有機1.苯與那些分子發(fā)生化學反應這些反應屬于那種

Benzene’sStructureAnd?2012WeiNanjingUniversityofScience&TechnologyQuestionAbout苯與那些分子發(fā)生化學反應？這些反應屬于那種Kekulé'sBallandStickModelStructureandStabilityofRecallthereactionofregularalkeneswithYougetrapidadditionatRTtogivethedibromo rapid rapidButundertheseconditions,benzenegivesno++pibondsBenzenewillreactwithBr2togivebromobenzene,butitrequiresheatandironcatalyst(slow).Thisissubstitutionratherthanadditionasseenwithregularalkenes,suggestingthatbenzenehasspecialQuestionAbout苯的結構準確嗎?它是怎樣得來的Benzene-AromaticHeatsofhydrogenationdemonstratetheextrastabilityofthebenzenemolecule.Some MichaelFaradayisolatesanewhydrocarbonfromilluminatinggas. EilhardtMitscherlichisolatessamesubstanceanddeterminesitsempiricalformulatobeC6H6.Compoundcomestobecalledbenzene. AugustW.vonHofmannisolatesbenzenefromcoaltar. AugustKekuléproposesstructureofFurthermore,onlyonemonobromosubstitutionproductwasknown;noisomersofC6H5Brhadbeenprepared.

C C

+H (substitutionproductOnfurtherreactionofC6H5BrwithBr2,disubstitutionproductsareobtained,andthreeisomericC6H4Br2compoundshadbeenprepared.AugustKekuléproposedin1865thatbenzeneconsistsofaringofcarbonatomsandcanbeformulatedas1,3,5-cyclohexatriene.Kekuléreasonedthatthisstructurewouldreadilyaccountfortheisolationofonlyasinglemonobromosubstitutionproduct,becauseallsixcarbonatomsandallsixhydrogensin1,3,5-cyclohexatrieneareequivalent.HHHHHHHHHTheobservationthatonlythreeisomericdibromosubstitutionproductswereknownwasmoredifficulttoexplainbecausefourstructurescanbewritten.

3Kekuléaccountedfortheformationofonlythreeisomersbyproposingthatthedoublebondsinbenzene“oscillate”rapidlybetweentwopositions.Thus,thetwo1,2-dibromo-cyclohexatrienescan’tbeseparated,accordingtoKekulé,becausetheyinterconverttoorapidlyThePuzzleaboutAlthoughbenzeneisclearly“unsaturated”theformulaC6H6requiresacombinationoffourdoublebonds/rings,itneverthelessfailstoundergoreactionscharacteristicofQuestionAboutKekulé的苯結構有缺陷，你有其他的建議Kekulé’sproposedstructureforbenzenewaswidelycriticizedatthetime.Althoughitsatisfactorilyaccountsforthecorrectnumberofmono-anddisubstitutedbenzeneisomers,itfailstoanswertwocriticalquestions:whyisbenzeneunreactivecomparedwithotheralkenes,andwhydoesbenzenegiveasubstitutionproductratherthananadditionproductonreactionwithBr2?Ladenburg...threedibromoisomersareOtherStructuresProposedof M.Berthelot

(1)O(2)

+2

(1)O

O

+ (2)QuestionAbout大膽假設 求證Benzene,Dreams,andCreativeAtceremoniesinBerlinin1890tocelebratethetwenty-fifth ofhisproposalofthestructureofbenzene,AugustKekulérecalledthethoughtprocesswhichledhimtoit.KekulébeganbyspeakingoftheideaofthestructuraltheorycomingtohimwhiledaydreamingonabusinLondon.Hewentontodescribetheoriginsofhisviewofthebenzen structureKekulé’sDreamandCreativeThereIsatandwroteformytextbook;butthingsdidnotgowell;mymindwasoccupiedwithothermatters.Iturnedthechairtowardsthefireplaceandbegantodoze.Onceagaintheatomsdancedbeforemyeyes.Thistimesmallergroupsmodestlyremainedinthebackground.Mymentaleye,sharpenedbyrepeatedapparitionsofsimilarkind,nowdistinguishedlargerunitsofvariousshapes.Longrows,frequentlyjoinedmoredensely;everythinginmotion,twistingandturninglikesnakes.Kekulé’sDreamandCreativeAndbehold,whatwasthat?One ofthesnakescaughtholdofitsowntailandmockinglywhirledroundbeforemyeyes. Iawoke,asifbylightning;thistime,too,Ispenttherestofnightworkingouttheconsequencesofthishypothesis.Kekulé’sDreamandCreativeConcludinghisremarks,Kekulémergedhisadvocacyof creativeimaginationwith rigorousstandardsofsciencebyremindinghisaudience:Letuslearntodream,thenperhapsweshallfindthetruth.Butletusbewareofpublishingourdreamsbeforetheyhavebeenputtotheproofbythewakingunderstanding.QuestionAbout什么是苯結構的現(xiàn)代觀點StabilityofBenzenehasfourdegreesofunsaturation,butundergoesnoneofthecommonreactionsofalkenes...noreactionnoveryslowHeatof(energyreleasedduringreductionwith

