有机化学-烯烃制备-消去反应.pptx

X XY Ydehydrogenation of alkanes:dehydrogenation of alkanes:X=Y=HX=Y=Hdehydration of alcohols:dehydration of alcohols:X=H;Y=OHX=H;Y=OHdehydrohalogenation of alkyl halides:dehydrohalogenation of alkyl halides:X=H;Y=Br,etc.X=H;Y=Br,etc.a ab bC CC CC CC C+X XY Yb b-Elimination Reactions Overview-Elimination Reactions Overviewlimited to industrial syntheses of ethylene,limited to industrial syntheses of ethylene,propene,1,3-butadiene,and styrene propene,1,3-butadiene,and styrene important economically,but rarely used in important economically,but rarely used in laboratory-scale syntheseslaboratoryscale syntheses750C750CCHCH3 3CHCH3 3750C750CCHCH3 3CHCH2 2CHCH3 3H H2 2C CCHCH2 2+H H2 2H H2 2C CCHCHCHCH3 3+H H2 2DehydrogenationDehydrogenation5.95.9Dehydration of AlcoholsDehydration of Alcohols(79-87%)(79-87%)(82%)(82%)H H2 2SOSO4 4160C160CCHCH3 3CHCH2 2OHOHH H2 2C CCHCH2 2+H H2 2OOH H2 2SOSO4 4140C140C+H H2 2OOC COHOHCHCH3 3CHCH3 3H H3 3C CH H2 2SOSO4 4heatheatCHCH2 2H H3 3C CC CH H3 3C C+H H2 2OODehydration of AlcoholsDehydration of AlcoholsOHOHtertiary:tertiary:most reactivemost reactiveprimary:primary:least reactiveleast reactiveR RRRRROHOHC CR RRRH HOHOHC CR RH HH HOHOHC CRelative Relative ReactivityReactivity5.105.10Regioselectivity in Alcohol Dehydration:Regioselectivity in Alcohol Dehydration:The Zaitsev RuleThe Zaitsev Rule10%10%90%90%HOHOH H2 2SOSO4 480C80C+RegioselectivityRegioselectivityA reaction that can proceed in more than one A reaction that can proceed in more than one direction,but in which one direction direction,but in which one direction predominates,is said to be regioselective.predominates,is said to be regioselective.RegioselectivityRegioselectivityA reaction that can proceed in more than one A reaction that can proceed in more than one direction,but in which one direction direction,but in which one direction predominates,is said to be regioselective.predominates,is said to be regioselective.16%16%H H3 3POPO4 4heatheat+84%84%CHCH3 3OHOHCHCH3 3CHCH3 3When elimination can occur in more than one When elimination can occur in more than one directionthe principal alkene is the one formed direction,the principal alkene is the one formed by loss of H from the by loss of H from the b b carbon having the carbon having thefewestfewest hydrogens.hydrogens.R ROHOHCHCH3 3C CC CH HR RCHCH2 2R Rthree protons on this three protons on this b b carbon carbonThe Zaitsev RuleThe Zaitsev RuleWhen elimination can occur in more than one When elimination can occur in more than one direction,the principal alkene is the one formed direction,the principal alkene is the one formed by loss of H from the by loss of H from the b b carbon having the carbon having thefewestfewest hydrogens.hydrogens.R ROHOHCHCH3 3C CC CH HR RCHCH2 2R Rtwo protons on this two protons on this b b carbon carbonThe Zaitsev RuleThe Zaitsev RuleWhen elimination can occur in more than one When elimination can occur in more than one direction,the principal alkene is the one formed direction,the principal alkene is the one formed by loss of H from the by loss of H from the b b carbon having the carbon having thefewestfewest hydrogens.hydrogens.R ROHOHCHCH3 3C CC CH HR RCHCH2 2R Ronly one proton on this only one proton on this b b carbon carbonThe Zaitsev RuleThe Zaitsev RuleWhen elimination can occur in more than one When elimination can occur in more than one direction,the principal alkene is the one formed direction,the