Conversion of Alkenes to Vicinal Halohydrins. Cl 2 anti addition: only product H2OH2OH2OH2O H2CH2CH2CH2C CH 2 BrCH 2 CH 2 OH + Br 2 H2OH2OH2OH2O (70%)

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Conversion of Alkenes to Vicinal Halohydrins

Cl 2 anti addition: only product H2OH2OH2OH2O H2CH2CH2CH2C CH 2 BrCH 2 CH 2 OH + Br 2 H2OH2OH2OH2O (70%) Examples H H OHOHOHOH ClHH

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

Cl 2 anti addition: only product H2OH2OH2OH2O H2CH2CH2CH2C CH 2 BrCH 2 CH 2 OH + Br 2 H2OH2OH2OH2O (70%) Examples H H OHOHOHOH ClHH

Cyclopentene + Cl 2

Chloronium ion

Water attacks chloronium ion from side opposite carbon-chlorine bond

trans-Stereochemistry in oxonium ion

trans-2-Chlorocyclopentanol

(77%) H3CH3CH3CH3CC CH 2 H3CH3CH3CH3C CH 3 OH C CH 2 Br CH 3 Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens. Br 2 H2OH2OH2OH2O Regioselectivity

CH 2 H3CH3CH3CH3C C H3CH3CH3CH3C 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. H3CH3CH3CH3C CH 2 H3CH3CH3CH3C Br ::  HO H .. Br ::HO H .. C   Explanation

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