有機(jī)化學(xué)英文chapter8課件

OrganicChemistryWilliamH.BrownChristopherS.FooteBrentL.IversonHaloalkanesChapter8StructureHaloalkane(alkylhalide):

acompoundcontainingahalogencovalentlybondedtoansp3hybridizedcarbon;giventhesymbolRXHaloalkene(vinylichalide):acompoundcontainingahalogenbondedtoansp2hybridizedcarbonHaloarene(arylhalide):acompoundcontainingahalogenbondedtoabenzenering;giventhesymbolArX(wedonotstudyvinylicorarylhalidesinthischapter)NomenclatureexamplesCommonnames:namethealkylgroupfollowedbythenameofthehalideNomenclatureseveralpolyhaloalkanesarecommonsolventsandaregenerallyreferredtobytheircommonortrivialnameshydrocarbonsinwhichallhydrogensarereplacedbyhalogensarecommonlynamedasperhaloalkanesorperhaloalkenesDipoleMomentsDipolemomentofRXdependson:thesizesofthepartialchargesthedistancebetweenthemthepolarizabilityoftheunsharedelectronsonhalogenvanderWaalsForcestheenergyminimumiswheretheattractiveforcesarethestrongestnonbondedinteratomicandintermoleculardistancesattheseminimacanbemeasuredbyx-raycrystallographyandeachatomandgroupofatomscanbeassignedavanderWaalsradiusnonbondedatomsinamoleculecannotapproacheachothercloserthanthesumoftheirvanderWaalsradiiwithoutcausingnonbondedinteractionstrainBoilingPointsForanalkaneandahaloalkaneofcomparablesizeandshape,thehaloalkanehasthehigherboilingpointthedifferenceisduealmostentirelytothegreaterpolarizabilityofthethreeunsharedpairsofelectronsonhalogencomparedwiththelowpolarizabilityofsharedelectronpairsofcovalentbondspolarizability:ameasureoftheeaseofdistortionofthedistributionofelectrondensityaboutanatominresponsetointeractionwithothermoleculesandions;fluorinehasaverylowpolarizability,iodinehasaveryhighpolarizabilityBoilingPointsamongconstitutionalisomers,branchedisomershaveamorecompactshape,decreasedareaofcontact,decreasedvanderWaalsattractiveforcesbetweenneighbors,andlowerboilingpointsDensityThedensitiesofliquidhaloalkanesaregreaterthanthoseofhydrocarbonsofcomparablemolecularweightahalogenhasagreatermasspervolumethanamethylormethylenegroupAllliquidbromoalkanesandiodoalkanesaremoredensethanwaterDi-andpolyhalogenatedalkanesaremoredensethanwaterBondLengths,StrengthsC-FbondsarestrongerthanC-Hbonds;C-Cl,C-Br,andC-IbondsareweakerHalogenationofAlkanesIfamixtureofmethaneandchlorineiskeptinthedarkatroomtemperature,nochangeoccursIfthemixtureisheatedorexposedtovisibleorultravioletlight,reactionbeginsatoncewiththeevolutionofheatSubstitution:

areactioninwhichanatomorgroupofatomsisreplacedbyanotheratomorgroupofatomsRegioselectivityRegioselectivityishighforbromination,butnotashighforchlorinationRegioselectivityRegioselectivityis3°>2°>1°forbromination,approximately1600:80:1forchlorination,approximately5:4:1

Example:

drawallmonobrominationproductsandpredictthepercentageofeachforthisreaction

EnergeticsBondDissociationEnthalpies(BDE)MechanismRadical:

anychemicalspeciesthatcontainsoneormoreunpairedelectronsradicalsareformedbyhomolyticbondcleavagetheorderofstabilityofalkylradicalsis3°>2°>1°>methylRadicalChainMechanismChaininitiation:astepinachainreactioncharacterizedbyformationofreactiveintermediates(radicals,anions,orcations)fromnonradicalornonchargedmoleculesRadicalChainMechanismChainpropagation:astepinachainreactioncharacterizedbythereactionofareactiveintermediateandamoleculetoformanewreactiveintermediateandanewmoleculeChainlength:

thenumberoftimesthecycleofchainpropagationstepsrepeatsinachainreactionChainPropagationStepsForanysetofchainpropagationsteps,theirequationsaddtotheobservedstoichiometryenthalpiesaddtotheobservedH0

