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OCRChemistryA AromaticCompounds

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AromaticCompoundsNamingAromaticcompoundscontainoneormorebenzenerings(whilealiphaticcompoundsdonotcontainbenzenerings).Anothertermforacompoundcontainingabenzeneringisarene.Thebasicbenzenering,C6H6iscommonlyrepresentedasahexagonwitharinginside.Youshouldbeawarethatthereisahydrogenateachcorneralthoughthisisnotnormallyshown.N.B.itisOKtousebenzeneinthisforminstructuralformulaetoo!Arenesoccurnaturallyinmanysubstances,andarepresentincoalandcrudeoil.Aspirin,forexample,isanaromaticcompound,anarene:Namingofsubstancesbasedonbenzenefollowsfamiliarrules:

benzene methylbenzene bromobenzenenitrobenzeneNumbersareneededtoidentifythepositionsofsubstituents.Thecarbonsaroundtheringarenumberedfrom1-6consecutivelyandthenamewhichgivesthelowestnumber(s)ischosen:2-bromomethylbenzene 4-bromomethylbenzene 1,2-dichlorobenzene1,3,5-trichlorobenzene

CH3

OH O

O

O

CH3

Br NO2

CH3Br

CH3

Br

ClCl

Cl

Cl Cl

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HC

HC C

C C CH

H

H

H

CCCCC

CH

H

HH

H

H

Determiningthestructureofbenzene(historical)1825–substancefirstisolatedbyMichaelFaraday,whoalsodetermineditsempiricalformulaasCH1834–RFMof78andmolecularformulaofC6H6determinedMuchspeculationoverthestructure.Manysuggestedstructureslike:1865–Kekulépublishessuggestionofaringwithalternatingdouble andsinglebonds: displayed skeletalThismodelpersisteduntil1922,butnotallchemistsacceptedthestructurebecauseitfailedtoexplainthechemicalandphysicalpropertiesofbenzenefully:ifC=CbondswerepresentasKekuléproposed,thenbenzenewouldreactlikealkenes.Forexamplebenzenewouldbeexpectedtodecolourisebrominewater.Infactbenzenedoesnotdothis,nordoesbenzenedotheotherelectrophilicadditionreactionsthatalkenesdo.Kekulé'sanswerwastorefinehismodeltoaccountforthislackofreactivity,suggestingthatthedoublebondsrapidlychangedpositionsroundthering(twoformsofbenzeneinrapidequilibrium)sothatapproachingelectrophilessuchasBr2couldnotbeattractedtoadoublebondbeforeitmovedwhenthestructurechanged.1922 X-raycrystallographyusedtomeasurebondlengthsinarenes.KathleenLonsdalediscoversthatalltheC-Cbondsaroundtheringarethesamelength: C-Cbondsinalkanes 0.153nm C=Cbondsinalkenes 0.134nm CtoCbondsinbenzene 0.139nm(allsixcarbon-carbonbonds)ThiswasimportantevidencethattheKekulémodelwasincorrect.

CCCCC

CH

H

HH

H

HCCCCC

CH

H

HH

H

H

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FurthermodernevidencethatKekulé'smodelwaswrong:Theenergychange(enthalpychange)whenthedoublebondinanalkeneishydrogenatedcanbemeasured. +H2! ΔH=-120kJmol-1 C6H10 C6H12 cyclohexene cyclohexaneThereforewhenthethreedoublebondsinKekulé'sbenzenearehydrogenatedweshouldseeanenthalpychangeofhydrogenationof3x-120=-360kJmol-1Predicted: +3H2! ΔH=-360kJmol-1 "cyclohexa-1,3,5-triene" cyclohexaneActually: +3H2! ΔH=-208kJmol-1 benzene cyclohexaneWhenbenzeneishydrogenatedtheenthalpychangeis-208kJmol-1,whichis152kJmol-1lessthanpredicted.TheconclusionhastobethattheactualstructureofbenzenehasmuchlessenergythantheproposedKekuléstructure–i.e.therealstructureis152kJmol-1morestable,whichhelpstoexplainwhybenzeneislessreactivethanalkenes.Wecanvisualisethisonanenergyleveldiagram:Theenergydifferencebetweentheexpectedenthalpyofhydrogenationandtheactualenthalpyofhydrogenation,whichgivesrisetotheadditionalstabilityofbenzene

