加州大学伯克利分校有机化学课件79548

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1、Chapter14DelocalizedPiSystemsTrigonalTheπbondise-rich:E+attack,R∙add)Thelobesofthep-orbitals:Perpendiculartothesigmaframeandparalleltoeachother.Recallthedoublebond2-Propenyl(Allyl)Question:Whataboutaddingathirdp-orbitaladjacenttothedoublebond?Istheresomething

2、special?Or:Isthereanyspecialreactivityatthecarbonsadjacenttoadoublebond?Ha.b.c.SN1reactivityofallyliccarbonlikethatofRsecX,eventhoughitisprimary!pKa~40:Acidic!50SN187kcalmol-1:Weak!101kcalmol-1Replacingoneofthehydrogensinethenewithanothersp2-hybridizedcarbongi

3、vesanallylicsystem.AllylicpositionObservations:HLHBBHHClearly:Allylic·arestabilized.-+Shortnotation:DottedlinesWhy?Resonance!MOPictureof2-Propenyl(Allyl)3pOs3MOsRecall:BondsmadebyoverlapH.H.HH++H++H+H+HInphase+H+-H-H+HOutofphaseBondingAnti-bondingNodeSignofthe

4、wavefunction,Notcharge!EEπσσ*π*CH2CH2Whathappenstothispicturewhenweinteractwithanotherporbital?Interactionwiththeπbondingorbitalcausesenergysplitting:theporbitallevelmovesupandtheπbondinglevelmovesdown.Interactionwiththeπantibondingorbitalcausestheenergylevelo

5、ftheporbitaltomovedowntowhereitwasoriginallyandtheenergyleveloftheπantibondingorbitaltomoveup.Thetwoeffectsontheporbitalcanceleachotheroutandthetwoπorbitalsarepushedapart.Interactionsofasinglyoccupiedp-orbitalwitheachoftheπmolecularorbitalspEππ*0ππ*updownuncha

6、ngedEtheneAllylpOrbitalNonbondingMOpE#ofedependson+,·,-ππ*0Resultingpicture:AndlocationatterminiReactivityofAllylicPositionA.RadicalHalogenationCH2CHCH3+Br2CH2CHCH2Br+HBrFasterthanaddition!Lowconc.Mech.1.2.Br22Br.hυorΔInitiation:Propagation:CH2CHCH3+Br.CH2CHCH

7、2.CH2CHCH2.+HBrCH2CHCH2.CH2CHCH2.Br2CH2CHCH2Br+Br.Termination:3.Br.+.CH2CHCH2BrCH2CHCH2Br.Br.+Br2CH2CHCH2.2CH2CHCH2CH2CHCH2Anythingthattrapsradicals,includingthe“dirt”onthewallsoftheflask,contributestotermination.TracesLowconc.Propenegeneratesasymmetricalallyl

8、icradical,butthisisnotalwaysthecase.Forunsymmetricalsystems:mixtures.Ratiosdependon%radicalcharacteroneachcarbonandTSsleadingtoproducts.Br2Br·+B.SN1:TheAllylicCationisStabilizedCH3