1、6.14Addition of Halogens to Alkenes+ X2XCXCCCelectrophilic addition to double bondforms a vicinal dihalideGeneral featuresBr2CHCl3 0CCH3CHCHCH(CH3)2CH3CHCHCH(CH3)2BrBr(100%)Examplelimited to Cl2 and Br2F2 addition proceeds with explosive violenceI2 addition is endothermic:vicinal diiodidesdissociate

2、 to an alkene and I2Scope6.15Stereochemistry of Halogen Additionanti additionHBr2HBrHHtrans-1,2-Dibromocyclopentane80% yield;only productBrExampleHHCl2HHCltrans-1,2-Dichlorocyclooctane73% yield;only productClExample6.16Mechanism of Halogen Addition toAlkenes:Halonium IonsBr2 is not polar, but it is

3、polarizabletwo steps(1)(2)formation of bromonium ionnucleophilic attack on bromonium ion by bromideMechanism is electrophilic additionethylene propene2-methylpropene2,3-dimethyl-2-buteneH2C=CH2 CH3CH=CH2 (CH3)2C=CH2(CH3)2C=C(CH3)21615400920,000More highly substituted double bonds react faster.Alkyl

4、groups on the double bond make it more “electron rich.”Relative Rates of Bromination+H2CCH2Br2BrCH2CH2Br?.+ C+: B.r:C: B.r :No obvious explanation for anti addition provided by this mechanism.Mechanism?+H2CCH2Br2BrCH2CH2Br.+: B.r:CC: Br :+Cyclic bromonium ionMechanismBrMutual polarizationof electron

5、 distributions of Br2 and alkeneBrFormation of Bromonium IonBr dElectrons flowfrom alkene toward Br2Brd+d+Formation of Bromonium IonBrp electrons of alkene displace Br from Br+BrFormation of Bromonium Ion. Br :.:.Br.+.Br :.:attack of Brfrom side oppositeCBr bond of bromonium ion givesanti addition.B

6、r :.StereochemistryHBr2HBrHHtrans-1,2-Dibromocyclopentane80% yield;only productBrExampleCyclopentene +Br2Bromonium ionde ion attacks omonium ionfrom side opposite carbon-bromine bondBromi the brtrans-Stereochemistry in vicinal dibromide6.17Conversion of Alkenes to Vicinal Halohydrins+ X2XCCCXCalkene

7、s react with X2 to form vicinal dihalides+ X2XCCCXCalkenes react with X2alkenes react with X2 halohydrinsto form vicinal dihalidesin water to give vicinal+ H2O+ X2XOHCCCC+ HXH2O+H2CCH2Br2BrCH2CH2OH(70%)HCl2H2OHOHClHHanti addition:only productExamplesO :.Br.+O.bromonium ion is intermediatewater is nu

8、cleophile that attacks bromonium ion.Br :.MechanismH2O+H2CCH2Br2BrCH2CH2OH(70%)HCl2H2OHOHClHHanti addition:only productExamplesCyclopentene + Cl2Chloronium ionater attacks loronium ion m side positerbon-chlorine ndWch fro op ca botrans-Stereochemistry in oxonium iontrans-2-ChlorocyclopentanolCH3H3CBr2H2OCH3CCCH2BrCH2H3COH(77%)Markovnikovs rule applied to halohydrinformation:the halogen adds to the carbonhaving the greater number of hydrogens.RegioselectivityHd+Hd+. H.OHOH3C H3CH Cd+d+3H3CCCH2CH2C: Br :d+: Br:d+transition state has for attack of water on