有机化学英文chapter6课件

OrganicChemistryWilliamH.BrownChristopherS.FooteBrentL.IversonReactions

ofAlkenesChapter6CharacteristicReactionsReactionMechanismsAreactionmechanismdescribeshowareactionoccurswhichbondsarebrokenandwhichnewonesareformedtheorderandrelativeratesofthevariousbond-breakingandbond-formingstepsifinsolution,theroleofthesolventifthereisacatalyst,theroleofacatalystthepositionofallatomsandenergyoftheentiresystemduringthereactionGibbsFreeEnergyGibbsfreeenergychange,DG0:athermodynamicfunctionrelatingenthalpy,entropy,andtemperatureexergonicreaction:areactioninwhichtheGibbsfreeenergyoftheproductsislowerthanthatofthereactants;thepositionofequilibriumforanexergonicreactionfavorsproductsendergonicreaction:areactioninwhichtheGibbsfreeenergyoftheproductsishigherthanthatofthereactants;thepositionofequilibriumforanendergonicreactionfavorsstartingmaterialsGibbsFreeEnergyachangeinGibbsfreeenergyisdirectlyrelatedtochemicalequilibriumsummaryoftherelationshipsbetweenDG0,DH0,DS0,andthepositionofchemicalequilibriumEnergyDiagramsEnergydiagram:agraphshowingthechangesinenergythatoccurduringachemicalreactionReactioncoordinate:

ameasureinthechangeinpositionsofatomsduringareactionActivationEnergyTransitionstate:

anunstablespeciesofmaximumenergyformedduringthecourseofareactionamaximumonanenergydiagramActivationEnergy,G‡:

thedifferenceinGibbsfreeenergybetweenreactantsandatransitionstateifG‡is

large,fewcollisionsoccurwithsufficientenergytoreachthetransitionstate;reactionisslowifG‡issmall,manycollisionsoccurwithsufficientenergytoreachthetransitionstate;reactionisfastEnergyDiagramaone-stepreactionwithnointermediateDevelopingaReactionMechanismHowitisdonedesignexperimentstorevealdetailsofaparticularchemicalreactionproposeasetorsetsofstepsthatmightaccountfortheoveralltransformationamechanismbecomesestablishedwhenitisshowntobeconsistentwitheverytestthatcanbedevisedthisdoesmeanthatthemechanismiscorrect,onlythatitisthebestexplanationweareabletodeviseWhyMechanisms?theyaretheframeworkwithinwhichtoorganizedescriptivechemistrytheyprovideanintellectualsatisfactionderivedfromconstructingmodelsthataccuratelyreflectthebehaviorofchemicalsystemstheyaretoolswithwhichtosearchfornewinformationandnewunderstandingElectrophilicAdditionshydrohalogenationusingHCl,HBr,HIhydrationusingH2OinthepresenceofH2SO4halogenationusingCl2,Br2halohydrinationusingHOCl,HOBroxymercurationusingHg(OAc)2,H2OfollowedbyreductionHBr+2-ButeneAtwo-stepmechanismStep1:protontransferfromHBrtothealkenegivesacarbocationintermediateStep2:reactionofthesec-butylcation(anelectrophile)withbromideion(anucleophile)completesthereactionHBr+2-ButeneAnenergydiagramforthetwo-stepadditionofHBrto2-butenethereactionisexergonicCarbocationsCarbocation:

aspeciesinwhichacarbonatomhasonlysixelectronsinitsvalenceshellandbearspositivechargeCarbocationsareclassifiedas1°,2°,or3°dependingonthenumberofcarbonsbondedtothecarbonbearingthepositivechargeelectrophiles;thatis,theyareelectron-lovingLewisacidsCarbocationStabilitya3°carbocationismorestablethana2°carbocation,andrequiresaloweractivationenergyforitsformationa2°carbocationis,inturn,morestablethana1°carbocation,methyland1°carbocationsaresounstablethattheyareneverobservedinsolutionCarbocationStabilityrelativestabilitymethylandprimarycarbocationsaresounstablethattheyareneverobservedinsolutionCarbocationStabilitywecanaccountfortherelativestabilityofcarbocationsifweassumethatalkylgroupsbondedtothepositivelychargedcarbonareelectronreleasingandtherebydelocalizethepositivechargeofthecationweaccountforthiselectron-releasingabilityofalkylgroupsby(1)theinductiveeffect,and(2)hyperconjugationHyperconjugationinvolvespartialoverlapofthe-bondingorbitalofanadjacentC-HorC-Cbondwiththevacant2porbitalofthecationiccarbontheresultisdelocalizationofthepositivechargeAdditionofH2Oadditionofwateriscalledhydrationacid-catalyzedhydrationofanalkeneisregioselective;hydrogenaddspreferentiallytothelesssubstitutedcarbonofthedoublebondHOHaddsinaccordancewithMarkovnikov’sruleCarbocationRearrangementsinadditionofHCltoanalkeneinacid-catalyzedhydrationofanalkeneCarbocationRearrangementsthedrivingforceisrearrangementofalessstablecarbocationtoamorestableonethelessstable2°carbocationrearrangestoamorestable3°oneby1,2-shiftofahydrideion++A3°carbocationCH3HCH3HCH3C-CHCH3CH3C-CHCH3fastCarbocationRearrangementsreactionofthemorestablecarbocation(anelectrophile)withchlorideion(anucleophile)completesthereaction-Cl::::2-Chloro-2-methylbutane++CH3CH3CH3C-CH2CH3CH3C-CH2CH3fastCl:::AdditionofCl2andBr2carriedoutwitheitherthepurereagentsorinaninertsolventsuchasCH2Cl2additionofbromineorchlorinetoacycloalkenegivesatrans-dihalocycloalkaneadditionoccurswithantistereoselectivity;halogenatomsaddfromtheoppositefaceofthedoublebondwewilldiscussthisselectivityindetailinSection6.7AdditionofCl2andBr2Step1:formationofabridgedbromoniumionintermediateAdditionofCl2andBr2Step2:attackofhalideion(anucleophile)fromtheoppositesideofthebromoniumion(anelectrophile)opensthethree-memberedringtogivetheproductAdditionofCl2andBr2foracyclohexene,anticoplanaradditioncorrespondstotransdiaxialadditiontheinitialtransdiaxialconformationisinequilibriumwiththemorestabletransdiequatorialconformationbecausethebromoniumioncanformoneitherfaceofthealkenewithequalprobability,bothtransenantiomersareformedasaracemicmixtureAdditionofHOClandHOBrTreatmentofanalkenewithBr2orCl2inwaterformsahalohydrinHalohydrin:

