranDielsAlderAdduct

5.5Summary

Arenesgivedifferentproductsdependingonwhetherthereactionisdoneinpresenceor

absenceoflight.

UnsaturatedarenasundergohalogenationinabsenceofLewisacidsjustlikealkenes.

ArenesarepreparedbyFrediel-Craftsacylationofbenzenefollowedbyreduction.

Halogensubstituentonbenzeneringdeactivatesittowardsnucleophilesbutitisortho-

paradirector.

Electronwithdrawinggroupsatorthoandparapositiononarylhalidemakesiteasierto

removethehalogenfromthering.

Forinactivatedarylhalide,thehalogencanberemovedviabenzyneintermediateathigh

temperatures.

Ahalogenontheringisadeactivatorbutortho-paradirector.

Arylhalidesareveryimportantprecursorforsynthesizingmanybenzenederivatives.


Questions

Q1.Withtheaidofreactionmechanisms,explainthefollowingobservation.

Reactionofethylbenzene(1)withbromineinpresenceoflightgivescompound2asthe

majorproductandcompound3astheminorproduct.

CH2CH3

1

Br2/light

CHBrCH3CH2CH2Br

+

23

Q2.Giventhefollowingreaction,

CF3

NaNH2/NH3

NH2

(i)Writeareactionmechanismleadingtotheaboveproduct.

(ii)Whyistheorthosubstitutedproductnotformed?

Q3.Whatistheproductinthefollowingtransformation?

Br

NO2

NaOCH3+-

NO2

Solutions

1.Seepage53.

2.(a)Thereactioninvolvesabenzyneintermediate.Attackofthebenzynebyamideion

wouldleadtoformationoftwonegativelychargedbenzenes(seebelow).The

compoundformedisgovernedbythestabilityofoneofthesecarboanions.Note

thatCF3groupisanelectron-withdrawinggroup,andthusthecloserthenegative

chargetoitthebetter-stabilizedcarbocation.Thatinductivewithdrawalof

electrons(notresonance)bytheCF3groupmorefavoredifthenegativechargeis

nearer.Thisleadstothemostfavoredproduct(lowenergypathway)

CF3

ClNH2-CF3

-

Cl

Cl-

CF3

Benzyne

NH2-

CF3CF3

NH2

-

AB

-

NH2

intermediate

NH3

CF3

CF3

NH2

NH2

(b)Theorthoproductisnotformedbecausethenegativechargeoftheintermediate

(B)isfarawayfromtheCF3groupthuslessstabilized.Thereforelessfavored

productduetohighenergypathway.

3.Thereactionisanucleophilicsubstitutionreaction,justlikethereactionin2

above.ThenucleophileisOCH3ion,whichreplacesthechlorineatom,thuslost

asachloride.Theproductis2,4-dinitroanisole.

OCH3

NO2

NO2

CHAPTER6

PHENOLS

6.0Introduction

Inchapterfive,welookedatarenesandarylhalides,theirreactionsandimportanceas

precursorsforvarietyofindustriallyusefulorganiccompounds.Inthischapter,phenols,

thatformthemostimportantderivativesofbenzene,willbestudied.Physicaland

chemicalpropertiesofphenolsthatmakethenuniquewillbedealtwithindetails.

Illustrationofsynthesisofcompoundsutilizingphenolswillalsobestudied.

Objectives


Explainthephysicalandchemicalpropertiesofphenols.

Describelaboratorypreparationandindustrialmanufactureofphenol.

Writeresonancestructurestoillustrateortho-paradirectingeffectofhydroxy-

substituentonbenzene.

Predictreactionunderdifferentconditionsproductsofphenols

Statesomeoftheusesofphenolsandtheirderivatives.

6.1Physicalpropertiesofphenols

Phenolsarearomaticalcoholsinwhichhydroxylgroupisdirectlyattachedtothe

aromaticnucleus.Phenolsresemblealiphaticalcoholsinmanyways.However,thereare

somecasewherebytherearedistinctivedifferencesbetweenthetwohydroxylcontaining

compounds.

Simplephenolsareliquidswithhighboilingpointsattributedtopowerfulhydrogen

bonding.Normallycolorlessunlessoxidizedtoquinones,whichisresponsibleforthe

colorofphenolwhenimpure.

