(理学)加州大学伯克利分校有机化学第七章

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1、Relative“rates”of4arrowsdifferCHCLBNu:-:-Chapter7:SN1,E1,E2,notjustSN2E1,E2SN2,SN1,E1,E2E1,E2SN2,SN1“Lingo”:αposition,ßposition;furtheraway:γ,δ,andsoon.SolventMeOHSolventDMF1©UniversityofCaliforniaCH3Br+H2OBut(CH3)3CBr(CH3)2CHBrorreact!+(CH3)3CBrHOH(CH3)

2、3COHAcetoneHBr+WeakNuDespitebeingα-branchedHydrolysis+(CH3)2CHBrHOCH3(CH3)2CHOCH3DMFHBr+MethanolysisGenerally:SolvolysisRecall:SN2slowswithweakNu(andbranching).Forexample:2©UniversityofCaliforniaMechanism:1.Rate=k[R-L],1storderunimolecular,onlyR-Linrate-d

3、eterminingTS:“bottleneck”.2.Stereochemistry:Racemization(extensive,althoughoftennotcomplete).BothobservationsinconsistentwithSN2mechanism3©UniversityofCalifornia3.Accelerateswithpolar(bestwithprotic,incontrasttoSN2)solvents:Hexane

4、celerateswithbetterLL:Cl>..CH3OH....(CH3)3OCH3..(CH3)3CN3wins4©UniversityofCaliforniaMechanism1.Electrondeficient!5©Unive

5、rsityofCalifornia2.6©UniversityofCalifornia3.7©UniversityofCaliforniaUnimolecularnucleophilicsubstitution:1stOrderratelawRacemizationAccelerationinpolarsolventsAccelerationwithbetterLFastproductdeterminingstepMechanismexplainsdata:SN1SN18©UniversityofCali

6、forniaSN1PEBottleneck:“Bottleneck”9©UniversityofCaliforniaIncompleteRacemizationMaystayclosetoformanionpairPlanarandachiral,butbothsidesnotequallyaccessibleinionpairwithBr-Ideally;inpractice:SlightenantiomerexcessSN1Racem10©UniversityofCaliforniaTheStrong

7、EffectofPolarSolventsontheSN1ReactionIncreasingsolventpolarityspeedsreactionIncreasingsolventpolarityretardsreaction11©UniversityofCaliforniaC+HCCH3CH3CH2(CH3)2CH(CH3)3C++++ofneighboringCHbondinteractswithempty2porbitalStabilizing(2e)!Betterthanradical(3e

8、).2pBondingMOEToounstableTheonlycationsfeasibleinsolution