Enthalpy (kJmol-1) expected:

-360 kJmol-1 ΔH = -208 kJmol-1

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comparedtotheKekulé'smodel,iscalledthedelocalisationenergy,orresonanceenergyofbenzene.CurrentlyacceptedstructureofbenzeneThedeficienciesinKekulé'smodelledtothecurrently-accepteddelocalisedmodelforthestructureofbenzene.Inthismodel:" thesixcarbonatomsarearrangedinaplanarhexagonalringwitheachcarbon

sigma-bondedtoonehydrogenandtwoothercarbons" theshapearoundeachcarbonistrigonalplanarwith120°bondanglesandeach

carbontocarbonbondisthesamelength" eachcarbonhas4outershellelectrons,threeofwhichareinvolvedinthesigma-

bondstothetwoothercarbonsandtheonehydrogen.Thisleavesafourthoutershellelectronina2porbitalaboveandbelowtheplaneofthering.

" Eachofthesep-orbitalsoverlapssidewayswiththep-orbitalsofthecarbonson

eitherside.Thisresultsinasystemofπ-bondsintheformofaringofelectrondensityaboveandbelowtheplaneofthebenzenering,inwhichthesixelectronsaredelocalised.

(Canyouthinkofwhereyou'vecomeacrossasimilarbondingconceptwithcarbonforming3bondsandhavingoneelectronabletobedelocalised?)Thisiswhatismeantbythecircleinsidethehexagonwhendrawingbenzene.

C C C

HC

p-orbital

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HowbenzenereactsBecauseitismorestablecomparedtoalkenes,benzenedoesnot(undernormalconditions)undergotheadditionreactionsexpectedofanalkene:

-itdoesn'tdecolourisebrominewater-itdoesn'treactwithhydrogenhalidessuchasHCl-reactwithotherhalogens(Cl2,I2)

Instead,benzeneandotherarenesundergosubstitutionreactionswhereoneatom/group(oftenoneofthehydrogenatoms)connectedtotheringisreplacedbyadifferentatomorgroup.Thisleavesthedelocalisedπ-systemintact(morestable),whereasanadditionreactionwouldhavebrokentheπ-systembyaddingtothering(lessstable).

SubstitutionreactionsofbenzeneTheregionofhighelectrondensityaboveandbelowtheplaneoftheringattractselectrophiles,sotheseareelectrophilicsubstitutions.1)NitrationofbenzeneEffect: Oneofthehydrogenatomsontheringisreplacedbyanitro(-NO2)groupConditions: mixtureofconc.HNO3andconc.H2SO4at50°C H2SO4isactingasacatalyst H2SO4Equation: +HNO3! +H2O 50°c benzenenitrobenzeneNotes:Ifthemixturegetshotterthan50°Cthenafurthernitro-groupmaybeaddedtothering(i.e.theproductreactswithfurthernitricacid)

NO2

Additionreaction–doesnothappenasthedelocalizedπ-systemgetsbroken,soorganicproductlessstablethanreactant

Substitutionreaction–doeshappenbecausethedelocalizedπ-systemispreserved,sotheproductisstable,likethereactant.