O

H

O

N+

O-

HO

O-O-

N+

O

HO

o-Nitrophenolp-Nitrophenol

Ortho-Nitrophenolhasintramolecularhydrogenbondingwhichlowersitssolubilityin

waterwhencomparedwithpara-nitrophenol.Generally,theortho-nitrophenolhaslower

boilingpointandmeltingpointthanmetaandparaisomersastheintramolecular

hydrogenbondingreducestheintermolecularones.Forthis,o-nitrophenolcanbesteam

distilledwhilemetaandparaisomerscannotbesteamdistilledeasily.

Phenolsareweaklyacidicduetoresonancestabilizationofthephenoxideion.This

makesthemstrongeracidswhencomparedwiththealiphaticalcohols.Thereforephenol

reactswithstrongbaseslikesodiumhydroxidebutnotwithweakoneslikebicarbonates.

Alcoholsdoreactwithneitherweaknorstrongbases.Thispropertycanbeusedto

identifyandseparatephenolsfromacidsandalcohols.

OHO-

-

OO

-

-

Presenceofelectronwithdrawinggroupsatorthoandparapositions(butnotmeta)

increasetheacidityofphenol.Thegroupsstabilizetheresultingphenoxideionbysharing

someofthenegativechargebyresonance.Themorethegroupsthereareinaphenol

moleculethestrongertheacid.

O-

O-

O

O

O-

-

+

-

ONO-

Phenolsaltsaresolubleinwater.Duetotheresonancestabilizationofthenegative

chargeonthephenoxideionitisapoornucleophile,butastrongbase.

Sincephenolicmoietyiscommontoallstructures,theeffectofelectrondelocalisationis

thesame.Thus,theotherfactorinoperationiseitherabilitytodonateelectronstothe

ringorwithdrawelectronsfromthering.

Thefollowingsubstitutedphenolsareusedtoexplaintheeffect:

OHOHOH

OCH3CH3

pH9.9510.3010.19

OH

Cl

OH

CHO

OH

NO2

pH9.387.667.14

Fromtheabovephenols,electron-donatinggroups(OCH3,CH3etc)reducestheacidityof

phenols.ThiscanbeexplainedintermsofhowharditistobreaktheO-Hbondsothat

H+isreleased.Sincethesegroupsincreasetheelectrondensityofoxygen,itishardfor

theO-Hbondtobreakorionize.Ontheotherhand,electronwithdrawinggroupsweaken

theO-Hbondbyreducingtheelectrondensityofoxygenandhenceeasytobreakor

ionize.Thesameeffectofsubstitutiononpkavaluesarealsoseeninsubstitutedbenzoic

acidsandsubstitutedprotonatedanilines.

Note:Thereisarelationshipbetweenthepkavaluesandreactivityofbenzenering

towardsEAS;themorestronglydeactivatingthesubstituentisthelowerthepka

values.

6.2.1Industrialmanufactureofphenol

Therearetwomainprocessesthatareusedinindustriesinmanufactureofphenol

normallyforindustrialuse.Dowprocess,alreadymentionedearlier,involvesalkyl

halide,chlorobenzeneheatedat3500Candhighpressuresinpresenceofsodium

hydroxide.Itisfirstconvertedtosodiumphenoxide,whichhydrolysisinpresenceofan

acidgivesphenol.

Cl

+NaOH3500C,H2O

Highpressure

ONa

H+

OH

Thesecondmethodinvolvescumene(isopropylbenzene)astherawmaterial.Cumeneis

airoxidizedtoformthecorrespondingorganicperoxide,whichonhydrolysisinpresence

ofanacidleadtoformationofphenol.

O2

Airoxidation

CH(CH3)2

CH3

H3CC

OOH

O

H2O/H++H3C

OHCH3

Cumene

PeroxidePhenolAcetone

6.2.2Laboratorypreparationofphenol

Variousmethodscanbeusedtopreparephenolsinthelaboratory.

1.Hydrolysisofdiazoniumsalts

N2+HSO4OH

H2O

+

H2SO4

+N2

Benzenediazonium

bisulphate

Question:Howcanyoupreparephenolfrombenzene,throughnitrobenzene?

2.Alkalifusionofsulphonates

SO3HNaOH,H2O

OH

3000C

Benzenesulphonic

acid

3.Oxidationofarylthalliumcompounds(highyield)

Tl(O2CCF3)2

(CF3COO)2TlPb(OAc)4

Ph3P

OCCF3

O

OH

H+

O-

H2O/OH-

4.Basichydrolysisofarylhalides(worksonlyifthereareelectron-withdrawinggroups

attachedatorthoorparawithrespecttothehalide.(Refertothereactionsofaryl

halides).

