Reactions of Alkenes (Halogenation)
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 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.