Chapter9ExercisesHydrocarbonscharacterizedbythepresenceofacarbon-carbontriplebond.Noncyclicalkynes’molecularformula:CnH2n-2Monosubstituented(terminal)Alkynes:RC≡CH§9.1SourcesofAlkynesIndustrialpreparationofacetyleneisbydehydrogenationofethyleneNaturallyOccurringAlkynes§9.2NomenclatureIfbothdoubleandtriplebondsarepresentintheparentchain,theendingbecomes–enyne.Numberingissuchastogivethelowestpossiblenumbertothedoubleandtriplebonds,irrespectiveofwhether–eneor–ynehasthelowernumber.Wheneithergroupcanbeassignedthesamenumber,-enehasthelower-numberpriority.-enyne-dienyne-trienyne-enediyne-diene-diyne3-penten-1-yne3-戊烯-1-炔(书写时先烯后炔)1,5-nonadien-8-yne1,5-壬二烯-8-炔§9.3PhysicalPropertiesofAlkyneslineargeometryforacetyleneStructuralFeatureofEthane,EtheneandAcetyleneThecarbonatomsofacetyleneareconnectedbya++triplebond.H3CHH2NHCH3C≡CHCH3CH2OHHOHPka49342515.915.74Thetwoseparateperpendicular(垂直的)pmolecularorbitalsOneofthetwobondsinacetyleneisshownhere.Thesecondbondisatrightanglestothefirst.Thisisthesecondofthetwobondsinacetylene.Theregionofhighestnegativechargeliesaboveandbelowthemolecularplaneinethylene.Thepbondsarearegionofhighelectrondensity(red)soalkynesaretypicallynucleophiles.Alkynestypicallyundergoadditionreactionsinwhichoneorbothofthep-bondsareconvertedtonewsbonds.Terminalalkynes,R-C≡C-H,arequiteacidic(indicatedbyblue)forhydrocarbons,pKa=26Deprotonationofaterminalacetylenegivesanacetylide乙炔化物ion.TheacetylideionisagoodnucleophileandcanbealkylatedtogivehigheralkynesCyclononyneisthesmallestcycloalkynestableenoughtobestoredatroomtemperatureforareasonablelengthoftime.Cyclooctynepolymerizesonstanding.2s2sp§9.5ReactionsofAlkynesAcidityandTerminalAlkynesAcetyleneisaweakacid,butnotnearlyasweakasalkanesoralkenes.Electronsinanorbitalwithmorescharacterareclosertothenucleusandmorestronglyheld.Objective:PrepareasolutioncontainingsodiumacetylideWilltreatmentofacetylenewithNaOHbeeffective?No.Hydroxideisnotastrongenoughbasetodeprotonateacetylene.Solution:Useastrongerba