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1、ThelifeofthechaindependsontheongoingpresenceofthehighlyreactiveClatomsandalkylradicals.Eliminatingthesespeciesendschains.4.2Cl.Cl–Cl5.2R.R–R6.R.+Cl.R-ClHeatorlightChainReactionMechanism1.Cl–Cl2Cl.2.R–H+Cl.R.+H-Cl3.R.+Cl–ClR–Cl+Cl.Repeat2,3,2,3,….Chainsteps.Chlorineatom.H
2、ighlyreactive,onlysevenelectronsinvalenceshellWeakCl-Clbondmaybebrokenbyheatorlight.Hydrogentobeabstracted.Tradebonds:R-HforH-ClAlkylradical,onlysevenelectronsaroundtheC,highlyreactivealkylradical.Tradebonds:weakCl-ClforastrongerC-ClRegeneratestheClatomusedinstep2Termina
3、tionsteps.InitiationEnergeticsoftheChainStepsChlorinationofethaneStep2,abstractionoftheH,controlstheregioselectivityofthereaction.IsothermicorslightlyexothermicforCl;endothermicforBr.Step3,attachmentofthehalogen,controlsthestereochemistry,whichsidethehalogenattaches.Exot
4、hermicStep2andBondDissociationEnergies,breakingbonds…Morehighlysubstitutedradicalsareeasiertomake.Thisgivesrisetoregioselectivity=non-randomreplacement.Nowbromination.Comparechlorinationandbrominationof2-methylpropane.BrominationismoreRegioselective.ExamineStep2onlyforre
5、gioselectivity.BDE,reflectingdifferentradicalstabilitiesSlightlyexothermicEndothermicH-ClisamorestablebondthanH-Br.Step2isexothermicforchlorination.EndothermicforBrFirstchlorination.TwokindsofH.Step2TransitionStateEnergetics,ClvsBrchlorinationExothermic,tertiaryradicalmo
6、restableEarlytransitionstates,littledifferenceinenergiesofactivation,ratesofabstractionandregioselectivitybrominationEndothermic,buttertiaryradicalstillmorestablebysameamount.Latetransitionstates,largerdifferenceinenergiesofactivation,ratesofabstractionandregioselectivit
7、y.R…H……….ClR……….H..BrHalogenationof2-methylpropaneyieldstwodiffererentradicals,primaryandtertiary.NowStep3:StereochemistryAlkylradical,sp2hybridization,oneelectronintheporbital.Mirrorobjects.Ifachiralcarbonhasbeenproducedwegetbothconfigurations.Step3hasthesecharacteristi
8、csDeterminesstereochemistryIsexothermicIsfast,notratedeterminingSimpleExample:monochlorinationof2-methy
