CHAPTER4,Alkynes1Carbon-carbontriplebondresultfromsporbitaloneachCformingasigmabondandunhybridizedpXandpyorbitalsformingaπbondTheremainingsporbitalsformbondstootheratomsat180otoC-Ctriplebond.ThebondisshorterandstrongerthansingleordoubleBreakingaπbondinacetylene(HCCH)requires318kJ/mole(inethyleneitis268kJ/mole)4.1StructureofAlkynes

234.2NamingAlkynesGeneralhydrocarbonrulesapplywuith“-yne”asasuffixindicatinganalkyneNumberingofchainwithtriplebondissetsothatthesmallestnumberpossibleincludethetriplebond4Diyines,Enynes,andTriynesAcompoundwithtwotriplebondsisadiyineAnenynehasadoublebondandtriplebondAtriynehasthreetriplebondsNumberfromchainthatendsnearestthefirstmultiblebond.Ifthereisachoice,doublebondsreceivelowernumbers.Alkynesassubstituentsarecalled“alkynyl”54.3ReactionsofAlkynes:Additionof

H2,HXandX2

AdditionreactionsofalkynesaresimilartothoseofalkenesIntermediatealkenereactsfurtherwithexcessreagentRegiospecificityaccordingtoMarkovnikov6AdditionofH2to

Alkynes:ReductionAdditionofH2overametalcatalyst(suchaspalladiumoncarbon,Pd/C)convertsalkynestoalkanes(completereduction)TheadditionofthefirstequivalentofH2producesanalkene,whichis

morereactivethanthealkyneso

thealkeneisnot

observed7ConversionofAlkynestocis-Alkenes

從炔烴制備順/反烯烴AdditionofH2usingchemicallydeactivatedpalladiumoncalciumcarbonateasacatalyst(theLindlarcatalystPd/CaCO3)orP-2Niproducesacisalkene

Thetwohydrogensaddsyn(fromthesamesideofthetriplebond)89ConversionofAlkynestotrans-AlkenesAnhydrousammonia(NH3)isaliquidbelow-33oC.Alkalimetals

dissolveinliquidammoniaandfunctionasreducingagentsAlkynesarereducedtotransalkeneswithSodiumorlithiuminliquidammonia10AdditionofHXtoAlkynes

InvolvesVinylicCarbocationsMarkovnikov’srule1112AdditionofBromine

andChlorineInitialadditiongivestransintermediateProductwithexcessreagentistetrahalide134.4AdditionofH2OtoAlkynesAdditionofH-OHasinalkenesMercury(II)catalyzesMarkovnikovorientedadditionHydroboration-oxidationgivesthenon-Markovnikovproduct14Mercury(II)-Catalyzed

HydrationofAlkynesAlkynesdonotreactwithaqueousproticacidsMercuricion(asthesulfate)isaLewisacidcatalystthatpromotesadditionofwaterinMarkovnikovorientation.Theimmediateproductisavinylicalcohol,orenol,whichspontaneouslytransformstoaketone15Keto-enolTautomerism16Hydrationof

UnsymmetricalAlkynesIfthetriplebondisatthefirstcarbonofthechain(thenHiswhatis

attachedtooneside)thisiscalledaterminalalkyne.Hydrationofaterminalalwaysgivesthemethylketone,whichisuseful.IfthealkylgroupsateitherendoftheC-Ctriplebondarenotthesame,

bothproductscanformandthisisnot

normallyuseful1718Hydroboration/Oxidation

ofAlkynes(炔烴的硼氫化反應)BH3(borane)addstoalkynestogiveavinylicboraneOxidationwithH2O2producesanenolthatconvertstotheketoneoraldehydeProcessconvertsalkynetoketoneoraldehydewithorientationoppositetomercuricioncatalyzedhydration192021OxidativeCleavageofAlkynes

(炔烴的氧化裂解)Strongoxidizingreagents(O3orKMnO4)cleaveinternalalkynes,producingtwocarboxylicacidsTerminalalkynesareoxidizedtoacarboxylicacidandcarbondioxideNeitherprocessisusefulinmodernsynthesis–wereusedtoelucidatestructuresbecausetheproductsindicatethestructureofthealkyneprecursor22234.5AlkyneAcidity:Formation

ofAcetylideAnions2425TerminalalkynesareweakBr?nstedacids(alkenesandalkanesaremuchlessacidicReactionofstronganhydrousbaseswithaterminalacetyleneproducesanacetylideion

Thesp-hydbridizationatcarbonholdsnegativechargerelativelyclosetothepositivenucleus26AlkylationofAcetylideAnionsAcetylideionscanreactasnucleophilesaswellasbasesReactionwithaprimaryalkylhalideproducesahydrocarbonthatcontainscarbonsfrombothpartners,providingageneralroutetolargeralkynes2728LimitationsofAlkyation

ofAcetylideIons

Reactionsonlyareefficientwith1oalkylbromidesandalkyliodidesAcetylideanionscanbehaveasbasesaswellasnucelophilesReactionswith2oand3oalkylhalidesgivesdehydrohalogenation,convertingalkylhalidetoalkene29304.6PreparationofAlkynes(炔烴的制備)

Treatmentofa1,2dihaloalkanewithKOHor