6.3Reactionsofphenols

Acidity:

Asmentionedearlierphenolsareacidicinnatureandwillreactwithstrongbaseslike

sodiumhydroxidetoformphenoxidesaltandwater.Theypartiallydissociateinwater.

OHO-Na+

+NaOH

Sodiumphenoxide

+H2O

OH

+H2O

O-

+H3O+

6.3.1Etherandesterformation

Phenolsandtheirsaltformetherswhenreactedwithalkylhalides(Williamssynthesis).

ArOH+CH3CH2IArOCH2CH3

ArO-Na++RXArOR+NaX

H3COH+BrH2CNO2H3COCH2

NO2

Phenolslikealiphaticalcoholsreactwithcarboxylicacidsandtheirderivativeslikeacyl

halidesandacidanhydridestoformesters.Thereactionwiththeacidishighlyreversible

andthusnotpreferredinesterformation.

RCOCl

OCOR

OH

or

(RCO)2O

RSO2ClOSO2R

6.3.2Nitrationandsulphonationofphenol

Phenolwillreactwithnitricacidtoformo-nitrophenolandp-nitrophenol.Sulphuricacid

isnotneededinthisreactionasthehydroxylgrouphighlyactivatestheringforEAS

reactions.Theorthoisomerisforminaslightlylargerproportionmoleculesare

stabilizedbyintramolecularhydrogenbonding.

OH

HNO3

OH

OH

+

NO2

O2N

Phenolreactswithsulphuricacidtoformtwoproductsdependingonthetemperature

underwhichthereactioniscarriedout.Atlowtemperatureso-hydroxybenzenesulphonic

acidisform.Thisproductistermedasakineticproduct.Athighertemperatures,onthe

otherhand,p-hydroxybenzenesulphonicacidisthemajorproduct.Thisproductis

termedasathermodynamicproduct.

OH

OH

0

SO3H

Kineticproduct

H2SO4

0

o-Hydroxybenzenesulphonicacid

OH

Thermodynamicproduct

SO3H

p-Hydroxybenzenesulphonicacid

6.3.3Brominationandacylationofphenol

Benzeneringishighlyactivatedbythehydroxylgroupsuchthatbrominationreaction

takesplaceinabsenceoftheLewisacidcatalyst.Threepositionsoftheringare

substitutedwhenbromineisintroducedinpresenceofwater.However,mono

brominationatparapositionwhenbromineisaccompaniedbysulphurcarbide(CS2)at

00C.

OH

OH

Br2,H2O

Br

Br

Br

2,4,6-Tribromophenol

OH

Br2,CS2

OH

p-Bromophenol

Br

Phenolanditsderivativecanundergoacylationinpresenceofacidanhydride.In

presenceofaLewisacidtheacylatedproductundergoesarearrangementwheretheacyl

groupmigratestotheorthoorparapositiondependingonthereactiontemperature.At

roomtemperature(250C)thegroupmigratestotheparapositionwithrespecttotheinitial

occupiedposition.Withtemperaturesofaround1600C,theorthoisomerisformed.This

rearrangementisknownasFriesrearrangement.

OH

OH

CH3

O

(CH3CO)2OOCCH3

CH3

AlCl3

250CCH3

COCH3

AlCl3

1600C

H3COC

OH

CH3

6.3.4Kolbeandcouplingreactionsofphenol

Phenolisconvertedtoortho-hydroxybenzoicacidwhenitistreatedwithsodium

hydroxideandlaterwithcarbondioxide.

OH

NaOHONaCO2

1250C,4-7Atm

OHH+

CO2Na

OH

CO2H

Phenolscouplewithdiazoniumsalttoformazocompounds.Theyarecolored

compoundsthatareusedasdyestuffpigments.

OH

+HONN

p-Hydroxyazobenzene

6.4UsesofPhenols

1.Phenolitselfisusedasantiseptic(antimicrobiol).Modernantisepticsstillcontain

phenolicgroups.

2.Phenolformaldehyderesin

3.Manufactureofdyes

4.Additiveforodorandflavorings

OH

OH

Cl

Cl

OH

ClClCl

Cl

HO

CH2(CH2)4CH3

Hexachlorophenen-Hexylresorciol

PhenolicAntiseptics

Somenaturallyoccurringphenolshavebeenusedforalongtimeinfoodindustries

becauseoftheiraromaandtaste,likethymol(2-isopropyl-5-methylphenol)andvanillin

(4-hydroxy-3-methoxybenzaldehyde)fromthymeandvanillabeans,respectively.

