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Chapter9

Alkynes:AnIntroductiontoOrganicSynthesisHydrocarbonsthatcontaincarbon-carbontriplebondsAcetylene,thesimplestalkyneisproducedindustriallyfrommethaneandsteamathightemperatureOurstudyofalkynesprovidesanintroductiontoorganicsynthesis,thepreparationoforganicmoleculesfromsimplerorganicmoleculesAlkynesWewillusealkynechemistrytobeginlookingatgeneralstrategiesusedinorganicsynthesisWhythischapter?Generalhydrocarbonrulesapplywith“-yne”asasuffixindicatinganalkyneNumberingofchainwithtriplebondissetsothatthesmallestnumberpossibleisassignedtothefirstcarbonofthetriplebond9.1NamingAlkynesTreatmentofa1,2-dihaloalkanewithKOHorNaOHproducesatwo-foldeliminationofHXVicinaldihalidesareavailablefromadditionofbromineorchlorinetoanalkeneIntermediateisavinylhalide9.2PreparationofAlkynes:

EliminationReactionsofDihalidesAdditionreactionsofalkynesaresimilartothoseofalkenesIntermediatealkenereactsfurtherwithexcessreagentRegioselectivityaccordingtoMarkovnikov9.3ReactionsofAlkynes:

AdditionofHXandX2Carbon-carbontriplebondresultsfromsporbitaloneachCformingasigmabondandunhybridizedpXandpyorbitalsformingπbonds.Theremainingsporbitalsformbondstootheratomsat180otoC-Ctriplebond.ThebondisshorterandstrongerthansingleordoubleBreakingaπbondinacetylene(HCCH)requires202kJ/mole(inethyleneitis269kJ/mole)ElectronicStructureofAlkynesInitialadditiongivestransintermediateProductwithexcessreagentistetrahalideAdditionofBromineandChlorineAdditionofH-XtoalkyneshouldproduceavinyliccarbocationintermediateSecondaryvinylcarbocationsformlessreadilythanprimaryalkylcarbocationsPrimaryvinylcarbocationsprobablydonotformatallNonethelss,H-BrcanaddtoanalkynetogiveavinylbromideiftheBrdoesnotenduponaprimarycarbonAdditionofHXtoAlkynesInvolvesVinylicCarbocationsAdditionofH-OHasinalkenesMercury(II)catalyzesMarkovnikovorientedadditionHydroboration-oxidationgivestheanti-Markovnikovproduct9.4HydrationofAlkynesAlkynesdonotreactwithaqueousproticacidsMercuricion(asthesulfate)isaLewisacidcatalystthatpromotesadditionofwaterwithMarkovnikovorientationTheimmediateproductisavinylicalcohol,orenol,whichspontaneouslytransformstoaketoneortoanaldehydeintheeventthatacetyleneisemployed.Mercury(II)-CatalyzedHydrationofAlkynesAdditionofHg(II)toalkynegivesavinyliccationWateraddsandlosesaprotonAprotonfromaqueousacidreplacesHg(II)MechanismofMercury(II)-

CatalyzedHydrationofAlkynesIsomericcompoundsthatcanrapidilyinterconvertbythemovementofaprotonarecalledtautomersandthephenomenoniscalledtautomerismEnolsrearrangetotheisomericketonebytherapidtransferofaprotonfromthehydroxyltothealkenecarbonTheketoformisusuallysostablecomparedtotheenolthatonlytheketoformcanbeobservedKeto-enolTautomerismIfthealkylgroupsateitherendoftheC-Ctriplebondarenotthesame,bothproductscanformandthisisnotnormallyusefulIfthetriplebondisatthefirstcarbonofthechain(thenHiswhatisattachedtooneside)thisiscalledaterminalalkyneHydrationofaterminalalkynealwaysgivesthemethylketone,whichisusefulHydrationofUnsymmetricalAlkynesBH3(borane)addstoalkynestogiveavinylicboraneOxidationwithH2O2producesanenolthatconvertstotheketoneoraldehydeProcessconvertsalkynetoketoneoraldehydewithorientationoppositetomercuricioncatalyzedhydrationHydroboration/Oxidation

ofAlkynesHydroboration/oxidationconvertsterminalalkynestoaldehydesbecauseadditionofwaterisanti-MarkovnikovTheproductfromthemercury(II)catalyzedhydrationconvertsterminalalkynestomethylketonesComparisonofHydrationof

TerminalAlkynesAdditionofH2overametalcatalyst(suchaspalladiumoncarbon,Pd/C)convertsalkynestoalkanes(completereduction)TheadditionofthefirstequivalentofH2producesanalkene,whichismorereactivethanthealkynesothealkeneisnotobserved9.5ReductionofAlkynesAdditionofH2usingchemicallydeactivatedpalladiumoncalciumcarbonateasacatalyst(theLindlarcatalyst)producesacisalkeneThetwohydrogensaddsyn(fromthesamesideofthetriplebond)ConversionofAlkynesto

cis-AlkenesAnhydrousammonia(NH3)isaliquidbelow–33oCAlkalimetalsdissolveinliquidammoniaandfunctionasreducingagentsAlkynesarereducedtotransalkeneswithsodiumorlithiuminliquidammoniaThereactioninvolvesaradicalanionintermediateConversionofAlkynesto

trans-AlkenesMechanismofLi/NH3Reduction

ofanAlkyneStrongoxidizingreagents(O3orKMnO4)cleaveinternalalkynes,producingtwocarboxylicacidsTerminalalkynesareoxidizedtoacarboxylicacidandcarbondioxideNeitherprocessisusefulinmodernsynthesis–wereusedtoelucidatestructuresbecausetheproductsindicatethestructureofthealkyneprecursor9.6OxidativeCleavageofAlkynesTerminalalkynesareweakBr?nstedacids(alkenesandalkanesaremuchlessacidic(pKa~25.SeeTable9.1forcomparisons))ReactionofstronganhydrousbaseswithaterminalalkyneproducesanacetylideionThesp-hydbridizationatcarbonholdsnegativechargerelativelyclosetothepositivenucleus(Figure9.5intext)9.7AlkyneAcidity:FormationofAcetylideAnionsAcetylideionscanreactasnucleophilesaswellasbases(seeFigure9-6formechanism)Reactionwithaprimaryalkylhalideproducesahydrocarbonthatcontainscarbonsfrombothpartners,providingageneralroutetolargeralkynes9.8AlkylationofAcetylideAnionsReactionsonlyareefficientwith1oalkylbromidesandalkyliodidesAcetylideanionscanbehaveasbasesaswellasnucelophilesReactionswith2oand3oalkylhalidesgivesdehydrohalogenation,convertingalkylhalidetoalkeneLimitationsofAlkyationof

AcetylideIonsOrganicsynthesiscreatesmoleculesbydesignSynthesiscanproducenewmoleculesthatareneededasdrugsormaterialsSynthesescanbedesignedandtestedtoimproveefficiencyandsafetyformakingknownmoleculesHighlyadvancedsynthesisisusedtotestideasandmethods,answeringchallengesChemistswhoengageinsynthesismayseesomeworkaselegantorbeautifulwhenitusesnovelideasorcombinationsofsteps–thisisverysubjectiveandnotpartofanintroductorycourse9.9AnIntroductiontoOrganicSynthesisInordertoproposeasynthesis

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