principal alkene is the one formed by loss of H from the by loss of H from the b b carbon having the carbon having thefewestfewest hydrogens.hydrogens.R RR RCHCH2 2R RCHCH3 3C CC CR ROHOHCHCH3 3C CC CH HR RCHCH2 2R Ronly one proton on this only one proton on this b b carbon carbonThe Zaitsev RuleThe Zaitsev Rule5.115.11Stereoselectivity in Alcohol DehydrationStereoselectivity in Alcohol DehydrationA stereoselective reaction is one in which A stereoselective reaction is one in which a single starting material can yield two or more a single starting material can yield two or more stereoisomeric products,but gives one of them stereoisomeric products,but gives one of them in greater amounts than any other.in greater amounts than any other.(25%)(25)(75%)(75%)+OHOHH H2 2SOSO4 4heatheatStereoselectivityStereoselectivity5.125.12The Mechanism of Acid-CatalyzedThe Mechanism of Acid-CatalyzedDehydration of AlcoholsDehydration of AlcoholsThe dehydration of alcohols and the reaction The dehydration of alcohols and the reaction of alcohols with hydrogen halides share theof alcohols with hydrogen halides share thefollowing common features:following common features:1)1)Both reactions are promoted by acidsBoth reactions are promoted by acids2)2)The relative reactivity decreases in theThe relative reactivity decreases in theorder tertiary secondary primaryorder tertiary secondary primaryThese similarities suggest that These similarities suggest that carbocationscarbocations are areintermediates in the acid-catalyzed dehydration ofintermediates in the acid-catalyzed dehydration ofalcohols,just as they are in the reaction of alcoholsalcohols,just as they are in the reaction of alcoholswith hydrogen halides.with hydrogen halides.A connecting point.A connecting point.first two steps of mechanism are identical tofirst two steps of mechanism are identical tothose for the reaction of those for the reaction of terttert-butyl alcohol with-butyl alcohol withhydrogen halides hydrogen halides Dehydration of tert-Butyl AlcoholDehydration of tert-Butyl AlcoholC COHOHCHCH3 3CHCH3 3H H3 3C CH H2 2SOSO4 4heatheatCHCH2 2H H3 3C CC CH H3 3C C+H H2 2OOStep 1:Step 1:Proton transfer to Proton transfer to terttert-butyl alcohol-butyl alcohol(CH(CH3 3)3 3C COOH H.:H HOO+H HOO:+(CH(CH3 3)3 3C CH H+fast,bimolecularfast,bimolecularterttert-Butyloxonium ion-Butyloxonium ion.H HH H+OOH H:H H:MechanismMechanismStep 2:Step 2:Dissociation of Dissociation of terttert-butyloxonium ion-butyloxonium ionto carbocation to carbocation+(CH(CH3 3)3 3C COOH H:H H+slow,unimolecularslow,unimolecular(CH(CH3 3)3 3C COOH H:H H:terttert-Butyl cation-Butyl cation+MechanismMechanismStep 3:Step 3:Deprotonation of Deprotonation of terttert-butyl cation.-butyl cation.fast,bimolecularfast,bimolecular+OOH H:H H:CHCH2 2+H H3 3C CC CH H3 3C CH HCHCH2 2H H3 3C CC CH H3 3C C+OOH H:H HH H+MechanismMechanismare intermediates in the acid-catalyzed dehydration are intermediates in the acid-catalyzed dehydration of tertiary and secondary alcohols of tertiary and secondary alcohols carbocations can:carbocations can:react with nucleophilesreact with nucleophileslose a lose a b b-proton to form an alkene-proton to form an alkeneCarbocationsCarbocationsavoidsavoids carbocation because primary carbocations carbocation because primary carbocations are too unstableare too unstableoxonium ion loses water and a proton in aoxonium ion loses water and a proton in abimolecular stepbimolecular