有机化学课件Chapter 4 Aromatic Compounds

有机化学课件Chapter4AromaticCompoundsTeachingObjectivesandRequirementsUnderstand:SubstitutedBenzene,PolycyclicandHeterocyclicAromaticCompounds.Grasp:Aromaticity,Hückel’sRule,Nomenclature,ElectrophilicAromaticSubstitution,InterpretationofRateEffectsandDirectingEffects,ReactionsofSideChains:Free-Radical,HalogenationandOxidation.KeypointsandAreasofDifficultyHückel’sRule,Nomenclature,ElectrophilicAromaticSubstitution,InterpretationofRateEffectsandDirectingEffectsReactionsofSideChains:Free-RadicalHalogenationandOxidation.4.1SubstitutedBenzeneCompoundsAromaticmeans“fragrant〞SomefragrantcompoundscontainbenzeneringInearlytime,that’showthenameofaromaticcompoundcomesfrom4.1SubstitutedBenzeneCompoundsManybenzenecompoundsarenotfragrantSix-memberedbenzeneringisthecharacter4.2AromaticityC6H6ishighlyunsaturatedcomparedwithhexaneC6H12Itdoesnotundergoadditionreactions.Benzenereactswithbromine,inthepresenceofironbromideasacatalyst,togiveasinglemonosubstitutedproduct,C6H5Br.4.2AromaticityKekule’sConceptofBenzeneIn1865,aGermanchemist,AugustKekuleBenzeneisaringofsixcarbonatomslinkedbyalternatingsingleanddoublebonds.Twostructuresdifferingonlyinthearrangementofsingleanddoublebonds4.2AromaticityOscillatearoundtheringRapidoscillationofsingleanddoublebondsaroundtheringmakesallsixcarbonatoms,andthereforeallsixhydrogenatoms,equivalent.AsinglestructureisobservedResonanceTheoryandBenzeneAplanarmolecule,sameC-Cbonds4.2AromaticityThebondanglesoftheringareall120degree.sp2-hybridizedcarbonatomsThreeσbonds4.2AromaticityThefourthelectronisina2porbitalperpendiculartotheplaneofthebenzenering.Asetofsix2porbitalsoverlaptosharetheirsixelectronsinaπsystemthatextends4.2AromaticityOvertheentireringofcarbonatoms.Thesharingofelectronsovermanyatomsiscalleddelocalization.TheHuckelRuleThreegeneralcriteriamustbemetifamoleculeistobearomatic.First,themoleculemustbecyclicandplanar.4.2AromaticitySecond,theringmustcontainonlysp2-hybridizedatomscontaining2porbitalsthatcanformadelocalizedsystemofπelectrons.Third,thenumberofπelectronsmustbeequalto4n+2,wherenisaninteger.4.3PolycyclicandHeterocyclicAromaticCompoundsPolycyclicaromatichydrocarbonsequalto“fused〞containtwoormorerings.4.3PolycyclicandHeterocyclicAromaticCompoundsCycliccompoundsthathaveoneormoreatomsotherthancarbonwithintheringaresaidtobeheterocycliccompounds.Heterocyclicaromaticcompounds:4n+2πelectrons.4.4NomenclatureofBenzeneCompoundsUsesthenamesofthesubstituentsasprefixestobenzene.Orthoisomerhastwogroupsonadjacentcarbonatoms,ina1,2relationship.Metaisomer:1,3relationshipParaisomer:1,4relationship4.4NomenclatureofBenzeneCompoundsNumberingthebenzeneringtogivethelowestpossiblenumberstothecarbonatomsbearingthesubstituents.4.4NomenclatureofBenzeneCompoundsCommonnameofamonosubstitutedaromaticcompoundsastheparentThepositionofthesubstituentoftheparentisautomaticallydesignated1.Butthenumberisnotusedinthename.4.4NomenclatureofBenzeneCompoundsInalphabeticalordertotheparentnamealongwithnumbersindicatingtheirlocations.Aromatichydrocarbonsbelongtoageneralclasscalledarenes.4.4NomenclatureofBenzeneCompoundsAnaromaticringresidueisanarylgroup,symbolizedasArTwogroupsphenylandbenzylareoftenconfused.4.5ElectrophilicAromaticSubstitutionAnelectrophile(E+)substitutesforH+MechanismofelectrophilicaromaticsubstitionTheelectrophileacceptsanelectronpairfromthearomaticring.4.5ElectrophilicAromaticSubstitutionTheprotonbondedtothesamecarbonatomasthenewlyarrivedelectrophileislost.Thearomaticπsystemrestored.TypicalelectrophilicsubstitutionreactionsBrominationandchlorination:Br2andCl2,Lewisacid,FeBr3andFeCl34.5ElectrophilicAromaticSubstitutionInnitration,anitrogroup,－NO2,isintroducedontoanaromaticringusingnitricacid,HNO3,withsulfuricacidasthecatalyst.4.5ElectrophilicAromaticSubstitutionSulfonation:Asulfonicacidgroup,－SO3Hcanbeintroducedontoanaromaticringbyelectrophilicaromaticsubstitution.Friedel-Craftsalkylation:analkylgroupcanbesubstitutedforahydrogenatomofanaromaticring.4.5ElectrophilicAromaticSubstitutionAlkylbromidesoralkylchlorides,LewisacidAlCl3doesnotoccuronaromaticrings:thathaveoneofthegroupsNO2,SO3H,CN,oranycarbonyl-containinggroup(aldehydes,carboxylicacids,andesters)bondeddirectlytothearomaticring.4.5ElectrophilicAromaticSubstitutionIsomerizationofcarbocationbyahydrideshift,bythetrendtoformmorestablecarbocation.Friedel-Craftsacylation:anacylgroupcanreplacehydrogeninanaromaticring.4.5ElectrophilicAromaticSubstitutionThesamelimitationasF-CalkylationReductionofphenoneresulttoalkylsubstituedbenzenewithoutrearrangement.Clemmensenreduction:reactinthepresenceofzinc/mercuryamalgamandHCl4.6StructureEffectsinElectrophilicAromaticSubstitutionActivatinggroups:makethearomaticringmorereactivecomparedtobenzene,suchashydoxyl,methylgroups.Deactivatinggroups:makethearomaticringlessreactive,suchaschloroandnitrogroups.OrientationeffectsofsubstituentsOrtho,paradirector:directsororientsthesubstituentintopositionortho,andpara4.6StructureEffectsinElectrophilicAromaticSubstitutionAllactivatinggroupsareortho,paradirector.Halogens,whichareweaklydeactivating,arealsoortho,paradirectors.Metadirectors:includenitro,trifluoro-methyl,cyano,sulfonicacid,andallcarbonyl-containinggroups.4.7InterpretationofRateEffectsTheabilityofthesubstituentstoeitherdonateorwithdrawelectronsfromthearomaticring.Substituentscandonateorwithdrawelectronsdensitybyinductiveorresonanceefectsoracombinationoftheseeffects.4.7InterpretationofRateEffectsInductiveeffectsofsubstituentsRelatedtotheelectronegativityAlkylgroupsareelectrondonating,stabilizecarbocationsTransferelectrondensitythroughtheσbondstosp2–hybridizedcarbonatomDonateelectrondensitytothebenzeneringbyaninductiveeffect4.7InterpretationofRateEffectsThetrifluoromethylgroup,whosefluorineatomspullelectrondensityawayfromthecarbonatomstowhichtheyarebonded.Aninductivelyelectronwithdrawing