28.655.449.886kcal/moleexpected;50kcal/moleobserved36kcal/molemorestablethanexpectedRepresentationofBenzene:TheResonanceApproachHowcanweaccountforbenzene’sproperties,andhowcanwebestrepresentitsstructure?ResonancetheoryanswersthisquestionbysayingthatbenzenecanbedescribedasahybridoftwoequivalentKekuléstructure.1.5bonds H

Benzenecan’tberepresentedaccuraybyeitherindividualKekuléstructureanddoesnotoscillatebackandforthbetweenthetwo.Thetruestructureissomewhereinbetweenthetwoextremesbutisimpossibletodrawwithourusualconventions.WemighttrytorepresentbenzenebydrawingitwithafullcircletoindicatetheequivalenceoftheC-Cbonds.ThereisasubtleyetimportantdifferencebetweenKekulé’srepresentationofbenzeneandtheresonancerepresentation.Kekuléconsideredbenzeneasrapidlyoscillatingbackandforthbetweentwocyclohexatrienestructures,whereasresonancetheoryconsidersbenzenetobeasingle“resonancehybrid”structure. MolecularOrbitalDescriptionofTheconjugatedsysteminthebenzeneringresultsintheformationofacontinuous-cloud,aboveandbelowtheplaneofthering.Anelectrostaticpotentialmapofbenzeneshowingenhancedelectrondensityaboveandbelowtheringplane.Thesixp-orbitalsofbenzenecombinetoformsix-molecularorbitalsofincreasingenergy.αα-αα+α+Thesixbondingelectronsoccupythethreelowestenergyα+E'π=2(α+2β)+4(α+β)EnergyLevelsofEtheneMolecularα-α-α+α+Eπ=2(α+β)EDE=E'π-3Eπ=Questionabout什么是芳香芳香性的標志是什分子中沒有苯結構是否也可以具有芳性芳香化合物的特高的C/H具有與脂肪族化合物明顯不同的IR，UVNMR化學意 如果某種化合物在化學性質上類似于苯 TheMolecularOrbitalsof66pAOscombinetogive63MOsarebonding;3areAllbondingMOsareNoelectronsAllbondingMOsareNoelectronsinantibondingAMolecularOrbitalDescriptionofAromaticityandAntiaromaticityAromaticvs.“anti-A.Deniz,et.al.,Science,5NovemberSP2雜化碳原子組成的平面如果在π體系中有偶數(shù)個P軌道，最高能量軌道只如果在π體系中有奇數(shù)個P軌道，最高能量的軌道HückelHückelrestrictedhis ysistoplanar, yconjugated,monocyclicpolyeneshefoundthatthemolecularorbitalsofthesecompoundshadadistinctiveoneorbitalwaslowestinenergy,anotherwashighestinenergy,andtheotherswerearrangedinpairsbetweenthehighestandthelowestHückelAmongplanar,monocyclic,comple conjugatedpolyenes,onlythosewith4n+2pElectronspossessspecialstability(arearomatic) 0216234CriteriaforThesystemmustbeThesystemmustbeplanarEachringatommustextendedpi Thecomple yextendedpisystemmustcontaintheHückel#ofpielectrons(4n+2)AromaticityStructurecriteriaReactivitycriteriaEnergycriteriaMagneticcriteria

chemicalshift(NICS)Schleyer(1996)

aromaticsmell(before1825)

DiscoveryofbenzeneFaraday(1825)Highcarbonratio(before1865)BenzeneMagneticsucseptibilityFlygareMagneticsucseptibilityDaubenRingcurrenteffectonNMRchemicalshiftPople

KekuléSubstitution>additionErlenmeyer(1866)ExalteddiamagneticsusceptibilityPascal(1910)ElectronsextetArmit-Robinson(1925)electrontotomagnetic

4n+2electronHückelLondon

RingcurrenttheoryPauling(1936)SchleyerandJiao,PureAppl.Chem.1996,68,209-Magnetic

Magneticcriteriaofchemicalshift(NICS)Schleyer(1996)

InducedmagneticfieldHeine(2004)MagneticsucseptibilityFlygareMagneticsucseptibilityDaubenRingcurrenteffectonNMRchemicalshiftPople

ExalteddiamagneticsusceptibilityPascal(1910)electrontocontributiontomagneticLondon

