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
2.8Questionsandsolutions23
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
3.10Questionsandsolutions33
Chapter4:Reactionsofbenzenederivatives35
4.0Introduction35
4.1EffectofsubstituentsonEASpatterns/orientation35
4.2Groupsthatdonateelectronstothering39
4.3Effectofgroupsthatdonatebyinductionandrelease
electronsbyresonance41
4.4Effectofathirdsubstituentonthebenzenering42
4.5Summary43
4.6Questionsandsolutions44
Chapter5:Reactionsofarenesandarylhalides47
5.0Introduction47
5.1Arenes47
5.2ReactionsofArenes48
5.3EASreactionsofarylhalide50
5.4Removalofhalidesubstituentfrominactivatedarylhalide53
5.5Summary54
5.6Questionsandsolutions55
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
6.6Questionsandsolutions64
Chapter7:Anilines66
7.0Introduction66
7.1Preparationandpropertiesofaniline66
7.2Reactionsofaniline68
7.3Diazoniumion70
7.4Summary72
7.5Questionsandsolutions72
Chapter8:Polynucleararomaticcompounds:Naphthalene74
8.0Introduction74
8.1Synthesisofnaphthalene74
8.2Reduction-oxidationreactionsofnaphthalene75
8.3OrientationandEASreactionsofnaphthalene76
8.4Electrophilicaromaticsubstitutionreactionsofnaphthalene77
8.5OrientationEASreactionsnaphthalenesderivatives79
8.6Summary80
8.7Questionsandsolutions81
Chapter9:AnthraceneandPhenanthrene83
9.0Introductionandpreparations83
9.1OxidationandReductionreactions84
9.2Diels-Alderreactions84
9.3Halogenationofanthraceneandphenanthrene85
9.4Sulphonationofanthraceneandphenanthrene85
9.5Alcoholsofanthracene86
9.6Positionalactivityofanthracene86
9.7Anthraquinones87
9.8Summary88
9.9Questionsandsolutions88
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
Bytheendofthislesson,youshouldbeableto:
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
Bytheendofthislesson,youshouldbeableto:
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
Bytheendofthislesson,youshouldbeableto:
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.
4.6Questionsandsolutions
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
Bytheendofthislesson,youshouldbeableto:
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.
5.6Questionsandsolutions
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
Bytheendofthislesson,youshouldbeableto:
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.
6.6Questionsandsolutions
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
Bytheendofthislesson,youshouldbeableto:
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.
7.5Questionsandsolutions
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
Bytheendofthislesson,youshouldbeableto:
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