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1、1111Spring2009Dr.HalliganCHM236Conjugation,Resonance,andDienesChapter162ConjugationConjugationoccurswheneverporbitalscanoverlaponthreeormoreadjacentatoms.Thefourporbitalsonadjacentatomsmakea1,3-dieneaconjugatedsystem.3Havingthreeormoreporbitalsonadjacentatomsallowsporbitalsto
2、overlapandelectronstodelocalize.Conjugation4Conjugation1,4-Pentadieneisanisolateddiene.Thebondsin1,4-pentadienearetoofaraparttobeconjugated.5ConjugationTheallylcarbocationisanotherexampleofaconjugatedsystem.Conjugationstabilizestheallylcarbocation.6ConjugationDrawingresonanc
3、estructuresfortheallylcarbocationisawaytoseehowtouseLewisstructurestoillustratehowconjugationdelocalizeselectrons.Thetrueallylcationisahybridofthetworesonanceforms.Inthehybrid,thepositivechargeisdelocalizedoverthetwoterminalcarbons.Delocalizingelectrondensitylowerstheenergyof
4、thehybrid,thusstabilizingtheallylcarbocationandmakingitmorestablethananormal1°carbocation.7ConjugationExperimentaldatashowthatthestabilityoftheallylcationiscomparabletoamorehighlysubstituted2°carbocation.Figure16.313Cchemicalshiftsforalocalizedandaresonance-stabilizedcarbocat
5、ion8CommonExamplesofResonance1.TheThreeAtom“Allyl”System,X=Y-Z*Examplesaretheallylcationandtheacetateanion.Thetworesonancestructuresdifferinthelocationofthedoublebond,andeitherthecharge,theradical,orthelonepair,generalizedby[*].9CommonExamplesofResonance2.ConjugatedDoubleBond
6、sCycliccompletelyconjugatedringslikebenzenehavetworesonancestructures,drawnbymovingtheelectronsinacyclicmanneraroundthering.Threeresonancestructurescanbedrawnforotherconjugateddienes,twoofwhichinvolveseparationofcharge.10CommonExamplesofResonance3.CationsHavingaPositiveCharge
7、AdjacenttoaLonePairTheoverallchargeisthesameinbothresonancestructures.Basedonformalcharge,aneutralXinonestructuremustbeara(+)chargeintheother.11CommonExamplesofResonance4.DoubleBondsHavingOneAtomMoreElectronegativeThantheOther12Theresonancehybridmorecloselyresemblesthemajorco
8、ntributor.13ElectronDelocalization,Hybridization,andGeometryConsider