Elimination reactions

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1、Eliminationreactions19ConnectionsBuildingon:Arrivingat:Lookingforwardto:•Mechanismsofnucleophilic•Eliminationreactions•Electrophilicadditionstoalkenes(thesubstitutionatsaturatedcarbonch17•Whatfactorsfavoureliminationoverreverseofthereactionsinthis•Conformationch18substitutionchapter)

2、ch20•Thethreeimportantmechanismsof•Howtocontroldouble-bondgeometryeliminationreactionsch31•Theimportanceofconformationineliminationreactions•Howtouseeliminationstomakealkenes(andalkynes)SubstitutionandeliminationSubstitutionreactionsoft-butylhalides,youwillrecallfromChapter17,invaria

3、blyfollowtheSN1mechanism.Inotherwords,therate-determiningstepoftheirsubstitutionreactionsisunimolecu-lar—itinvolvesonlythealkylhalide.Andthismeansthat,nomatterwhatthenucleophileis,theLreactiongoesatthesamerate.Youcan’tspeedthisSRemembertheturnstilesattherailwayN1reactionup,forexample

4、,byusinghydroxidestation(p.000).insteadofwater,orevenbyincreasingtheconcentrationofhydroxide.‘You’dbewastingyourtime,’wesaid(p.000).nucleophilicsubstitutionreactionsoft-BuBrrate=k[t-BuBr]reactiongoesattheslowfastsameratewhateverBrHOorHOOHthenucleophile2t-butylbromidet-butanolYou’dals

5、obewastingyouralkylhalide.Thisiswhatactuallyhappensifyoutrythesubstitutionreactionwithaconcentratedsolutionofsodiumhydroxide.reactionoft-BuBrwithconcentratedsolutionofNaOHrate=k[t-BuBr][HO–]eliminationreaction+HO++HOHBrformsalkeneBrt-butylbromideisobutene(2-methylpropene)Thereactions

6、topsbeingasubstitutionandanalkeneisformedinstead.Overall,HClhasbeenlostfromthealkylhalide,andthereactioniscalledanelimination.Inthischapterwewilltalkaboutthemechanismsofeliminationreactions—asinthecaseofsubsti-tutions,thereismorethanonemechanismforeliminations.Wewillcompareeliminatio

7、nswithsubsti-tutions—eitherreactioncanhappenfromalmostidenticalstartingmaterials,andyouwilllearnhowtopredictwhichisthemorelikely.MuchofthemechanisticdiscussionrelatesverycloselytoChapter17,andwesuggestthatyoushouldmakesureyouunderstandallofthepointsinthatchapterbeforetack-lingthisone

8、.Thischapter