哥伦比亚大学有机化学经典课件04_08_14

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1、4.8PreparationofAlkylHalidesfromAlcoholsandHydrogenHalidesROH+HXRX+H2OReactionofAlcoholswithHydrogenHalidesROH+HXRX+HOHHydrogenhalidereactivityHIHBrHClHFmostreactiveleastreactiveReactionofAlcoholswithHydrogenHalidesROH+HXRX+HOHAlcoholreactivityR3COHR2CHOHRCH2OHCH3OHTertiarySecondary

2、PrimaryMethanolmostreactiveleastreactivePreparationofAlkylHalides(CH3)3COH+HCl(CH3)3CCl+H2O78-88%+H2O73%CH3(CH2)5CH2OH+HBrCH3(CH2)5CH2Br+H2O87-90%25°C80-100°C120°COH+HBrBrPreparationofAlkylHalidesCH3CH2CH2CH2OHCH3CH2CH2CH2BrNaBrH2SO470-83%heatAmixtureofsodiumbromideandsulfuricacidmaybeusedin

3、placeofHBr.4.9MechanismoftheReactionofAlcoholswithHydrogenHalidesCarbocationThekeyintermediateinreactionofsecondaryandtertiaryalcoholswithhydrogenhalidesisacarbocation.Acarbocationisacationinwhichcarbonhas6valenceelectronsandapositivecharge.CRRR+CarbocationThekeyintermediateinreactionof

4、secondaryandtertiaryalcoholswithhydrogenhalidesisacarbocation.Theoverallreactionmechanisminvolvesthreeelementarysteps;thefirsttwostepsleadtothecarbocationintermediate,thethirdstepistheconversionofthiscarbocationtothealkylhalide.CRRR+Example(CH3)3COH+HCl(CH3)3CCl+H2O25°CCarbocationinterme

5、diateis:CH3CCH3CH3+tert-Butylalcoholtert-Butylchloridetert-ButylcationMechanismStep1:ProtontransferfromHCltotert-butylalcohol(CH3)3COH..:HCl:....+HO:+(CH3)3CHCl:....:–+fast,bimoleculartert-ButyloxoniumionMechanismStep2:Dissociationoftert-butyloxoniumion+(CH3)3COH:H+slow,unimolecular(CH3)3COH:

6、H:tert-Butylcation+MechanismStep3:Captureoftert-butylcationbychlorideion.fast,bimolecular+(CH3)3C+tert-Butylchloride(CH3)3CCl:....Cl::....–4.10Structure,Bonding,andStabilityofCarbocationsFigure4.8Structureofmethylcation.Carbonissp2hybridized.Allfouratomslieinsameplane.Figure4.8Structureofm

7、ethylcation.Empty2porbital.Axisof2porbitalisperpendiculartoplaneofatoms.CarbocationsMostcarbocationsaretoounstabletobeisolated.WhenRisanalkylgroup,thecarbocationisstabilizedcomparedtoR=H.CRRR+CarbocationsCHHH+MethylcationleaststableCarboc