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Otheraromaticcompoundse.g.methylbenzene(toluene)canalsobenitratedinthisway.Thisreactionisfasterthanwithbenzeneandcanleadtotheformationof2,4,6-trinitromethylbenzene(TNT)!!Mechanism(“AttackoftheNitrylCations”)Theconc.sulphuricacidisusedtogenerateanelectrophile,NO2

+fromtheconc.nitricacid.Thisioniscalledthenitroniumion(ornitrylcation) HNO3 + H2SO4 ! +NO2 + HSO4

- + H2O Thenitrylcationistheelectrophilewhichattacksthebenzenering:

FinallytheH+whichisproducedreactswiththeHSO4

-ion,regeneratingtheH2SO4catalyst: H+ + HSO4

- ! H2SO4 2)HalogenationofbenzeneBenzenedoesn'treactwithhalogensontheirown,butdoesreactwithhalogensifthereisahalogen-carrierpresentasacatalyst.e.g.whenbrominewaterisaddedtobenzeneandalittleFeCl3isadded,thebromineisdecolourisedandwhitefumesofHBrareseen.CommonhalogencarriersforchlorinationareFeCl3,AlCl3.FemetalcanbeusedasitreactswiththeCl2presenttoformFeCl3insitu.Similarlyforbromination,thecommonhalogencarrierswouldbeFeBr3orAlBr3orFe.Effect:ahydrogenisreplacedonthebenzeneringbyahalogenatom

+NO2

two of the delocalised electrons from the ring are donated to the electrophile, forming a covalent bond (Remember, curly arrows show the movement of an electron PAIR)

H NO2

+

unstable intermediate

NO2

the C-H bond breaks and the two electrons are returned to the delocalised ring

+ H+

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Conditions:halogensreactwithbenzeneatroomtemperatureandpressureinthepresenceofasuitablehalogencarrier FeCl3orAlCl3Equation:e.g. +Cl2! +HCl benzene chlorobenzeneMechanism:Theroleofthehalogen-carrieristogenerateCl+orBr+ions,whicharemorepowerfulelectrophilesthanCl2orBr2(seelater).e.g. AlBr3 + Br2 ! Br+ + AlBr4

- TheCl+orBr+ionistheelectrophilewhichattacksthebenzenering:e.g.FinallytheH+reactswiththeFeBr4-orAlBr4-regeneratingthehalogencarriercatalyst:e.g. H+ + AlBr4

- ! AlBr3 + HBr3)Alkylationofbenzene(Friedel-Crafts)Thisreactionisveryimportantasitresultsinabondbeingformedbetweenacarbonatomandanaromaticring.Thisallowsthecarbon‘skeleton’ofamoleculetobemodifiedbyextendingitduringsynthesis.Effect:ahydrogenonthebenzeneringisreplacedbyanalkylgroup. Conditions: ahaloalkane,inthepresenceofahalogencarriere.g.AlCl3Equation: AlCl3e.g. +CH3CH2Cl! +HCl benzene ethylbenzene

Cl

Br+ H Br

+

Br

+ H+

C2HCH3

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Mechanism:Theroleofthehalogen-carrieristogenerateanalkylcation,e.g.+CH2CH3,whichactsasanelectrophileinattackingthearomaticring. AlCl3 + CH3CH2Cl ! +CH2CH3 + AlCl4- FinallytheH+reactswiththeAlCl4-regeneratingthehalogencarriercatalyst:e.g. H+ + AlCl4- ! AlCl3 + HClYoumaybegiveninformationaboutanunfamiliarelectrophilicsubstitutionreaction,buttheprinciplewillbethesame.Anelectrophilewillbegeneratedandalloftherestofthemechanismwillthesameasintheseexamples,withtheelectrophilebeingsubstitutedontothering.ComparingandcontrastingthereactionsofalkeneswiththoseofbenzeneItisimportanttobeabletodiscussthedifferencesandsimilaritiesinhowanalkene(e.g.cyclohexene)reactwithanelectrophilesuchasBr2comparedtohowbenzenereacts. CYCLOHEXENEreactsbyELECTROPHILICADDITION

Thetwoelectronsintheπ-bondofanalkenearelocalisedbetweenthetwocarbonatoms.Thehighelectrondensityherecanpolarisethebromine,causingthereaction.