6.5.Summary

Phenolsreactjustlikealiphaticalcohols.

PhenolundergoesEASreactionsandisanortho-paradirector.

Phenolisaprecursorformanyindustriallyusefulcompoundssuchasaspirin.Thismakes

itoneofthemostimportantderivativesofbenzene.


Questions

Q1.Byuseofresonancestructuresandreactionmechanisms,explainwhypresenceof

anelectronwithdrawinggroupatorthoandparapositionsincreasetheacidic

characterofphenols.

Q2.Outlinesynthesisof2,6-Dichlorophenolfromphenol

Q3.

Suggestthemajororganiccompounds(A-C)inthefollowingtransformations.

OH

(i)1).CHCl3,OH-

2).H2O/H+

OH

(ii)NO2H2SO4,H2O

Heat

SO3H

OH

A

B

Solutions

1.Seepage59

2.

(iii)NH2

Br

1).NaNO2,HCl

2).CuCl,Heat

C

OH

Conc.H2SO4

1000C

OH

Cl2

FeCl3

OH

ClCl

1).dil.H2SO4Cl

Heat

OH

Cl

3.

SO3H

SO3H

2).NaOH

(i)

OH

CHO(ii)

OH

NO2

OH

(iii)

Br

Cl

CHAPTER7

ANILINES

7.0Introduction

Inchapter6,phenolswerediscussed.Itwasnotedthatphenolsareimportant

intermediatesinorganicsynthesis.Inthischapter,anilineswillbedealtwith.Theseare

compoundswithanaminogroupattacheddirectlytobenzene.Mostofitspropertiesare

similartothoseofaliphaticaminesincludingitssmell.Howeverpresenceofabenzene

ringmakesitdifferentinsomeways.

Objectives


Explainthephysicalandchemicalpropertiesofanilines.

Explainusingequationsdifferentreactionsshownbyaniline.

Describehowdiazoniumionispreparedfromvariousstartingorganic

compounds.

Describereactionsofdiazoniumions.

Statesomeoftheusesofanilinesandtheirderivatives.

7.1Preparationandpropertiesofaniline

Whennitrobenzeneisreducedbymetal-acidreduction(ZnandHCl)anilineisformed.

Reductionwithsodiumborohydridecanalsobecarriedoutbutinpresenceacatalyst

poisonPd/C.Theycanalsobepreparedbyheatingazocompounds.Derivatized

halobenzenescanalsobeasourceofanilinesformedthroughnucleophilicaromatic

substitutionreactions.

NO2

NO2

Sn/HCl

(Reduction)

NaBH4

Pd/C

NH2

NH2

O2N

Cl

NO2

NH3

O2NNH2

NO2

NO2

NO2

Incertainsituations,selectiveandspecialreagentsmayberequiredtobringaboutthe

reductionofthenitrogrouptoaminogroup,dependingonthetypeofgroup(s)attached

tothebenzenering.Afewexamplesarecitedbelow.

CH3

CH3

NO2

Sn/HCl

NH2

NO2

NO2

NH4HS

(NH3/HS)

NH2

CH3

NH2

NO2

NO2

Ni/H2

200-4000Ccompoundusedwhenacid

NHCOCH3

CHO

NHCOCH3

CHO

sensitivegroup

SnCl2/HClMethodusedwhenthe

substituenthasactive

NO2NH2

unsaturation.

Anilinehasthefollowingfeatures:

1.Anilineislessbasicthanalkylamines,duetoresonancedelocalisationofthe

lonepairofelectronofnitrogen.

2.Basicityisenhancedbypresenceofelectrondonatinggroupatorthoand

para.Electronwithdrawinggroupsmakeanilinelessbasicthananilineitself

3.Aminogroup(NH2)isastrongactivator.Presenceofalkylgroupsattachedto

theringreducestheactivatingpowerduetostericfactors.

4.Electronwithdrawinggroupsattachedtonitrogenreducetheactivating

property.Agoodexampleisanamidegroup.Thelonepairofnitrogenisnot

alwaysavailabletoactivatethebenzenering,asitcanalsoresonatewiththe

doublebondofthecarbonyl.Nitrogenwithaquaternarysystem(-NR3),like

protonationofaminogroup,isafullydeactivatinggroup.

NHCCH3

O

+

NH=CCH3

O-

7.2Reactionsofaniline

Anilinecanundergotwotypesofreactions;thosethatinvolvetheaminogroupandthose

thatinvolvethebenzenering.

