Conversion of Alkenes to Vicinal Halohydrins

In aqueous solution chlorine and bromine react with alkenes to form vicinal halohy-drins, compounds that have a halogen and a hydroxyl group on adjacent carbons. 

Anti addition occnrs. The halogen and the hydroxyl group add to opposite faces of the double bond. 

The regioselectivity of addition is established when water attacks one of the carbons of the haloninm ion. In the reaction shown, the structure of the product tells us that water attacks the more highly substituted carbon. 

This suggests that, as water attacks the bromonium ion, positive charge develops on the carbon from which the bromine departs. The transition state has some of the character of a carbocation. We know that more highly substitnted carbocations are more stable than less highly snbstituted ones therefore, when the bromonium ion ring opens, it does so by breaking the bond between bromine and the more snbstituted carbon. 

FIGURE 6.13 Mechanism of bromohydrin formation from cyclopentene. A bridged bromonium ion is formed and is attacked by a water molecule from the side opposite the carbon-bromine bond. The bromine and the hydroxyl group are trans to each other in the product. 

The idea that a cyclic bromonium ion was an intermediate was a novel concept when it was first proposed. Much additional evidence, including the isolation of a stable cyclic bromonium ion, has been obtained since then to support it. Similarly, cyclic chloronium ions are believed to be involved in the addition of chlorine to alkenes. In the next section we shall see how cyclic chloronium and bromonium ions (halonium ions) are intermediates in a second reaction involving alkenes and halogens. 

Halohydrin formation, as depicted in Mechanism 6.7, is mechanistically related to halogen addition to alkenes. A halonium ion intermediate is formed, which is attacked by water in aqueous solution. 

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Regioselectivity of Hydrogen Halide Addition 6.17 Conversion of Alkenes to Vicinal Halohydrins 253... 

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