acompoundcontaining-OHand-XonadjacentcarbonsAdditionofHOClandHOBrreactionisbothregiospecific(OHaddstothemoresubstitutedcarbon)andantistereoselectivebothselectivitiesareillustratedbytheadditionofHOBrto1-methylcyclopentenetoaccountfortheregioselectivityandtheantistereoselectivity,chemistsproposethethree-stepmechanisminthenextscreenAdditionofHOClandHOBr Step1:formationofabridgedhaloniumionintermediate Step2:attackofH2Oonthemoresubstitutedcarbonopensthethree-memberedringAdditionofHOClandHOBrStep3:protontransfertoH2OcompletesthereactionAstheelpotmaponthenextscreenshowstheC-Xbondtothemoresubstitutedcarbonislongerthantheonetothelesssubstitutedcarbonbecauseofthisdifferenceinbondlengths,thetransitionstateforringopeningcanbereachedmoreeasilybyattackofthenucleophileatthemoresubstitutedcarbonAdditionofHOClandHOBrbridgedbromoniumionfrompropeneOxymercuration/ReductionOxymercurationfollowedbyreductionresultsinhydrationofacarbon-carbondoublebondoxymercurationreductionOxymercuration/Reductionanimportantfeatureofoxymercuration/reductionisthatitoccurswithoutrearrangementoxymercurationoccurswithantistereoselectivityOxymercuration/ReductionStep1:dissociationofmercury(II)acetateStep2:formationofabridgedmercuriniumionintermediate;atwo-atomthree-centerbondOxymercuration/ReductionStep3:regioselectiveattackofH2O(anucleophile)onthebridgedintermediateopensthethree-memberedringStep4:reductionoftheC-HgOAcbondOxymercuration/ReductionAntistereoselectiveweaccountforthestereoselectivitybyformationofthebridgedbromoniumionandantiattackofthenucleophilewhichopensthethree-memberedringRegioselectiveofthetwocarbonsofthemercuriniumionintermediate,themoresubstitutedcarbonhasthegreaterdegreeofpartialpositivecharacteralternatively,computermodelingindicatesthattheC-Hgbondtothemoresubstitutedcarbonofthebridgedintermediateislongerthantheonetothelesssubstitutedcarbontherefore,thering-openingtransitionstateisreachedmoreeasilybyattackatthemoresubstitutedcarbonHydroboration/OxidationHydroboration:

theadditionofborane,BH3,toanalkenetoformatrialkylboraneBoranedimerizestodiborane,B2H6BoraneHBHH3CH2=CH2CH3CH2BCH2CH3CH2CH3Triethylborane(atrialkylborane)+BoraneDiborane2BH3B2H6Hydroboration/OxidationboraneformsastablecomplexwithetherssuchasTHFthereagentisusedmostoftenasacommerciallyavailablesolutionofBH3inTHFHydroboration/OxidationHydroborationisbothregioselective(borontothelesshinderedcarbon)andsynstereoselectiveCH3HBH3BR2HH3CH+1-Methylcyclopentene(SynadditionofBH3)(R=2-methylcyclopentyl)Hydroboration/OxidationconcertedregioselectiveandsynstereoselectiveadditionofBandHtothecarbon-carbondoublebondtrialkylboranesarerarelyisolatedoxidationwithalkalinehydrogenperoxidegivesanalcoholandsodiumborateHydroboration/OxidationHydrogenperoxideoxidationofatrialkylboranestep1:hydroperoxideion(anucleophile)donatesapairofelectronstoboron(anelectrophile)step2:rearrangementofanRgroupwithitspairofbondingelectronstoanadjacentoxygenatomHydroboration/Oxidationstep3:reactionofthetrialkylboranewithaqueousNaOHgivesthealcoholandsodiumborateOxidation/ReductionOxidation:

