Chlorine Atom／Benzene System. 1 .The Role of the 6-Chlorocyclohexadienyl Radical.docx

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1、6300J. Am. Chem. Soc. 1986, 108, 6300-6311ChlorineAtom/BenzeneSystem.1.TheRoleofthe6-ChlorocyclohexadienylRadicalPhilipS.SkeU,*HarryN.Baxter,III,JamesM.Tank。,an(VenkatasuryanarayanaCheboluContributionfromtheDepartmentofChemistry,ThePennsylvaniaStateUniversity,UniversityPark,Pennsylvania16802.Receive

2、dSeptember20,1985.RevisedManuscriptReceivedMay28,1986S22CBpoqs =qndolln-s AQlEE-l-cb。-oiYoq cosuopdoSQU一Qp-nBcnu-ceqs/SJO.SJBSqnd/dsL-00s65)与斜2.0 1C3zzoz ,6二套 co ?MOHUOOS BPOPBOcModAbstract:TheconceptofradicalreactivitymediatedbysolvationhasrestedmainlyontheIterationofClepropertiesbyaromaticsolvents

3、.Forthisreason,thefullscopeofthebenzene/Crsystemhasbeenreexaminedtoevaluatethedescriptionofthatsystembasedlargelyonar-complex(solvation).Atthepresenttime,ther-complexdescriptionrestsnarrowlyontheassignmentofa490-nmabsorption,which,evenifcorrect,couldnotprovideanunambiguousstructureassignment.Results

4、arepresentedwhichdescribetheselectivitiesinalkanesubstitutionsasafunctionoftheconcentrationsofbothbenzeneandthealkane.Selectivitiesincreasewithdecreasingalkaneconcentrations,reachingaplateaubelow0.1Malkane.Thechangeinselectivityistheresultofvariablecontributionsofbothalow-andahigh-selectivityinterme

5、diate.LSIandHSI,respectively.TheobservedselectivityatagivenPhHandRHistheconsequenceofauniqueLSI/HSIratio.ArangeofsubstratesandtheireffectonDMBselectivitywerestudied,andfromtheseresultsdetailsregardingthechemistryoftheHSIwereextracted.SeveralfeaturesoftheLSI/HSIequilibratingsystemarerealized,(1)React

6、ionofalkylradicalswithCl2inbenzeneproducestheLSI,(2)theLSIdoesnotexhibitthecharacteristicsoffreechlorineatom,and(3)atalkaneconcentrationsW0.1MequilibriumbetweentheLSIandHSIisattained.TheLSI/HSIequilibratingsystemismostreadilyunderstoodiftheHSIisassignedtothe6-chlorocyclohexadienylradical(CCH),Itissh

7、ownthatatfixedPhHandDMB,addedreagents(T)whichreactwithCCH,suchasmaleicanhydride(MA)orCl2,bringaboutanincreaseintheLSI/HSIratio.Low-selectivityhydrogenabstractions(LSIfunction)arebestascribedtoamixtureofchlorineatomandthechlorineatom/benzene7r-complex.ThechemistryofCCHisasfollows:(1)lossoftheipsoHtoO

8、2yieldingPhClandHO2(2)reactionsofCl2or(3)maleicanhydridewiththearomaticnucleusofCCHresultinginadditionstothering,(4)thetransferofCltoalkenes,and(5)thehighlyselectiveretardationofratesofreactionwithalkanesproducingalkylradicals,HCLandbenzene.Theresultsofakineticanalysis,accountingfortheeffectofPhH,RH

9、J,andCCHtrappingagents(T),arepresented.ForCCH,thefollowingreactivityorderisestablished:maleicanhydride(6)fra/u-dichloroethene(5)2,3-dimethylbutane(2)pentane(1)Cl21.00neopentane(2(27)10LOO.ThesepropertiescanberationalizedwithcanonicalstructuresforCCHwhereinspindensityatcarbon,chlorine,andtheipsohydro

10、genmakescontributionstothehybrid.I.IntroductionA.Overview.Wherechlorineatomsareproducedinthepresenceofbenzene,threepotentialintermediateshavebeenconsideredinordertoexplaintheresultantchemistry:freechlorineatom(Cr)achlorineatom/benzene-complex(tC),andthe6-chlorocyclohexadienylradical(CCH)JTherelation

11、ship(s)itisalsoreasonabletoconsiderthepossibilitythatthe6-chlorocyclohexadienylradicalhasthecapacitytoreactwithanalkanetomakeanalkylradical,HCl,andbenzene.ci-freeClatomClatomir-complex(nC)6-ch1orocyc1ohexadlenylradical(CCH)betweenthesespecieshasbeenfuzzy.Historically,reactionswhoseproductsinvolvesub

12、stitutionoradditiononthearomaticnucleus(ie,reactionatcarbon)havegenerallybeeninterpretedasin-volvingCCHJ?whereasselectivehydrogenabstractionsfromalkaneswereattributedto%C.,t3Theoriginalreasonsforattributingselectivehydrogenab-stractionstoirCarenolongervalid(videinfra).Furthermore,thenotionofasolvent

13、effectonthereactivityofanunchargedradicalhasdependedalmostentirelyonacceptanceoftheas-signmentofttCstructureforCPinbenzene.3Itisdisconcertingthatthereisnoindicationthatsolvationplaysaroleindeterminingthepropertiesofotherneutralradicals.Thus,despitetheuniversalacceptanceoftheconceptfor30years,itisnec

14、essarytoreconsidertheevidenceforthe/-complexassignment.FormationofCCHisbothreasonableandconsistentwithavailabledataintheliterature.Since6-halocyclohexadienylradicalsarereportedtohaveappreciablespindensityathalogen,B.HistoricalChlorinedoesnotreactwithbenzeneinthedark.Thelight-catalyzedreactionleading

15、tothehexachlorocyclohexanesisrecognizedasafree-radicalchainreaction,proceedingwiththe6-chlorocyclohexadienylradical(CCH),madebyadditionofachlorineatomtobenzene.1Inthe1950sRussellintroducedathirdspecies,their-complex(xC),asthepostulatedintermediatetoexplainthemoreselectivechlorinationofalkanesinthepresenceofbenzene.9-121Currentaddress:DepartmentofChemistry,VirginiaPolytechnicInstituteandStateUniversity.Blacksburg.VA24061.(1) Foracomprehensivereviewoffree-radicalchlorination,see:Poutsma,M.L.InMethodsinFreeRadicalChemistry;Huyser,E.S.，Ed.;Marcel