7.2.1Reactionsthatinvolvetheaminogroup

K

NH-K+

NH2

3RX

1.CHCl3

+

NCH+

n-quaternarysalt

2.3OH-anisocyanide

NH2CHO

+N=CH

Schiff'sbase(Imine)

NH2N2+

NaNO2

H+/00C

Diazoniumion

NH2

(CH3CO)2O

CH3COONa(N-Phenylethanamide)

H2O/H+

NH-CH2

7.2.2Reactionsinvolvingthebenzenering

Anilineistooreactiveforthehalogenationreactionsthatpolyhalogenationtakesplaceat

theparaandtwoorthopositions.Tointroduceonlyonegroup,theaminogroupmaybe

acetylatedfirst.

NH2

NH2

+

Br

3Br22,4,6-Tribromoaniline

Br

NH2

(CH3CO)2ONHCOCH3Br2

CH3COOH

NHCOCH3

+

NHCOCH3

Br

Br

NH2H2O

H+

80%20%

Br

Theorthoisomercanbepreparedbyintroductionattheparapositionfirst.Meta

substitutedanilineproductcanbepreparedfromnitrobenzene.Iodogrouptoaniline

occursatparapositionduetoselectivityprincipleduetostericfactor.

NH2

H2SO4NH2

HO3S

(CH3CO)2ONHCOCH3

HO3S

Br2

H2O

NH2

H2O/H+

NHCOCH3

0

HO3SBr

NH2

I2

NaHCO3/H2O

I

NH2

NO2

Br2

NO2

Sn/HCl

NH2

Lewisacid

Br

Br

7.3Diazoniumsalts

Oneofthemostimportantreactionsofanilinesistheformationofdiazoniumsalts.In

turndiazoniumioncanbeconvertedtoavarietyoforganiccompound,whichotherwise

wouldrequiremanystepstoachievefromotheraromaticcompounds.

NH2

NaNO2

2HCl/00C

N2+Cl-

+NaCl+H2O

Diazoniumsalt

Diazoniumsaltundergoestwotypesofreactions;

1.ReplacementofN2(lossofN2)

2.CouplingreactionswhereN2isretained

7.3.1ReplacementofN2(lossofN2)

VariousnucleophilescanbeusedtoreplacetheN2groupofthediazoniumsalt.Some

replacementscanbeintermediatetootherimportantaromaticcompounds.Direct

halogenationwithfluorineisnotpossiblebutcanbecarriedoutfromthediazoniumsalt

reactedwithfluoroboricacid(HBF4).ReplacementoftheN2groupcanbecarriedoutby

useofhypophosphousacid(H3PO2)atlowtemperatures.

N2+

:ZZ

Diazoniumion

+

N2

ArN2+

CuCl

CuBr

ArCl

ArBr

Diazoniumion

CuCN

KI

ArCN

ArI

Cl

CuCl

CN

CH3

CO2H

NH2

N2+Cl-

CuCN

CH3

H2O/H+

CH3

CH3NaNO2

CH3

HCl/00C

HBF4N2+BF4-

CH3Heat

F

CH3

NaOHOH

CH3

H3PO2

H2O

0-250C

CH3

+H3PO3

7.3.2CouplingreactionswhereN2isretained

Diazoniumsaltsundergocouplingreactionstoformazocompounds.Theyarestrongly

coloredcompoundsthathavebeenusedforalongtimeasdyestuffs.Thearomatic

compoundthatisundergoingattachmentmusthaveapowerfulelectron-releasinggroup

like-OH,-NR2,-NHRand-NH2.

N=N

N(CH3)2++N2SO3-Na+

(H3C)2N

SO3-Na+

Methylorange

Redinacidicmedia

Yellowinbasicmedia

OH

++N2NO2

'ParaRed'Dye

OH

N=N

NO2

7.4Summary

Anilineislessbasicthanalkylaminesduetoresonancedelocalisationoflonepairof

electronsonnitrogen.

Anilineshowsreactionbothattheaminogroupandatthebenzenering.

Diazoniumioncanbeconvertedtomanyorganiccompoundswhichotherwisewould

requiremanysteps.Thismakesdiazoniumionveryimportantinorganicsynthesis.


Q1.Byuseofresonancestructuresandreactionmechanism,explainthefollowing.

(i)Introductionofanacetylgroup(COCH3)tonitrogenreducestheactivating

propertiesofaniline.

