【精品】Reactions of Alkenes精品ppt课件.ppt

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1、ReactionsofAlkenes Addition of Strong Brnsted Acids Asillustratedbytheprecedinggeneralequation,strongBrnstedacidssuchasHCl,HBr,HI&H2SO4,rapidlyaddtotheC=Cfunctionalgroupofalkenestogiveproductsinwhichnewcovalentbondsareformedtohydrogenandtotheconjugatebaseoftheacid.WeakBrnstedacidssuchaswater(pKa=15.

2、7)andaceticacid(pKa=4.75)donotnormallyaddtoalkenes.However,theadditionofastrongacidservestocatalyzetheadditionofwater,andinthiswayalcoholsmaybepreparedfromalkenes.Forexample,ifsulfuricacidisdissolvedinwateritiscompletelyionizedtothehydroniumion,H3O(+),andthisstronglyacidic(pKa=-1.74)specieseffectshy

3、drationofetheneandotheralkenes.CH2=CH2+H3O(+)HCH2CH2OH+H(+)Theimportanceofchoosinganappropriatesolventfortheseadditionreactionsshouldnowbeclear.IftheadditionofHCl,HBrorHIisdesired,waterandalcoholsshouldnotbeused.ThesestrongacidswillionizeinsuchsolventstogiveROH2(+)andthenucleophilicoxygenofthesolven

4、twillcompetewiththehalideanionsinthefinalstep,givingalcoholandetherproducts.Byusinginertsolventssuchashexane,benzeneandmethylenechloride,thesecompetingsolventadditionsareavoided.Becausetheseadditionsproceedbywayofpolarorionicintermediates,therateofreactionisgreaterinpolarsolvents,suchasnitromethanea

5、ndacetonitrile,thaninnon-polarsolvents,suchascyclohexaneandcarbontetrachloride.Regioselectivity and the Markovnikov RuleOnlyoneproductispossiblefromtheadditionofthesestrongacidstosymmetricalalkenessuchasetheneandcyclohexene.However,ifthedoublebondcarbonatomsarenotstructurallyequivalent,asinmolecules

6、of1-butene,2-methyl-2-buteneand1-methylcyclohexene,thereagentconceivablymayaddintwodifferentways.Thisisshownfor2-methyl-2-buteneinthefollowingequation.EmpiricalrulesliketheMarkovnikovRuleareusefulaidsforrememberingandpredictingexperimentalresults.Indeed,empiricalrulesareoftenthefirststeptowardpracti

7、calmasteryofasubject,buttheyseldomconstitutetrueunderstanding.TheMarkovnikovRule,forexample,suggeststherearecommonandimportantprinciplesatworkintheseadditionreactions,butitdoesnottelluswhattheyare.Thenextstepinachievinganunderstandingofthisreactionmustbetoconstructarationalmechanisticmodelthatcanbet

8、estedbyexperiment.AllthereagentsdiscussedherearestrongBrnstedacidsso,asafirststep,itseemssensibletofindabasewithwhichtheacidcanreact.Sinceweknowthattheseacidsdonotreactwithalkanes,itmustbethepi-electronsofthealkenedoublebondthatserveasthebase.Asshowninthediagram,thepi-orbitalextendsintothespaceimmed

9、iatelyaboveandbelowtheplaneofthedoublebond,andtheelectronsoccupyingthisorbitalmaybeattractedtotheprotonofaBrnstedacid.Theresultingacid-baseequilibriumgeneratesacarbocationintermediate(theconjugateacidofthealkene)whichthencombinesrapidlywiththeanionicconjugatebaseoftheBrnstedacid.Thistwo-stepmechanis

10、misillustratedforthereactionofethenewithhydrogenchloridebythefollowingequations.Anenergydiagramforthistwo-stepadditionmechanismisshown.Fromthisdiagramweseethatthesloworrate-determiningstep(thefirststep)isalsotheproductdeterminingstep(theanionwillnecessarilybondtothecarbocationsite).Electrondonatingd

11、oublebondsubstituentsincreasethereactivityofanalkene,asevidencedbytheincreasedrateofhydrationof2-methylpropene(twoalkylgroups)comparedwith1-butene(onealkylgroup).Evidently,alkylsubstituentsacttoincreasetherateofadditionbyloweringtheactivationenergy,E1oftheratedeterminingstep,anditishereweshouldlookf

12、orarationalizationofMarkovnikovsrule.Asexpected,electronwithdrawingsubstituents,suchasfluorineorchlorine,reducethereactivityofanalkenetoadditionbyacids(vinylchlorideislessrectivethanethene).Thecarbocationintermediateformedinthefirststepoftheadditionreactionnowassumesakeyrole,inthatitdirectlyinfluenc