Regioselectivity?Theregioselectivityofchlorinationandbrominationcanbeaccountedforintermsoftherelativestabilitiesofalkylradicals(3°>2°>1°>methyl)Buthowdoweaccountforthegreaterregioselectivityofbromination(1600:80:1)comparedwithchlorination(5:4:1)Hammond’sPostulateinhalogenationofanalkane,hydrogenabstraction(therate-determiningstep)isexothermicforchlorinationbutendothermicforbrominationHammond’sPostulateBecausehydrogenabstractionforchlorinationisexothermic:thetransitionstateresemblesthealkaneandachlorineatomthereislittleradicalcharacteroncarboninthetransitionstateregioselectivityisonlyslightlyinfluencedbyradicalstabilityHammond’sPostulateBecausehydrogenabstractionforbrominationisendothermic:thetransitionstateresemblesanalkylradicalandHBrthereissignificantradicalcharacteroncarboninthetransitionstateregioselectivityisgreatlyinfluencedbyradicalstabilityradicalstabilityis3°>2°>1°>methyl,andregioselectivityisinthesameorderHammond’sPostulateStereochemistryWhenradicalhalogenationproducesachiralcenterortakesplaceatahydrogenonachiralcenter,theproductisamixtureofRandSenantiomersasaracemicmixtureforsimplealkylradicals,thecarbonbearingtheradicalissp2hybridizedandtheunpairedelectronoccupiestheunhybridized2porbital(seenextscreen)StereochemistryAllylicHalogenationAllyliccarbon:

aCadjacenttoaC-CdoublebondAllylichydrogen:anHonanallyliccarbonanallylicC-HbondisweakerthanavinylicC-HbondAllylicBrominationAllylicbrominationusingNBSAllylicBrominationAradicalchainmechanismChaininitiationChainpropagationAllylicBrominationchainterminationBr2isprovidedbythereactionofNBSwithHBrTheAllylRadicalAhybridoftwoequivalentcontributingstructures(Equivalentcontributingstructures)??CH2CHCH2CHCH2CH2TheAllylRadicalMolecularorbitalmodeloftheallylradicalTheAllylRadicalUnpairedelectronspindensitymapoftheallylradicaltheunpairedelectrondensity(greenlobes)appearsonlyoncarbons1and3AllylicHalogenationExample8.5Accountforthefactthatallylicbrominationof1-octenebyNBSgivestheseisomericproductsRadicalAutoxidationAutoxidation:oxidationrequiringoxygen,O2,andnootheroxidizingagentoccursbyaradicalchainmechanismsimilartothatforallylichalogenationinthissection,weconcentrateonautoxidationofthehydrocarbonchainsofpolyunsaturatedtriglyceridesthecharacteristicfeatureofthefattyacidchainsinpolyunsaturatedtriglyceridesisthepresenceof1,4-dienesradicalabstractionofadoublyallylichydrogenofa1,4-dieneformsaparticularlystableradicalRadicalAutoxidationautoxidationbeginswhenaradicalinitiator,X?,abstractsadoublyallylichydrogenthisradicalisstabilizedbyresonancewithbothdoublebondsRadicalAutoxidationthedoublyallylicradicalreactswithoxygen,itselfadiradical,toformaperoxyradicaltheperoxyradicalthenreactswithanother1,4-dienetogiveanewradical,R?,andahydroperoxidevitaminA,anaturallyoccurringantioxidant,reactspreferentiallywiththeinitialperoxyradicaltogivearesonance-stabilizedphenoxyradical,whichisveryunreactive,andscavengesanotherperoxideradicalRadicalAutoxidationvitaminEasanantioxidantRadicalAdditionofHBrtoAlkenesAdditionofHBrtoalkenesgiveseitherMarkovnikovadditionornon-Markovnikovaddi