Q2.Suggestthemajororganiccompounds(A-F)inthefollowingtransformations.

OH

(i)+

NNCl-+

A

CH3

Cl2/H2O

(ii)NH2B

(iii)O2N

CH3NH4HS

NO2

Q3.Outlinesynthesisofthefollowingcompounds.

(i)2,6-Dichlorophenolfromphenol

(ii)o-Nitroanilinefromaniline

(iii)m-chlorophenolfrombenzene

Solutions

1.(a)Seepage67

2.

OH

Cl

(i)

N=N

(ii)

Cl

NH2

Cl

CH3

(iii)H2NCH3

NO2

3.

(i)

OH

Conc.H2SO4

1000C

OH

Cl2

FeCl3

Cl

OH

Cl

1).dil.H2SO4

Heat

Cl

OH

Cl

SO3H

SO3H

2).NaOH

(ii)

NH2

(CH3CO)2O

NHCOCH3

H2SO4

NHCOCH3

HNO3

H2SO4

NHCOCH3

NO2

H2O/H2SO4

0

NH2

NO2

SO3H

SO3H

(iii)HNO3

H2SO4

NO2

Cl2/FeCl3NO2

Cl

Sn/HCl

OH-

NH2

Cl

OH

Cl

NaOH

N2+Cl-

Cl

NaNO2/H+

HCl/00C

CHAPTER8

POLYNUCLEARAROMATICCOMPOUNDS:NAPHTHALENE

8.0Introduction

Inchapterssixandseven,welookedatphenolsandanilinesandtheirderivativesand

theirimportanceasprecursorsforsynthesisofmanyusefulorganiccompounds.Inthis

chapterwillfocusmainlyonEASreactionsofnaphthalenes,di-nucleararomatic

compoundsandtheirderivatives.Orientationpatternsofsubstituentsonthesecompounds

aswellastheirimportanceinorganicsynthesiswillalsobediscussed.

Objectives


Describestepsinvolvedinsynthesisofnaphthalenefrombenzene

Writedownoxidationandreductionproductsofnaphthalene

Describeusingmechanisms,EASorientationpatternsofnaphthalenes

PredictEASproductsofnaphthaleneswithvariousnucleophiles

Statetheeffectontherateofreactionandofasubstituentonnaphthaleneon

incomingnucleophile

Discussthereactionsofsubstitutednaphthalenes

8.1SynthesisofNaphthalene

Naphthalenehastwobenzeneringsfusedtogether.Naphthalenemainsourceiscoaltar.

Ithasresonanceenergyof61Kcalmol-1.Ashasalreadybeenmentionedinchapter2that

naphthalenehasthreecarboncentersthatarenotequivalent.Itisthereforeexpectedto

givemoreproductsduringchemicalreactions.

Thisisamultiplestepssyntheticroute.Itinvolveselectrophilicsubstitutionofbenzene.

Thisstepworkswellifbenzeneisderivatizedwithanalkylorhalidegroups.Secondstep

involvesringclosurewhilethethirdstepisforaromatizationoftheextra-formedring.

X

O

+O

AlCl3

X

HO

O

Zn/Hg/HCl

X

HO

O

O

Clemmensen

OReduction

HF

or

X

Heat/PdXX

O

H3PO4

Heat

Wolff-Krishner

Reduction

8.2Reductionandoxidationreactionsofnaphthalene

Naphthalenecanundergooxidationandreductionreactionstogiveavarietyofproducts

dependingonthereagentused.

8.2.1ReductionReactions

Whennaphthaleneisrefluxedwithethanolinpresenceofsodiumat780C,themajor

productis1,4-dihydronaphthalene.Additionoffourhydrogensisachievedwhenitis

refluxedwithpentanolinpresenceofsodiumat1320Ctoform1,2,3,4-

tetrahydronaphthalene.However,hydrogeninpresenceofmetalcatalystnickel,platinum

orpalladium,naphthaleneisfullyreducedtodecahydrodecalin.

Na/CH3CH2OH

Reflux(780C)

Na/C5H11OH

0

H2

1,4-Dihydro

naphthalene

1,2,3,4-Tetrahydro

naphthalene

Decahydrodecalin

NiorPtorPd

Themechanismofformationof1,4-dihydronaphthalenefollowstheBirchreduction.The

intermediateinvolvedhastheresultingnegativechargeandtheradicalarefarapartand

thusmorestable.Thealcoholsuppliesthehydrogensinvolvedinthereaction.

Na.-.-

.

Morestable

EtOH

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