13、estheactivationenergyforthisstep.Independentresearchshowsthatthestabilityofcarbocationsvarieswiththenatureofsubstituents,inamannersimilartothatseenforalkylradicals.Theexceptionalstabilityofallylandbenzylcationsistheresultofchargedelocalization,andthestabilizinginfluenceofalkylsubstituents,althoughle

14、sspronounced,hasbeeninterpretedinasimilarfashion.Fromthisinformation,wearriveataplausiblerationalizationofMarkovnikovsrule.When an unsymmetrically substituted double bond is protonated,we expect the more stable carbocation intermediate to be formed faster than the less stable alternative,becausethea

15、ctivationenergyofthepathtotheformeristhelowerofthetwopossibilities.Thisisillustratedbythefollowingequationfortheadditionofhydrogenchloridetopropene.Notethattheinitialacid-baseequilibriumleadstoapi-complexwhichimmediatelyreorganizestoasigma-bondedcarbocationintermediate.Themorestable2-carbocationisfo

16、rmedpreferentially,andtheconjugatebaseoftheBrnstedacid(chlorideanionintheexample)thenrapidlybondstothiselectrophilicintermediatetoformthefinalproduct.8-2Rearrangement of CarbocationsTheformationofcarbocationsissometimesaccompaniedbyastructuralrearrangement.Suchrearrangementstakeplacebyashiftofaneigh

17、boringalkylgrouporhydrogen,andarefavoredwhentherearrangedcarbocationismorestablethantheinitialcation.TheadditionofHClto3,3-dimethyl-1-butene,forexample,leadstoanunexpectedproduct,2-chloro-2,3-dimethylbutane,insomewhatgreateryieldthan3-chloro-2,2-dimethylbutane,theexpectedMarkovnikovproduct.Thissuppr

18、isingresultmaybeexplainedbyacarbocationrearrangementoftheinitiallyformed2-carbocationtoa3-carbocationbya1,2-shiftofamethylgroup.8-3AdditionofLewisAcids(ElectrophilicReagents)Theprotonisnottheonlyelectrophilicspeciesthatinitiatesadditionreactionstothedoublebond.Lewisacidslikethehalogens,boronhydrides

19、andcertaintransitionmetalionsareabletobondtothealkenepi-electrons,andtheresultingcomplexesrearrangeorareattackedbynucleophilestogiveadditionproducts.Theelectrophiliccharacterofthehalogensiswellknown.Althoughfluorineisuncontrollablyreactive,chlorine,bromineandtoalesserdegreeiodinereactselectivelywith

20、thedoublebondofalkenes.Theadditionofchlorineandbrominetoalkenes,asshowninthefollowinggeneralequation,proceedsbyaninitialelectrophilicattackonthepi-electronsofthedoublebond.Iodineaddsreversiblytodoublebonds,buttheequilibriumdoesnotnormallyfavortheadditionproduct,soitisnotausefulpreparativemethod.Diha

21、lo-compoundsinwhichthehalogensarejuxtaposedinthemannershownarecalledvicinal,fromtheLatinvicinalis,meaningneighboring.R2C=CR2+X2R2CX-CR2XOtherhalogencontainingreagentswhichaddtodoublebondsincludehypohalousacids,HOX,andsulfenylchlorides,RSCl.Thesereagentsareunsymmetrical,sotheiradditiontounsymmetrical

22、doublebondsmayinprincipletakeplaceintwoways.Inpractice,theseadditionreactionsareregioselective,withoneofthetwopossibleconstitutionallyisomericproductsbeingfavored.Theelectrophilicmoietyofthesereagentsisthehalogen.(CH3)2C=CH2+HOBr(CH3)2COH-CH2Br(CH3)2C=CH2+C6H5SCl(CH3)2CCl-CH2SC6H5Theregioselectivity

23、oftheabovereactionsmaybeexplainedbythesamemechanismweusedtorationalizetheMarkovnikovrule.Thus,bondingofanelectrophilicspeciestothedoublebondofanalkeneshouldresultinpreferentialformationofthemorestable(morehighlysubstituted)carbocation,andthisintermediateshouldthencombinerapidlywithanucleophilicspeci

24、estoproducetheadditionproduct.Thisisillustratedbythefollowingequation.Toapplythismechanismweneedtodeterminetheelectrophilicmoietyineachofthereagents.Byusingelectronegativitydifferenceswecandissectcommonadditionreagentsintoelectrophilicandnucleophilicmoieties,asshownontheright.Inthecaseofhypochlorous

