Alkenes Structure and Reactivity6烯的結(jié)構(gòu)和反應(yīng)性

CH6:AlkenesStructureandReactivityReneeY.BeckerCHM2210ValenciaCommunityCollege2Alkene-HydrocarbonWithCarbon-CarbonDoubleBondAlsocalledanolefinbutalkeneisbetterIncludesmanynaturallyoccurringmaterialsFlavors,fragrances,vitamins3WhythisChapter?C-CdoublebondsarepresentinmostorganicandbiologicalmoleculesToexamineconsequencesofalkenestereoisomerismTofocusongeneralalkenereaction:electrophilicaddition4IndustrialPreparationandUseofAlkenesEthyleneandpropylenearethemostimportantorganicchemicalsproduced5NamingofAlkenesNametheparenthydrocarbonNumbercarbonsinchainsothatdoublebondcarbonshavelowestpossiblenumbersRingshave“cyclo〞prefixIf2doublebonds–diene,If3doublebonds–triene……..678ManyAlkenesAreKnownbyCommonNames9Nameme10Drawme2,3,5-trimethyl-1,3-hexadiene3,3-diethyl-1-pentene4-sec-butyl-3,5-dimethyl-2,5-heptadiene3,3-dimethylcyclopentene3,6-dimethyl-3-octene11Cis-TransIsomerisminAlkenesCarbonatomsinadoublebondaresp2-hybridizedThreeequivalentorbitalsat120oseparationinplaneFourthorbitalisatomicporbitalCombinationofelectronsintwosp2orbitalsoftwoatomsformsbondbetweenthemAdditiveinteractionofporbitalscreatesabondingorbitalSubtractiveinteractioncreatesaanti-bondingorbital

12Cis-TransIsomerisminAlkenesOccupiedorbitalpreventsrotationabout-bondRotationpreventedbybond-highbarrier,about268kJ/moleinethylene13RotationofBondIsProhibitiveThispreventsrotationaboutacarbon-carbondoublebond(unlikeacarbon-carbonsinglebond).Createspossiblealternativestructures14Thepresenceofacarbon-carbondoublebondcancreatetwopossiblestructurescisisomer-twosimilargroupsonsamesideofthedoublebondtransisomer-similargroupsonoppositesidesEachcarbonmusthavetwodifferentgroupsfortheseisomerstooccur15Cis,TransIsomersRequireThatEndGroupsMustDifferinPairs180°rotationsuperposesBottompaircannotbesuperposedwithoutbreakingC=C16SequenceRules:TheE,ZDesignationNeithercompoundisclearly“cis〞or“trans〞SubstituentsonC1aredifferentthanthoseonC2Weneedtodefine“similarity〞inaprecisewaytodistinguishthetwostereoisomersCis,transnomenclatureonlyworksfordisubstituteddoublebonds17E,ZStereochemicalNomenclaturePriorityrulesofCahn,Ingold,andPrelogComparewherehigherprioritygroupsarewithrespecttobondanddesignateasprefixE-entgegen,oppositesidesZ-zusammen,togetheronthesameside18RankingPriorities:Cahn-Ingold-PrelogRulesRULE1MustrankatomsthatareconnectedatcomparisonpointHigheratomicnumbergetshigherpriorityBr>Cl>S>P>O>N>C>H19RULE2Ifatomicnumbersarethesame,compareatnextconnectionpointatsamedistanceCompareuntilsomethinghashigheratomicnumberDonotcombine–alwayscompareExtendedComparison20RULE3SubstituentisdrawnwithconnectionsshownandnodoubleortriplebondsAddedatomsarevaluedwith0ligandsthemselvesDealingWithMultipleBonds:21AssignEorZconfigurations22StabilityofAlkenesCisalkenesarelessstablethantransalkenesCompareheatgivenoffonhydrogenation:HoLessstableisomerishigherinenergyAndgivesoffmoreheattetrasubstituted>trisubstituted>disubstituted>monosusbtitutedhyperconjugationstabilizes23ComparingStabilitiesofAlkenesEvaluateheatgivenoffwhenC=CisconvertedtoC-CMorestablealkenegivesofflessheattrans-Butenegenerates5kJlessheatthancis-butene24HyperconjugationElectronsinneighboringfilledorbitalstabilizevacantantibondingorbital–netpositiveinteractionAlkylgroupsarebetterthanH25ElectrophilicAdditionofAlkenesGeneralreactionmechanism:electrophilicadditionAttackofelectrophile(suchasHBr)onbondofalkeneProducescarbocationandbromideionCarbocationisanelectrophile,reactingwithnucleophilicbromideion26TwostepprocessFirsttransitionstateishighenergypointElectrophilicAdditionEnergyPath27ElectrophilicAdditionforpreparationsThereactionissuccessfulwithHClandwithHIaswellasHBrHIisgeneratedfromKIandphosphoricacid28OrientationofElectrophilicAddition:Markovnikov’sRuleInanunsymmetricalalkene,HXreagentscanaddintwodifferentways,butonewaymaybepreferredovertheotherIfoneorientationpredominates,thereactionisregiospecificMarkovnikovobservedinthe19thcenturythatintheadditionofHXtoalkene,theHattachestothecarbonwiththemostH’sandXattachestotheotherend(totheonewiththemostalkylsubstituents)ThisisMarkovnikov’srule2930AdditionofHClto2-methylpropeneRegiospecific–oneproductformswheretwoarepossibleIfbothendshavesimilarsubstitution,thennotregiospecificExampleofMarkovnikov’sRule31Markovnikov’sRule(restated)MorehighlysubstitutedcarbocationformsasintermediateratherthanlesshighlysubstitutedoneTertiarycationsandassociatedtransitionstatesaremorestablethanprimarycations32CarbocationStructureandStabilityCarbocationsareplanarandthetricoordinatecarbonissurroundedbyonly6electronsinsp2orbitalsThefourthorbitaloncarbonisavacantp-orbitalThestabilityofthecarbocation(measuredbyenergyneededtoformitfromR-X)isincreasedbythepresenceofalkylsubstituents33Inductivestabilizationofcationspecies34TheHammondPostulateIfcarbocationintermediateismorestablethananother,whyisthereactionthroughthemorestableonefaster?Therelativestabilityoftheintermediateisrelatedtoanequilibriumconstant(DGo)Therelativestabilityofthetransitionstate(whichdescribesthesizeoftherateconstant)istheactivationenergy(DG?)Thetransitionstateistransientandcannotbeexamined35TransitionStateStructuresAtransitionstateisthehighestenergyspeciesinareactionstepBydefinition,itsstructureisnotstableenoughtoexistforonevibrationButthestructurecontrolstherateofreactionSoweneedtobeabletoguessaboutitspropertiesinaninformedwayWeclassifythemingeneralwaysandlookfortrendsinreactivity–theconclusionsareinthe