4.7InterpretationofRateEffectsThehalogensdirectlybondedtoanaromaticringwithdrawelectrondensityanddeactivatethecompoundtowardelectrophilicaromaticsubstitution.Theothergroupshavethesamerule.Conjugatingeffects4.7InterpretationofRateEffectsOrbitalsoverlapeachother,thelonepairelectronsintheporbitalofoxygenatomcanbedonatedtoaromaticringbythisway.Iftheatombondeddirectlytoaromaticringisattachedwithlonepairelectrons,thesubstituentcanactivatethearomaticcompound.4.8InterpretationofDirectingEffectsAttackateithertheorthoortheparapositionresultsinoneresonancestructurewithapositivechargeontheringcarbonatombondedtothemethylgroup.Thecarbocationisclosemostlytotheelectrondonatinggroup,andismoststable.4.8InterpretationofDirectingEffectsFurthermorethecarbocationistertiarycarbon.Attackateitherthemetapositionresultsonlysecondarycarbocationresonancestructure.4.8InterpretationofDirectingEffectsSubstituentwithunsharedpairofelectronscanmakethecarbocationstablebysharingthelonepairelectrons.4.8InterpretationofDirectingEffectsMetasubstitutionisfavoredbyavoidingthepositivechargeofcarbocationneartothenitrogroup.Thehalogensubstituentsareweaklydeactivatingbutyetareortho,parodirectors.4.8InterpretationofDirectingEffectsBecausethehalogenshavelonepairelectrons,theycandonateelectronstothecarbocationintermediate.4.9ReactionsofSideChainsCarbonatomsdirectlybondedtothearomaticringareinfluencedbythering.Benzylcarbocationhasapostivelychargedcarbonatomattachedtoabenzenering.Morestablethanprimaryandsecondarycarbocationsandofcomparablestabilitytoatertiarycarbocation.Alsothesameforbenzylradical4.9ReactionsofSideChains4.9ReactionsofSideChainsOxidationofsidechain:inspiteofthesidechainlength,theproductisbenzoicacid.4.10FunctionalGroupModificationOnlyafewfunctionalgroupscanbeplaceddirectlyonanaromaticringbyelectrophilicaromaticsubstitution.OxidationofmethyltocarboxylicacidReductionofacyltoalkylwithamalgam

4.10FunctionalGroupModificationAdiazoniumion,ismadebytreatingananilinewithnitrousacid,HNO2,preparedbyreactionofsodiumnitritewithsulfuricacid:diazotization.Aromaticdiazoniumionsareextremelyreactive.Theyreactwithnucleophiles,replacingthediazoniumgroupandliberatingnitrogengas.

4.10FunctionalGroupModificationSandmeyerreaction:In1884,theGermanchemistT.SandmeyerfoundthatdiazoniumionsreactwithnucleophilessuppliedintheformofaCu(Ⅰ)salt.4.10FunctionalGroupModificationCuproussaltsofthecyanideionresultintheformationofarylnitriles.Phenolscanbesynthesizedbyreactionofthearyldiazoniunc