Chapter11ExercisesArenesarehydrocarbonsbasedonthebenzeneringasastructureunit.§11.1BenzaneSomehistory芳烃AromaticHydrocarbon苯和脂肪烃基取代的苯只含有一个苯环，称为单环芳烃。苯环上可以是一元取代，也可以是多元取代；脂肪烃基可以是饱和的，也可以是不饱和的。例如：Thearomaticcharacterofbenzeneinfersaninherentstabilitytothesystem.Asaresult,benzeneisnotparticularlyreactivecomparedtoalkenes.Thisstabilitycanbeexpressedbytheresonanceenergy,whichforbenzeneisabout36kcal/mol环己烯与苯的对比氢化热比较(KJ/mole)Heatofhydrogenationofcyclogenationofcyclohexene,1,3-cyclohexadiene,ahypothetical1,3,5-cyclohextriene,andbenzene.(KJ/mole)ResonanceEnergy共振能苯分子的共振能§11.3KekuléandtheStructureofBenzeneBenzenehastheshapeofaregularhexagon.Inwhichall6Catomsin“aromatic”ringaresp2hybridised,withremaining6e-inunhybridisedporbitalsdelocalisedoverentirering,formingdoughnut/torusshapede-cloudsabove&belowringplaneAll12atomsinbenzene,C6H6,lieinthesameplane.Benzenehasaplanar,cyclic,conjugatedstructure.IfonedrawsbenzeneasalternatingC=CandC-CthenthetwodifferentKekuléstructuresareobtained.Thesearetwoequallyvalidresonancecontributors.Alternatively,thesetwoformscanbecombinedintheresonancehybridandthepsystemrepresentedbyacircleasintheRobinsonstructure.NotethatalloftheC-Cbondsare1.4Å(betweentypicalC=CandC-Cdistances).140pmStructure§11.3AResonanceDescriptionofBondinginBenzeneKekulé的两种结构式符合共振条件：两种结构式仅在电子排列上结构不同根据共振理论,苯是Ⅰ，Ⅱ的杂化体，由于二式完全等同，因此具有相同的稳定性，对杂化体的参与程度相等,共振作用大大提高了苯识所引起的稳定性Circle-in-a-ringnotationstandsforresonancedescriptionofbenzene(hybridoftwoKekuléstructures).§11.4AnOrbitalHybridizationModelofBondinginBenzene碳上未参与杂化的轨道电子与相邻碳上电子产叠，形成平面上下两个连续的面包圈形电子云Themoreredanareais,thehighertheelectrondensityandthemoreblueanareais,thelowertheelectrondensity.Notethenucleophiliccharacterofthearomaticpsystem.Benzeneismuchmorestablethanwouldbeexpectedbasedoncalculationsfor“cyclohexatriene”Areasonablepredictionfortheheatofhydrogenationofhypotheticalcyclohexatrieneis-360kJmol-1(3timesthatofcyclohexene,-120kJmol-1)Theexperimentallydeterminedheatofhydrogenationforbenzeneis-280mol-1,152kJmol-1morestable