Br Brδ+ δ-

Br

+

:Br-

Br

Br

+CH2CH3 H CH2CH3

+

CH2CH3

+ H+

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WhereasBENZENEreactsbyELECTROPHILICSUBSTITUTIONBr2+AlBr3!Br++AlBr4-H++AlBr4-!AlBr3+HBrThesixelectronsintheπ-systemaboveandbelowtheplaneofthebenzeneringaredelocalisedoverthesixcarbonatoms,sotheelectrondensityislower.Thebrominecannotbepolarisedsufficientlytoreact,andthelowerelectrondensitydoesnotattracttheelectrophilesostrongly.Keydifferences:Benzenehasdelocalisedπ-electronsspreadoverallsixcarbonatomsinthering(6electronsspreadover6bonds).Alkeneshaveπ-electronslocalisedinthedoublebond(2electronslocalisedin1bond),sobenzenehasalowerπ-electrondensitythanalkenes.Whenanon-polarmoleculesuchasbromineapproachesthebenzeneringthereisinsufficientπ-electrondensityaboveandbelowanytwocarbonatomscausethenecessarypolarisationofthebrominemolecule,soahalogencarrierisneededtogenerateBr+toattacktheringTheπ-electrondensityinanalkene'sdoublebondissufficienttopolarisethebrominemoleculesothatitcanactasanelectrophilewithoutneedingahalogencarrier.

Br+ H Br

+

Br

+ H+

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PhenolsInaphenolthereisan–OHgroupdirectlyattachedtothebenzenering.The–OHgroupistakenasbeinginposition1forthepurposesofnamingaphenol,althoughinsomearomaticcompoundsthe-OHcanbeasubstituentgroupandisindicatedby‘hydroxy-‘inthename.e.g. phenol 2-ethylphenol salicylicacid (2-hydroxybenzoicacid)Salicylicacidisaphenolusedinthepreparationofaspirinandotherpharmaceuticals.Notethatanaromaticcompoundwherethe–OHgroupisnotattachedtothebenzeneringwouldbeanaromaticalcohol,notaphenols.e.g. 2-phenylethanol (notaphenol)Propertiesofphenols" Slightlysolubleinwaterbecausethe–OHgroupformshydrogenbondswithwater,

butthepresenceofthebenzeneringsmakesphenolslesssolublethanalcohols." Weaklyacidicinaqueoussolution: C6H5OH(aq) ⇌ C6H5O-

(aq) + H+(aq)

Theacidicnatureofphenolsmeansthattheycanbeneutralisedbyalkalissuchassodiumhydroxide,toformasaltandwater: C6H5OH+NaOH ! C6H5O-Na+ +H2O phenolsodiumhydroxidesodiumphenoxide waterHowever,phenolsaresuchweakacidsthatunlikeotheracidstheydonotreactwithcarbonatessuchassodiumcarbonate.ReactionswithelectrophilesLikebenzene,phenolundergoeselectrophilicsubstitutions.Unlikebenzenethereactionstakeplaceatroomtemperatureandwithouttheneedforahalogencarriercatalyst.

OH OHCH2CH3

CH2CH2OH

O

OH

OH

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Whyareelectrophilicsubstitutionsofphenoleasierthanwithbenzene?" Alonepairoccupyingap-orbitalontheoxygenatomofthe–OHgroupis

delocalisedintothebenzenering(addingtwomoreelectronstotheπ-system)." Thiscreatesahigherelectrondensityintheringstructure–theringisactivated." Theincreasedelectrondensitymakestheringmoresusceptibletoattackby

electrophiles.

