《有机化学》英文教学课件：Chap.04 Aromatic hydrocarbon(2012).ppt

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1、Section2Aromatichydrocarbonnonbenzenoid:benzenoidtheelectronsinthemoleculesettleforHückel’srulemonocyclicringpolycyclicringpolyphenylsubstitutedaliphatichydrocarbonpolyphenylcondensedaromaticring1Hückel’srule(ruleof4n+2):Allplanar,cyclic,fullyconjugatedmolec

2、uleswith4n+2(n=0,1,2…)electronswillbearomatic(especiallystable).Theruleworksbecausesuchmoleculeswillhavemolecularorbitalsystemsinwhichallbondingmolecularorbitalsarecompletelyfull.nonbenzenoidaromatichydrocarbon2electronselectronsaromaticityaromaticity24466

3、88103Fullerene：C6041.KekuléformsⅠ.StructureofbenzeneTheGermanchemistKekulé(1829-1896)suggestedbenzeneshouldbetheringstructureincyclohexatrieneformin1865.Eachcarbonattachesahydrogen,andcarbonsconnectbyalternativesinglebondsanddoublebonds.56AccordingtotheKekulé

4、forms,1,2-dimethylbenzeneshouldhavetwostructures.Butonlyoneformexistactually.76C-CbondshavesamelengthBenzeneundergoessubstitutionreactionsratherthantheadditionreactionsthataretypicalofalkenes.C-C:0.154nm,C=C:0.134nm,benzen:0.139nm8Theconceptualcontradictionpr

5、esentedbyahighdegreeofunsaturation(lowH:Cratio)andhighchemicalstabilityforbenzeneandrelatedcompoundsremainedanunsolvedpuzzleformanyyears.9Thepresentlyacceptedstructureofaregular-hexagonal,planarringofcarbonswasadopted,andtheexceptionalthermodynamicandchemical

6、stabilityofthissystemwasattributedtoresonancestabilizationofaconjugatedcyclictriene.2.Moderntheoryforthestructureofbenzene:10Analternativerepresentationforbenzene(circlewithinahexagon)emphasizesthepi-electrondelocalizationinthismolecule,andhastheadvantageofbe

7、ingasinglediagram.Incasessuchasthese,theelectrondelocali-zationdescribedbyresonanceenhancesthestabilityofthemolecules,andcompoundscomposedofsuchmoleculesoftenshowexceptionalstabilityandrelatedproperties.1112Eachcarbonishybridizedsp2andthereis,therefore,a2porb

8、italoneverycarbon,extendingaboveandbelowtheplanecontainingthesixcarbonsandtheirattachedhydrogens,overlaptoformacircularcloudofelectrondensityaboveandbelowtheplaneofthering.Allcarbon-carbo