25、andhypobromousacids(HOX),theseweakBrnstedacids(pKasca.8)donotreactasprotondonors;andsinceoxygenismoreelectronegativethanchlorineorbromine,theelectrophilewillbeahalidecation.Thenucleophilicspeciesthatbondstotheintermediatecarbocationisthenhydroxideion,ormorelikelywater(theusualsolventforthesereagents

26、),andtheproductsarecalledhalohydrins.Sulfenylchloridesaddintheoppositemannerbecausetheelectrophileisasulfurcation,RS(+),whereasthenucleophilicmoietyischlorideanion(chlorineismoreelectronegativethansulfur).Ifyouunderstandthismechanismyoushouldbeabletowriteproductsforthefollowingreactions:Theadditionp

27、roductsformedinreactionsofalkeneswithmercuricacetateandboronhydrides(compoundsshownatthebottomofofthereagentlist)arenormallynotisolated,butinsteadareconvertedtoalcoholsbyasubstitutionreaction.Theseimportantsynthetictransformationsareillustratedfor2-methylpropenebythefollowingequations,inwhichtheelec

28、trophilicmoietyiscoloredredandthenucleophileblue.Thetopreactionsequenceillustratestheoxymercurationprocedureandthebottomisanexampleofhydroboration.Thelightblueverticallineseparatestheadditionreactionontheleftfromthesubstitutionontheright.Theatomsorgroupsthathavebeenaddedtotheoriginaldoublebondarecol

29、oredorangeinthefinalproduct.Inbothcasestheoverallreactionistheadditionofwatertothedoublebond,buttheregioselectivityisreversed.TheoxymercurationreactiongivestheproductpredictedbyMarkovnikovsrule;hydroborationontheotherhandgivestheanti-Markovnikovproduct.Complementaryreactionssuchastheseareimportantbe

30、causetheyallowustodirectamoleculartransformationwhicheverwayisdesired.Mercuryandboronareremovedfromtheorganicsubstrateinthesecondstepofoxymercurationandhydroborationrespectively.Thesereactionsareseldomdiscussedindetail;however,itisworthnotingthatthemercurymoietyisreducedtometallicmercurybyborohydrid

31、e(probablybywayofradicalintermediates),andboronisoxidizedtoboratebythealkalineperoxide.Additionofhydroperoxideaniontotheelectrophilicboranegeneratesatetra-coordinateboronperoxide,havingthegeneralformulaR3B-O-OH(-).Thisundergoessuccessiveintramolecularshiftsofalkylgroupsfromborontooxygen,accompaniedi

32、neacheventbyadditionalperoxideadditiontoelectrondeficientboron.Theretentionofconfigurationofthemigratingalkylgroupisattributedtotheintramolecularnatureoftherearrangement.Sincetheoxymercurationsequencegivesthesamehydrationproductasacid-catalyzedadditionofwater,wemightquestionwhythistwo-stepprocedurei

33、susedatall.Thereasonliesinthemilderreactionconditionsusedforoxymercuration.Thestrongacidusedfordirecthydrationmaynotbetoleratedbyotherfunctionalgroups,andinsomecasesmaycausemolecularrearrangement.Theadditionofborane,BH3,requiresadditionalcomment.InpureformthisreagentisadimericgasB2H6,calleddiborane,

34、butinetherorTHFsolutionitisdissociatedintoasolventcoordinatedmonomer,R2O-BH3.Althoughdiboraneitselfdoesnotreacteasilywithalkenedoublebonds,H.C.Brown(Purdue,NobelPrize1979)discoveredthatthesolvatedmonomeraddsrapidlyundermildconditions.Boronandhydrogenhaverathersimilarelectronegativities,withhydrogenb

35、eingslightlygreater,soitisnotlikelythereissignificantdipolarcharactertotheB-Hbond.Sinceboroniselectrondeficient(itdoesnothaveavalenceshellelectronoctet)thereagentitselfisaLewisacidandcanbondtothepi-electronsofadoublebondbydisplacementoftheethermoietyfromthesolvatedmonomer.Asshowninthefollowingequati

36、on,thisbondingmightgenerateadipolarintermediateconsistingofanegatively-chargedboronandacarbocation.Suchaspecieswouldnotbestableandwouldrearrangetoaneutralproductbytheshiftofahydridetothecarbocationcenter.Indeed,thishydrideshiftisbelievedtooccurconcurrentlywiththeinitialbondingtoboron,asshownbythetra

37、nsitionstatedrawnbelowtheequation,sothediscreteintermediateshownintheequationisnotactuallyformed.Nevertheless,thecarbocationstabilityrulecitedaboveremainsausefulwaytopredicttheproductsfromhydroborationreactions.Notethatthisadditionisuniqueamongthosewehavediscussed,inthatitisasingle-stepprocess.Also,