1)WithbromineWhenbrominewaterisaddedtoanaqueoussolutionofphenol,thebromineisdecolourisedandawhiteprecipitateof2,4,6-tribromophenolisformed(steamyfumesofHBrmayalsobeseen).NohalogencarrierisrequiredbecausetheincreasedelectrondensityintheringincreasesthepolarisationofBr2molecules,whicharethusattractedmorestronglytowardstheringthaninbenzene,andabletoattackaselectrophiles.Equation:

+3Br2! +3HBr phenol 2,4,6-tribromophenol

OH OHBrBr

Br

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Mechanism2)WithnitricacidPhenolreadilyundergoesnitrationtoformamixturecontainingmainly2-nitrophenol,andsome4-nitrophenol.Nitrationofphenolsdoesnotrequireconc.nitricacidoraconc.sulphuricacidcatalyst,andcanbecarriedoutusingdilutenitricacidalone.Equation:

+HNO3! +H2O phenol 2-nitrophenolDirectingeffectsThepresenceofgroupsbondedtothearomaticringdoesnotonlyaffecthowreactivetheringistowardselectrophiles,butalsoinfluenceswherearoundtheringsubstitutiontakesplace.Werefertotheseasdirectingeffects.Groupssuchas–OHand–NH2areelectron-donatingtowardsthering,andhavea2-and4-directingeffectwhenelectrophilicsubstitutiontakesplace(aswellasactivatingthering,increasingitsreactivity).Electron-withdrawinggroupssuchas–NO2havetheoppositeeffect.Theyare3-directing(aswellasdecreasingthereactivityofthering).Youmaybegiveninformationinexamsabouttheelectron-donatingorelectron-withdrawingpropertiesofothergroups,andshouldbeabletopredicttheirdirecting

OHNO2

OH

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propertiesaccordinglyinelectrophilicsubstitutionreactionsofsubstitutedaromaticcompounds.Workedexamples:Q1)Phenylamine,C6H5NH2canreactwithbromineinanelectrophilicsubstitutionreaction.Predicttheproductformed,andtherelativeeaseofbrominationcomparedtothebrominationofbenzene.Ans: The–NH2group,likethe–OHgroup,iselectron-donating.Itistherefore2-and4-directing.Wecanpredictthattheringwillbeactivatedlikeinphenolandthereforethereactionwilltakeplacereadilywithoutneedforahalogencarrier.Likewithphenolwemightexpectmultiplesubstitutionstoform2,4,6-tribromophenylamine.Q2)Nitrobenzene,C6H5NO2canreactwithbromineinanelectrophilicsubstitutionreaction.Predicttheproductformed,andcommentontherelativeeaseofbrominationcomparedtothatofbenzene.Ans: TheNO2-groupiselectron-withdrawing.Itistherefore3-directingandtheproductweexpectwouldbe3-bromonitrobenzene.Thereactionwillbeslowerthanwithbenzene,requiringbothahalogencarriercatalystandahightemperature.Q3)Benzoicacidcomprisesacarboxylicacidgroupbondedtoabenzenering.The-COOHgroupiselectron-withdrawing.Benzoicacidcanbenitratedusingamixtureofnitricandsulphuricacids.Identifythemono-substitutedproductthatwouldbeformed,andwriteanequationforthereactionthattakesplace.Ans: Electron-withdrawinggroupsare3-directing,sotheproductwouldbepredictedtobe3-nitrobenzoicacid.Q4)Methylbenzeneisbrominatedmorereadilythanbenzenebecausethealkylgroupiselectron-donating.Predictthemono-substitutedproductsthatwillbeformed,andtherelativeproportionsofeach.Ans:Electron-donatinggroupsare2-and4-directing,sotheproductswouldbeexpectedtobe2-bromomethylbenzeneand4-bromomethylbenzene.The6-positionisequivalenttothe2-position,sothereareeffectivelytwiceasmany2-positionsas4-positions.Therewillthereforebetwiceasmuch2-bromomethylbenzeneas4-bromomethylbenzene.

COOH COOH

NO2

HNO3 H2O