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Conversion of Alkenes to Vicinal Halohydrins

Conversion of Alkenes to Vicinal Halohydrins. H. H. O H. Cl. H. H. Examples. H 2 O. +. Br 2. Br CH 2 CH 2 O H. H 2 C. CH 2. (70%). Cl 2. H 2 O. anti addition: only product. O. Br. O. Mechanism. :. bromonium ion is intermediate

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Conversion of Alkenes to Vicinal Halohydrins

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  1. Conversion of Alkenes to Vicinal Halohydrins

  2. H H OH Cl H H Examples H2O + Br2 BrCH2CH2OH H2C CH2 (70%) Cl2 H2O anti addition: only product

  3. O .. .. Br .. O .. Mechanism : • bromonium ion is intermediate • water is nucleophile that attacks bromonium ion + + .. : Br ..

  4. H H OH Cl H H Examples H2O + Br2 BrCH2CH2OH H2C CH2 (70%) Cl2 H2O anti addition: only product

  5. Cyclopentene + Cl2

  6. Chloronium ion

  7. Water attacks chloronium ion from side opposite carbon-chlorine bond

  8. trans-Stereochemistry in oxonium ion

  9. trans-2-Chlorocyclopentanol

  10. CH3 H3C Br2 CH3 C C CH2Br CH2 H2O H3C OH Regioselectivity • Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens. (77%)

  11. H H .. .. d+ H d+ O H O Explanation • Both transition states (showing the attack of water to either bridged carbon) are possible, since both carbons have carbocationic character. However, the more stable TS is the one which resembles the more stable (the more highly substituted) carbocation. H3C H3C d+ d+ H3C H3C CH2 CH2 C C : : : : Br Br d+ d+

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