stepH H2 2SOSO4 4160C160CCHCH3 3CHCH2 2OHOHH H2 2C CCHCH2 2+H H2 2OODehydration of Primary AlcoholsDehydration of Primary AlcoholsStep 1:Step 1:Proton transfer from acid to ethanolProton transfer from acid to ethanolH H.:H HOO+H HOO:+H H+fast,bimolecularfast,bimolecularEthyloxonium ionEthyloxonium ionOOCHCH3 3CHCH2 2CHCH3 3CHCH2 2.H HH HOOH H:H H:MechanismMechanismStep 2:Step 2:Oxonium ion loses both a proton and Oxonium ion loses both a proton and a water molecule in the same step.a water molecule in the same step.slow,bimolecularslow,bimolecular+OOH H:H H:H HOO:H H+CHCH2 2CHCH2 2H HOOH H:H H:OOH HH H:H H+H H2 2C CCHCH2 2+MechanismMechanismSometimes the alkene product does not have Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol.the same carbon skeleton as the starting alcohol.5.135.13Rearrangements in Alcohol DehydrationRearrangements in Alcohol DehydrationOHOHH H3 3POPO4 4,heat,heat3%3%64%64%33%33%+ExampleExample3%3%CHCH3 3CHCHCHCH3 3CHCH3 3CHCH3 3+C Ccarbocation can lose a carbocation can lose a proton as shownproton as shownor it can undergo a or it can undergo a methyl migrationmethyl migrationCHCH3 3 group migrates group migrates with its pair of with its pair of electrons to adjacent electrons to adjacent positively charged positively charged carboncarbonRearrangement involves alkyl group migrationRearrangement involves alkyl group migration3%3%CHCH3 3CHCHCHCH3 3CHCH3 3+CHCH3 397%97%CHCH3 3CHCHCHCH3 3CHCH3 3CHCH3 3+C CC Ctertiary carbocation;tertiary carbocation;more stablemore stableRearrangement involves alkyl group migrationRearrangement involves alkyl group migration3%3%CHCH3 3CHCHCHCH3 3CHCH3 3+CHCH3 397%97%CHCH3 3CHCHCHCH3 3CHCH3 3CHCH3 3+C CC CRearrangement involves alkyl group migrationRearrangement involves alkyl group migrationCHCH3 3CHCH2 2CHCH2 2CHCH2 2OHOHH H3 3POPO4 4,heat,heat12%12%+mixture of mixture of ciscis(32%)(32%)and and transtrans-2-butene(56%)-2-butene(56%)CHCH2 2CHCH3 3CHCH2 2CHCHCHCHCHCH3 3CHCH3 3CHCHAnother rearrangementAnother rearrangementoxonium ion can loseoxonium ion can losewater and a protonwater and a proton(from C-2)to give(from C-2)to give1-butene1-butenedoesnt give a doesnt give a carbocation directlycarbocation directlybecause primarybecause primarycarbocations are toocarbocations are toounstableunstableCHCH3 3CHCH2 2CHCH2 2CHCH2 2OOH HH H+:CHCH2 2CHCH3 3CHCH2 2CHCHRearrangement involves hydride shiftRearrangement involves hydride shifthydrogen migrates hydrogen migrates with its pair of with its pair of electrons from C-2 to electrons from C-2 to C-1 as water is lostC-1 as water is lostcarbocation formed by carbocation formed by hydride shift is hydride shift is secondarysecondaryCHCH3 3CHCH2 2CHCH2 2CHCH2 2OOH HH H+:CHCH2 2CHCH3 3CHCH2 2CHCHCHCH3 3CHCH2 2CHCHCHCH3 3+Rearrangement involves hydride shiftRearrangement involves hydride shiftCHCH3 3CHCH2 2CHCH2 2CHCH2 2OOH HH H+:CHCH2 2CHCH3 3CHCH2 2CHCHCHCH3 3CHCH2 2CHCHCHCH3 3+mixture of mixture of ciscisand and transtrans-2-butene-2-buteneCHCHCHCH3 3CHCH3 3CHCHRearrangement involves hydride shiftRearrangement involves hydride shiftH HH HCHCH3 3CHCH2 2C CH HC CH H2 2OOH H+:+CHCH3 3CHCH2 2C CH HC CH H2 2+H HOOH HH H:Hydride ShiftHydride Shiftreact with nucleophilesreact with nucleophileslose a proton from the lose a proton from the b b-carbon to form an alkene-carbon to form an alkene rearrange(less stable to more stable)rearrange(less stable to more stable)Carbocations can.Carbocations can.