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Reactions of Alkenes (Halogenation)

Halohydrin Reaction

If halogen addition to alkenes is carried out in the presence of other nucleophiles, these nucleophiles compete with the halide ions to open the intermediate three-membered ring. The bromination of ethene in the presence of chloride ions yields 1,2-dibromoethane as well as 1-bromo-2-chloroethane, for example.

Influence of solvents
If the bromination or chlorination of alkenes is carried out in a nucleophilic solvent such as H2O or an alcohol, the nucleophilic ring opening of the bromonium or chloronium ion proceeds by attack of a solvent molecule rather than by a halide ion.

Therefore, if water is used as a solvent the main result of the halogenation is not a vicinal dihalide but an α-halogenated alcohol, the halohydrin. The reaction is called the halohydrin reaction.

Halohydrin reaction.

Halohydrin reactions are useful for investigating the regioselectivity of the anti addition. If asymmetrically-substituted alkenes are employed, the two C-X bonds of the halonium ion possess different lengths. The halonium ion with the weaker and thus longer C-X bond to the higher-substituted carbon atom is energetically favored, because the higher partially positive charge is more stabilized on the higher-substituted carbon atom by the +I effect of the alkyl substituents and hyperconjugation. Therefore, the activation energy for the bond cleavage of this C-X bond is lower and the halonium ion ring is opened by the nucleophilic attack of a solvent molecule on the higher substituted carbon.


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