thelossofelectronsalternatively,thelossofH,thegainofO,orbothReduction:

thegainofelectronsalternatively,thegainofH,thelossofO,orbothRecognizeusingabalancedhalf-reaction1.writeahalf-reactionshowingonereactantanditsproduct(s)2.completeamaterialbalance;useH2OandH+inacidsolution,useH2OandOH-pleteachargebalanceusingelectrons,e-Oxidation/Reductionthreebalancedhalf-reactionsOxidationwithOsO4OsO4oxidizesanalkenetoaglycol,acompoundwithOHgroupsonadjacentcarbonsoxidationissynstereoselectiveOxidationwithOsO4OsO4isbothexpensiveandhighlytoxicitisusedincatalyticamountswithanotheroxidizingagenttoreoxidizeitsreducedformsand,thus,recycleOsO4OxidationwithO3TreatmentofanalkenewithozonefollowedbyaweakreducingagentcleavestheC=CandformstwocarbonylgroupsinitsplacePropanal(analdehyde)Propanone(aketone)2-Methyl-2-penteneCH3OOCH3C=CHCH2CH31.

O32.

(CH3)2SCH3CCH3

+

HCCH2CH3OxidationwithO3theinitialproductisamolozonidewhichrearrangestoanisomericozonideAcetaldehyde2-ButeneOCH3CH=CHCH3O3(CH3)2SCH3CHCH3CH-CHCH3OOOOOCOCHCH3HH3CAmolozonideAnozonideReductionofAlkenesMostalkenesreactwithH2inthepresenceofatransitionmetalcatalysttogivealkanescommonlyusedcatalystsarePt,Pd,Ru,andNitheprocessiscalledcatalyticreductionor,alternatively,catalytichydrogenationadditionoccurswithsynstereoselectivity+H2PdCyclohexeneCyclohexane25°C,3atmReductionofAlkenesMechanismofcatalytichydrogenationReductionofAlkeneseventhoughadditionsynstereoselectivity,someproductmayappeartoresultfromtransadditionreversalofthereactionaftertheadditionofthefirsthydrogengivesanisomericalkene,etc.H0ofHydrogenationReductionofanalkenetoanalkaneisexothermicthereisnetconversionofonepibondtoonesigmabondH0dependsonthedegreeofsubstitutionthegreaterthesubstitution,thelowerthevalueofH°

H0foratransalkeneislowerthanthatofanisomericcisalkeneatransalkeneismorestablethanacisalkeneH0ofHydrogenationReactionStereochemistryInseveralofthereactionspresentedinthischapter,chiralcentersarecreatedWhereoneormorechiralcentersarecreated,istheproductoneenantiomerand,ifso,whichone?apairofenantiomersasaracemicmixture?amesocompound?amixtureofstereoisomers?Aswewillsee,thestereochemistryoftheproductforsomereactionsdependsonthestereochemistryofthestartingmaterial;thatis,somereactionsarestereospecificReactionStereochemistryWesawinSection6.3Dthatbromineaddsto2-butenetogive2,3-dibromobutanetwostereoisomersarepossiblefor2-butene;apairofcis,transisomersthreestereoisomersarepossiblefortheproduct;apairofenantiomersandamesocompoundifwestartwiththecisisomer,whatisthestereochemistryoftheproduct?ifwestartwiththetransisomer,whatisthestereochemistryoftheproduct?Brominationofcis-2-Butenereactionofcis-2-butenewithbromineformsbridgedbromoniumionswhicharemesoandidenticalBrominationofcis-2-Buteneattackofbromideionatcarbons2and3occurswithequalprobabilitytogiveenantiomericproductsasaracemicmixtureBrominationoftrans-2-ButenereactionwithbromineformsbridgedbromoniumionintermediateswhichareenantiomersBrominationoftrans-2-Buteneattackofbromideionineithercarbonofeitherenantiomergivesmeso-2,3-dibromobutaneBrominationof2-ButeneGiventheseresults,wesaythatadditionofBr2orCl2toanalkeneisstereospecificbrominationofcis-2-butenegivestheenantiomersof2,3-dibromobutaneasaracemicmixturebrominationoftrans-2-butenegivesmeso-2,3-dibromobutaneStereospecificreaction:areactioninwhichthestereochemistryoftheproductdependsonthestereochemistryofthestartingmaterialOxidationof2-ButeneOsO4oxidationofcis-2-butenegivesmeso-2,3-butanediolOxidationof2-ButeneOsO4oxidationofanalkeneisstereospecificoxidationoftrans-2-butenegivestheenantiomersof2,3-butanediolasaracemicmixture(opticallyinactive)andoxidationofcis-2-butenegivesmeso2,3-butanediol(alsoopticallyinactive)ReactionStereochemistryWehaveseentwoexamplesinwhichreactionofachiralstarti