RingcurrenttheoryPauling(1936)SchleyerandJiao,PureAppl.Chem.1996,68,209-A.Hückel4n+2AAplanar,cyclic,conjugatedsystemwillbearomaticwhenithas[4n+2]electrons(n=0,1,2,3...). 6 10 14not

4 8Hückel4n+2138 “tub”1510

nonaromaticsavoiddiradicalconfigurationbyadoptingstructuresotherthanplanar,regularpolygons closed-AromaticB.Aromatic """

pKa~pKa~

""""NpNC.HeterocyclicaromaticNpNNNH

contributesonesp2orbitalperpendiculartopsystem;nointeractionsp2

OcontributestwopelectronsN sp2Nhybridizestosp2toallowsixelectronsinpQuestionDeterminewhethereachofthefollowingstructuresisaromaticorNONHNSONSQuestionDeterminewhethereachofthefollowingstructuresisaromaticor NONHnot(non-planar)not(notconjugated)arom-10 not-8NSONSarom-10 arom-10 not-8 not-8Questionabout那些分子有芳香性是否只有平面結構的分子才具有芳香性芳香性概念的發(fā)展情況TypesofAromaticBefore1958,2Dplanarpolycyclicaromatichydrocarbons(PAHs)reducibletomoleculescontainingsixp-electrons,e.g.,After1)Monocyclichydrocarbonscontainingupto30p-electrons,e.g.,[n]annulenes2.3Dboronandcarboraneclustermoleculesbasedupontriangularfacepolyhedra,e.g.,

3)LargecarbonclustersillustratedbythefamousbuckminsterfullereneC60anditshomologues.富勒烯Curl、KrotoandSmalley NobelPrizeMaindevelopmentsaroundthearomaticityNatureofthearomaticityLikeotherusefulandpopularchemicalconcepts(chemicalbonds,charges,electronegativities,hyperconjugationsetc.),aromaticityisnon-reductive,andlacksofclearphysicalbases.Aromaticityisnotaphysicalobservable,havingnopreciseexperimentaldefinition.Aromaticityisjustliketodefinebeautyinourdailylife!Easilytorecognize(butnotManyHardtoDifficult Variousopinions,nogeneralInterpretedBeauty(Aromaticity)isintheeyeoftheAromaticityisatime-dependentconcept,ofwhichnewaspectsarependingfordiscovery.AromaticityisapropertyassociatedwithextrastabilityandmanyotherunusualAromaticity:Derivativesof SomePolycyclicAromatic SomeOtherNotableAromaticCompounds...Monosubstitutedbenzenederivativesarenamesasotherhydrocarbons,usingbenzeneastheparentname.

ortolueneMonosubstitutedbenzenederivativesarenamesasotherhydrocarbons,usingbenzeneastheparentname.Disubstitutedbenzenesarenamedusingortho-,para-andmeta-todescribethesubstitutionpattern(1,21,4and1,3respectively)orsimplybynumberingthesubstituents.or1,3-diethylbenzeneor1,2-

orpara-xyleneMonosubstitutedbenzenederivativesarenamesasotherhydrocarbons,usingbenzeneastheparentname.Disubstitutedbenzenesarenamedusingortho-,para-andmeta-todescribethesubstitutionpattern(1,21,4and1,3respectively)orsimplybynumberingthesubstituents.Substituentsarenumberedtogivethelowestpossiblenumbersequenceatthefirstpointofdifference,assigningprioritiesalphabeticallyifthereisa“tie”.Monosubstitutedbenzenederivativesarenamesasotherhydrocarbons,usingbenzeneastheparentname.Disubstitutedbenzenesarenamedusingortho-,para-andmeta-todescribethesubstitutionpattern(1,21,4and1,3respectively)orsimplybynumberingthesubstituents.Substituentsarenumberedtogivethelowestpossiblenumbersequenceatthefirstpointofdifference,assigningprioritiesalphabeticallyifthereisa“tie”.Whenabenzeneringisasubstituentonanotherchain,itisreferredtoasa“phenyl”group.1- 1-chloro-3-methylbenzene3-

1-methyl-4-nitrobenzenevideIUPACnamesforeachofthemoleculesshown ...na5clf當含有不同官能團時，一般按下列次序確， 面優(yōu)先考慮—COOH、—SO3H、—COOR、—COCl—CONH2、—CN、—CHO、—COR、—OH—NH2、—C=CR、—C=CHR、—OR、—R—Cl、—Kekulé’sDreamandCreativeTheimaginaryofawhirlingcircleofsnakesevokesavividpicturethatengagesone’sattentionwhenfirstexposedtoKekulé’smodelofthebenzenes