The-Art-of-writing-reasonable-organic-mechanism-3有

3.1AnswersToChapter3Problems.1.(a)Inordertocompareitdirectlywiththeothertwocarbocations,thecarbocationderivedfromthefirstcompoundshouldbedrawnintheresonanceforminwhichtheemptyorbitalislocatedonthe3°C.Itcanthenclearlybeseenthatthethreecarbocationsareall3°carbocationsthatdifferonlyinthethirdcarbo-cationsubstituent.Theorderofsubstituentstabilizingabilityislonepairπbondσbonds.MeMet-BuMeMeMeOMeMeBrBrBr123(b)Thefirstcompoundgivesanantiaromaticcarbocation.Amongtheothertwo,thesecondcompoundgivesacationwiththeelectrondeficiencydelocalizedacrossone2°andtwo1°C’s,whilethethirdcompoundgivesacationwiththeelectrondeficiencydelocalizedacrossthree2°C’s.MeMeClClCl123(c)Theorderofstabilityofalkylcationsis3°2°1°.321OTsOTsOTs(d)Thesecondcompoundgivesalone-pair-stabilizedcarbocation.Amongtheothertwo,1°alkylcarbo-cationsaremorestablethan1°alkenylcarbocations.213H3COTfH3CNOTfH3CH3COTfH3C3.2(e)ThefirstcompoundgeneratesacationthatcanbestabilizedbythelonepaironN.Thesecondcom-poundgeneratesacationthatcannotbestabilizedbythelonepaironNduetogeometricalconstraints(wouldformbridgeheadπbond,ano-no).ThereforetheinductiveeffectofNdestabilizesthecarbocationderivedfromthesecondcompoundrelativetothecarbocationfromthethirdcompound,inwhichtheNismoreremote.NNNTsOTsOTsO123(f)Thesecondandthirdcompoundsgeneratecationsthatcanbedirectlystabilizedbyresonancewiththelonepairsontheheteroatoms,withNmorestabilizingthanO,whilethecationfromthefirstcompoundisn’tstabilizedbyresonancewiththeheteroatomatall.213AcHNAcHNBrBrAcOBr(g)Thesecondcompound(atriptycene)providesnoπstabilizationtothecorrespondingcation,becausetheporbitalsofthephenylringsareperpendiculartotheemptyporbital.Thefirstcompoundismorelikelytoionizethanthethirdfortworeasons.(1)Thephenylringsinfirstcompoundaremoreelectron-rich(alkyl-substituted).(2)Inthefirstcompound,twoofthephenylringsareheldinacoplanararrangementbythebridgingCH2,sotheyalwaysoverlapwiththeemptyporbitalofthecation.Inthethirdcompound,thereisfreerotationabouttheC–Phbonds,sothereisgenerallylessoverlapbetweenthePhπcloudsandtheemptyporbitalofthecationiccenter.3.3HOPhOHPhPhPhOH1322.(a)Excellentcarbocation,nucleophilicsolvent,∴SN1.Br–leavesspontaneouslytogiveacarbocation,whichcombineswithsolventtogiveaprotonatedether,whichlosesH+togivetheproduct.(b)Excellentcarbocation,nucleophilicsolvent,∴SN1.FirstOisprotonated,thenOH2leavestogivecarbocation,Next,thecarbonylOofAcOHaddstothecarbocation,andthenH+islostfromOtogivetheproduct.(c)Excellentcarbocation,nonnucleophilicsolvent,∴E1.FirstOisprotonated,thenOH2leavestogivecarbocation.Finally,H+islostfromtheCadjacenttotheelectron-deficientCtogivethealkene.(d)Goodcarbocation,nucleophilicsolvent,∴SN1.Theproductisracemic.Br–leavesspontaneouslytogiveaplanar,achiralcarbocation;thenthecarbonylOofHCO2Haddstothecarbocationfromeitherenantioface.Finally,H+islostfromOtogivetheproduct.(e)Excellentcarbocation,nucleophilicsolvent,∴SN1.HerethenucleophileisCl–,becauseadditionofH2Osimplygivesbackstartingmaterial.FirstOisprotonated,thenOH2leavestogivecarbocation,thenCl–addstocarbocationtogivetheproduct.(f)Excellentcarbocation,nucleophilicsolvent,∴SN1.FirsttheOoftheOHgroupisprotonated,thenOH2leavestogiveanO-stabilizedcarbocation.Next,theOofCH3OHaddstothecarbocation,andfinallyH+islostfromtheOofOCH3grouptogivetheproduct.Notethattheringoxygencouldalsoactasaleavinggrouptogiveanacycliccompound,butentropyfavorsthelossoftheOHgroup(becausetwoproductsareformedfromone).(g)Awfulcarbocation,socan’tbeSN1.Stronglyacidicconditions,excellentnonbasicnucleophile,∴SN2.FirstOisprotonated,thenBr–doesanucleophilicdisplacementofOH2togivetheproduct.(h)So-socarbocation,excellentnonbasicnucleophile.CouldbeSN1orSN2.FirstOisprotonated;3.4then,eitherBr–displacesOfromCtogiveproduct,orOleavestoformcarbocation,andthenBr–addstothecarbocation.Theregiochemistryisdeterminedbytheformationofthestablercarbocation.(EveninSN2reaction,thecentralCinthetransitionstatehassomecarbocationiccharacter,sothemoresubstitutedCundergoessubstitutionunderacidicconditions.)(a)OEt(b)OAcPh(c)PhPhOCHOHCH3ClOOCH3Br(d)(e)(f)(g)HOn-BuBr(h)3.NumbertheC’sin1.Weseethatthefirstsetofcompounds,2-4,areallobtainedbyformationofabondbetweenC4andC8.TomaketheC4–C8bond,wecouldmakeC4electrophilicandC8nucleophilic,orviceversa.IfwemakeC8electrophilicbyprotonationofC9,thenafterattackofC4,weendupwitha1°carbocationonC5—veryunstableandnotwhatwewant.Ontheotherhand,ifwemakeC4electrophilicbyprotonatingC5,thenafterattackofC8onC4,weendupwitha3°carbocationonC9.Ascompounds2-4differonlyinthelocationoftheπbondtoC9,suggestingthatlossofH+fromaC9carbocationisthelaststep,thisiswhatweneedtodo.H3CH3CHHH3C2-445891011H3CH3CCH3HH12356789101112134H+1H3CH3CCH3HH58910114H+H3CH3CH3CCH3HH3.5H3CH3CH3CCH3HHAHHH81011–H112–H83–H104Thenextsetofproducts,5-9,mustbeformedfrom2-4.Togetfrom2-4to5-9,wemustbreaktheC4–C8bondagain.ThisiseasytodoifweregeneratecarbocationA.CleavageoftheC4–C8bondgivesaC8=C9πbondandacarbocation,B,atC4.LossofH+fromC5orC3ofBgivesproduct5or9,respectively.Compounds5and9canthenpartlyisomerizetocompounds7and6,respe