MechanismandTheoryinOrganicChemistryT.H.LowryHarper&Row1987諢魁宏、高宏賓、任責忠編高等教育出版社1988．8Part1:THECOVALENTBONDLewisStructuresResonanceHybridorbitalsAromaticityFunctionalGroupsLewisstructure:theelectron-pairbondTheLewisstructuralformulacanserveasaconvenientstartingpointtounderstandingthevalence-bondtheoryofmolecularstructure.I.LewisStructuresIonic,covalent,andpolarbondsionicbonds:transferofelectronscovalentbonds:sharingofelectronselectronegativity,:relativeattractionforelectronsinabond-increasesgoingupandtotherightintheperiodictableThevalence-shelloccupancymustnotexceed2forhydrogenandmustnotexceed8foratomsofthefirstrowoftheperiodictable.Forelementsofthesecondandlaterrows,thevalence-shelloccupancymayexceed8.SF6,H2SO4Theheatofhydrogenationofbenzeneislessexothermicbyabout21kcalmol-1(88kjmol-1)thanonewouldhaveexpectedfromLewisstructure1onthebasisofknownpropertiesofsingleanddoublebonds.I.LewisStructuresResonancestructuresmorestablemajorcontributorlessstableminorcontributorDrawresonancestructuresforthefollowingspecies.Ifthestructuresarenotequivalent,indicatewhichwouldbethemajorcontributor. CH3NO2 CH2CHO–III.ValenceBondModelA.AtomicandmolecularorbitalsH?+H?H—HTwoelectronsins1sarelowerenergythanintheseparateatomscovalentbondShapeandOrientation

Theelectrondistributionmusthavethecharacteristicsofboths(spherical)andp(node)orbitals.Thefournewelectrondistributions(orbitals)willorientasfarawayfromeachotheraspossible.Towardthecornersofatetrahedron.III.ValenceBondModelB.Hybridatomicorbitals1.sp3hybridizationCH4facts:tetrahedral,4equivalentbondsIII.ValenceBondModelHybridatomicorbitals1.sp3hybridizationIII.ValenceBondModelB.Hybridatomicorbitals2.sp2hybridizationC2H4facts:allsixatomslieinsameplanetrigonalplanar=sp2III.ValenceBondModelB.Hybridatomicorbitals2.sp2hybridizationIII.ValenceBondModelB.Hybridatomicorbitals3.sphybridizationC2H2facts:linear=spIII.ValenceBondModelB.Hybridatomicorbitals3.sphybridizationIII.ValenceBondModelB.HybridatomicorbitalsWhatisthehybridizationofeachindicatedatominthefollowingcompound?17-ethynylestradiol(“ThePill”)IV.FunctionalGroupsAtomsorgroupsofatomsthatbehavesimilarly,regardlessofthestructuretowhichtheyareattached.methanolethanolb-phenethylalcohol(lilacs)geraniolretinol(vitA)IV.FunctionalGroupsHydrocarbons(C&Honly)HeteroatomiccompoundsaliphaticaromaticalkanesalkenesalkynescycliccompoundsalcoholsethersaldehydesketonescarboxylicacidsestersaminesamidesIntheallylsystemeachcarbonistrigonal,andeachusessp2hybridstomakebondstoitsneighbors.DelocalizedπbondingResonanceandAromaticity1.Benzeneismuchlessreactivetowardelectrophiles,suchashalogens,thanaresimplealkenes,andwhenitreactsitgivessubstitutionratherthanaddition.2.NMRofbenzeneanditsderivativesshowstheprotonsbondedtotheringtobeexperiencingastrongereffectivemagneticfieldthandoprotonsattachedtosimplealkenes.3.Theheatofhydrogenationofbenzeneislessexothermicbyabout21kcalmol-1(88kjmol-1)thanonewouldhaveexpectedfromLewisstructure1onthebasisofknownpropertiesofsingleanddoublebonds.AROMATICITYAromaticcompoundsarecyclicunsaturatedmoleculescharacterizedbycertainmagneticeffectsandbysubstantiallydifferentchemicalreactivityandgreaterthermodynamicstabilitythanwouldbeexpectedfromanoncyclicmodel.Antiaromaticcompoundsarecyclicunsaturatedcompoundsthatarethermodynamicallylessstablethanwouldbeexpectedfromanoncyclicmodel.Capableofexistenceonlywhenimmobilizedbyfreezinginaninertmatrixatverylowtemperature.

Thoughastablecompd.,doesnothaveaplanarringThesimpleresonancetheoryfailstoexplainthecontrastbetweenthepropertiesofbenzeneontheonehandandofcyclobutadieneandcyclooctatetraeneontheother,butwemayturntomolecularorbitalsforthesolution.Huckeltheory:anyregularplanepolygonconjugatedringIfthenumberofpairsiseven(4n),thelastpairwillbetheonlyoccupantsofadoublydegeneratelevelandsooneelectronwillgointoeachorbitalwithspinsparallel.Itisfoundthatthe4n+2ringsarestabilizedcomparedwiththeopenchainanalog,whereasthe4nsystemsaredestabilized.Theorythusexplainstheclassificationofcyclicconjugatedsystemsasaromaticorantiaromaticaccordingtotheenergycriterion,whichwemaydefineasfollows:amoleculeisaromaticifitisthermodynamicallymorestablethanexpectedfortheopen-chainanalog,

anditisantiaromaticifitisthermodynamicallylessstable.Acompoundshowingneitherstabilizationnordestabilizationwouldbeclassedasnonaromatic.Theorysuggeststhatanappliedmagneticfieldperpendiculartoaconjugatedringthathas4n+2electronswillcausetheelectronstobehaveasthoughtheywerecirculatingaroundtheringandgeneratingtheirownsmallmagneticfield.Thiscirculationiscalledtheringcurrent.(p.40,Fig.1.27)芳香性概念Ⅰ.分子高度不飽和（C/H比值很高）Ⅱ.高度穩定：①難加成②難氧化③易親電取代芳香化合物均具有相當大的共振能（離域能）（芳香性←→分子能量低）NMR：芳香性：在外界磁場的作用下，產生一個與外界磁場方向相反、并與環平面垂直的磁場，使環外質子的化學位移向低場（δ值較大）芳香性的表現歸結為：獨特的化學反應性能；獨特的熱力學穩定性和獨特的光譜性質（主要是NMR）HomoaromaticityTheyaresystemsinwhichtheπsystemsisinterruptedatoneormorepointsbyasaturatedcenterbutinwhichgeometrystillpermitssignificantoverlapoftheporbitalsacrosstheinsulatinggap.同芳體系是指被一個或兩個Csp3隔開的環狀∏體系，其分子的幾何