6.14 Addition of Halogens to Alkenes General features C C + X2 X C C X electrophilic addition to double bond forms a vicinal dihalide Example Br2 CH3CH CHCH(CH3)2 CH3CHCHCH(CH3)2 CHCl3 0°C Br Br (100%) Scope limited to Cl2 and Br2 F2 addition proceeds with explosive violence I2 addition is endothermic: vicinal diiodides dissociate to an alkene and I2 6.15 Stereochemistry of Halogen Addition antianti additionaddition Example H Br2 H Br H Br H trans-1,2-Dibromocyclopentane 80% yield; only product Example H H Cl Cl2 H H Cl trans-1,2-Dichlorocyclooctane 73% yield; only product 6.16 Mechanism of Halogen Addition to Alkenes: Halonium Ions Mechanism is electrophilic addition Br2 is not polar, but it is polarizable two steps (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide Relative Rates of Bromination ethylene H2C=CH2 1 propene CH3CH=CH2 61 2-methylpropene (CH3)2C=CH2 5400 2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 920,000 More highly substituted double bonds react faster. Alkyl groups on the double bond make it more “electron rich.” Mechanism? + H2C CH2 + Br2 BrCH2CH2Br ? + .. – + : Br: C C .. : Br: .. No obvious explanation for anti addition provided by this mechanism. Mechanism + H2C CH2 + Br2 BrCH2CH2Br .. – + : Br: C C .. : Br: + Cyclic bromonium ion Formation of Bromonium Ion Br Mutual polarization Br of electron distributions of Br2 and alkene Formation of Bromonium Ion d– Electrons flow Br d from alkene toward Br2 Br d+ d+ Formation of Bromonium Ion – Br p electrons of alkene displace Br– from Br + Br Stereochemistry .. – : Br : .. .. Br + .. .. : Br : .. attack of Br– from side opposite C—Br bond of bromonium ion gives .. anti addition Br : .. Example H Br2 H Br H Br H trans-1,2-Dibromocyclopentane 80% yield; only product Cyclopentene +Br2 Bromonium ion – – Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond – trans-Stereochemistry in vicinal dibromide 6.17 Conversion of Alkenes to Vicinal Halohydrins C C + X2 X C C X alkenes react with X2 to form vicinal dihalides C C + X2 X C C X alkenes react with X2 to form vicinal dihalides alkenes react with X2 in water to give vicinal halohydrins C C + X2 + H2O X C C OH + H—X Examples H2O + H2C CH2 + Br2 BrCH2CH2OH (70%) H Cl2 H OH H2O H Cl H anti addition: only product Mechanism O : .. .. Br + .. + O .. bromonium ion is intermediate .. water is nucleophile that Br : attacks bromonium ion .. Examples H2O + H2C CH2 + Br2 BrCH2CH2OH (70%) H Cl2 H OH H2O H Cl H anti addition: only product Cyclopentene + Cl2 Chloronium ion Water attacks chloronium ion from side opposite carbon-chlorine bond trans-Stereochemistry in oxonium ion trans-2-Chlorocyclopentanol Regioselectivity CH3 H3C Br2 C CH2 CH3 C CH2Br H2O H3C OH (77%) Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens. Explanation H H .. .. H O d+ d+ O H H C H C 3 3 d+ H C d+ H C C H3C 3 CH2 C CH2 : Br: : Br: d+ d+ transition state has for attack of water on bromonium ion has carbocation character; more stable transition state (left) has positive charge on more highly substituted carbon.