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1、CH221CLASS15CHAPTER8:ALKYNES–ANINTRODUCTIONTOORGANICSYNTHESISSynopsis.Class15givesanaccountofthepreparation,characteristicsandreactionsofalkynes,endingwithabriefintroductiontostrategyinorganicsynthesis,baseduponthechemistryofalkynesandalkenes.Centaurea
2、cyanusIntroductionAlkynesarehydrocarbonsthatpossessacarbon-carbontriplebond.Theyarenotascommonasalkenesinnature(butseebelow),butasignificantnumberofnaturalproducts,suchassomecarotenoidsandunsaturatedfattyacids(seebelow),includeCºCintheirstructures.Acet
3、ylene(CHºCH,thesimplestalkyne)wasoncethefoundationofmuchoftheorganicchemicalindustry,baseduponcoal.Thatrolehaslongbeentakenoverbyethyleneandpropylene(basedonpetroleum),butacetyleneisstillusedasstartingmaterialforacrylicpolymers.Itisproducedbythesteam-p
4、yrolysisofmethane(fromnaturalgas):ElectronicStructureofAcetyleneIthasbeenshowninclass2thatthestructureofacetyleneislinear,basedonsphybridizationofcarbon.Itsstructureissummarizedbelow.TheCºCbondinacetyleneistheshortestandstrongestknowncarbon-carbonbond.
5、NamingAlkynesAlkynestakethesuffix–yneandthepositionofthetriplebondisindicatedbygivingitthelowestpossiblenumber,exceptincertaincases(seebelow).E.g.Ifthereisalsoadoublebondpresent,andifthereisachoiceofnumbering,thenthedoublebondispreferredforthelowestnum
6、ber.E.g.Ifthereisamoreseniorgroup,suchasacarbonylgroup,thenthispreferentiallytakesthelowestnumber:PreparationofAlkynes:EliminationReactionsof1,2-DihalidesTreatmentof1,2-dihalides(alsocalledvicinalorvic-dihalides)withanexcessofstrongbase,suchasKOH,C2H5O
7、NaorNaNH2,resultsindoubleeliminationofhydrogenhalidetogivealkynes.1,2-Dihalidesthemselvesarereadilypreparedbythehalogenationofalkenes:Alternatively,alkynescanbesynthesizedbyhydrogenhalideeliminationfromvinylhalides:ReactionsofAlkynes:AdditionofHydrogen
8、HalidesandHalogensAlkynesreactwithhydrogenhalidesandhalogensinasimilarmannertoalkenes,asillustratedbytheexamplebelow.NotetheMarkovnikovorientation.Hydrohalogenationoften(butnotalways)involvesantiaddition:Notethathydrohalogenationofalken