38、allthreehydrogensinboranearepotentiallyreactive,sothatthealkylboraneproductfromthefirstadditionmayserveasthehydroborationreagentfortwoadditionalalkenemolecules.StereoselectivityinAdditionReactionstoDoubleBondsAsillustratedinthedrawing,thepi-bondfixesthecarbon-carbondoublebondinaplanarconfiguration,a

39、nddoesnotpermitfreerotationaboutthedoublebonditself.Weseethenthatadditionreactionstothisfunctionmightoccurinthreedifferentways,dependingontherelativeorientationoftheatomsorgroupsthataddtothecarbonsofthedoublebond:(i)theymaybondfromthesameside,(ii)theymaybondfromoppositesides,or(iii)theymaybondrandom

40、lyfrombothsides.Thefirsttwopossibilitiesareexamplesofstereoselectivity,thefirstbeingtermedsyn-addition,andthesecondanti-addition.Sinceinitialelectrophilicattackonthedoublebondmayoccurequallywellfromeitherside,itisinthesecondstep(orstage)ofthereaction(bondingofthenucleophile)thatstereoselectivitymayb

41、eimposed.Ifthetwo-stepmechanismdescribedaboveiscorrect,andifthecarbocationintermediateissufficientlylong-livedtofreely-rotateaboutthesigma-bondcomponentoftheoriginaldoublebond,wewouldexpecttofindrandomornon-stereoselectiveadditionintheproducts.Ontheotherhand,iftheintermediateisshort-livedandfactorss

42、uchassterichindranceorneighboringgroupinteractionsfavoronesideinthesecondstep,thenstereoselectivityinproductformationislikely.Thefollowingtablesummarizestheresultsobtainedfrommanystudies,theformulaHXreferstoallthestrongBrnstedacids.Theinterestingdifferencesinstereoselectivitynotedhereprovidefurtheri

43、nsightintothemechanismsoftheseadditionreactions.1.Brnsted Acid AdditionsThestereoselectivityofBrnstedacidadditionissensitivetoexperimentalconditionssuchastemperature,solventandreagentconcentration.Theselectivityisoftenanti,butreportsofsynselectivityandnon-selectivityarenotuncommon.Ofallthereagentsdi

44、scussedhere,thesestrongacidadditions(E=Hinthefollowingequation)comeclosesttoproceedingbytheproposedtwo-stepmechanisminwhichadiscretecarbocationintermediateisgeneratedinthefirststep.Suchreactionsaremostpronetorearrangementwhenthisisfavoredbythealkenestructure.2.Addition Reactions Initiated by Electro

45、philic HalogenThehalogenschlorineandbromineaddrapidlytoawidevarietyofalkeneswithoutinducingthekindsofstructuralrearrangementsnotedforstrongacids(firstexamplebelow).Thestereoselectivityoftheseadditionsisstronglyanti,asshowninmanyofthefollowingexamples.Animportantprincipleshouldberestatedatthistime.Th

46、ealkenesshownhereareallachiral,buttheadditionproductshavechiralcenters,andinmanycasesmayexistasenantiomericstereoisomers.Intheabsenceofchiralcatalystsorreagents,reactionsofthiskindwillalwaysgiveracemicmixturesiftheproductsareenantiomeric.Ontheotherhand,iftwochiralcentersareformedintheadditionthereac

47、tionwillbediastereomerselective.Thisisclearlyshownbytheadditionofbrominetotheisomeric2-butenes.Anti-additiontocis-2-butenegivestheracemicproduct,whereasanti-additiontothetrans-isomergivesthemeso-diastereomer.Wecanaccountbothforthehighstereoselectivityandthelackofrearrangementinthesereactionsbypropos

48、ingastabilizinginteractionbetweenthedevelopingcarbocationcenterandtheelectronrichhalogenatomontheadjacentcarbon.Thisinteraction,whichisdepictedforbromineinthefollowingequation,delocalizesthepositivechargeontheintermediateandblockshalideionattackfromthesyn-location.Thestabilizationprovidedbythishalog

49、en-carbocationbondingmakesrearrangementunlikely.Inafewcasesthree-memberedcyclichaloniumcationshavebeenisolatedandidentifiedastrueintermediates.Aresonancedescriptionofsuchabromoniumionintermediateisshownbelow.Thepositivechargeisdelocalizedoveralltheatomsofthering,butshouldbeconcentratedatthemoresubst

50、itutedcarbon(carbocationstability),andthisisthesitetowhichthenucleophilewillbond.Becausetheyproceedbywayofpolarion-pairintermediates,chlorineandbromineadditionreactionsarefasterinpolarsolventsthaninnon-polarsolvents,suchashexaneorcarbontetrachloride.However,inordertopreventsolventnucleophilesfromcom