Reactions of Alkenes (Halogenation)
Halogenation of Alkenes
Electrophiles add to the double bond of alkenes breaking the π bond. In contrast to the hydrogenation of alkenes, the addition of molecular bromine or chlorine to produce vicinal dihalogenides does not require catalysts. Bromine or chlorine are the best choices for the halogenation of alkenes. Fluorine reacts very vigorously to form not only fluorinated but also several oxidation products. The addition of iodine to alkenes is thermodynamically unfavorable.
The addition of halogens (bromine or chlorine) to the double bond of alkenes could follow several mechanisms.
- A mechanism analogous to the hydrogenation of alkenes would add the halogen atoms from the the same side and only yield the cis product.
- A two-step mechanism proceeding through an intermediate carbenium ion similar to the addition of hydrogen halides to double bonds would form the cis as well as the trans product. For example, the bromination of cyclopentene would yield cis-1,2-cyclopentane and trans-1,2-cyclopentane.
However, only the trans product is observed, indicating that anti addition occurs selectively. Therefore the actual mechanism of the halogenation is obviously different from that of hydrogenation and HX addition.
Movie: Addition of bromine to cyclohexene on a laboratory scale.
Polarizability of halogens
A bromine molecule is polarized by the nucleophilic π electron cloud on approaching the alkene thus shifting the electron density of the bromine molecule. The result is a partially positive and a partially negatively charged bromine atom.
Subsequently, the bromine-bromine bond is cleaved heterolytically and an intermediate cyclic bromonium ion is formed by the addition of the partially positively charged bromine atom to the double bond. The bromonium ion is a three-membered ring in which the two single-occupied p orbitals of the bromine cation overlap with the two single-occupied p orbitals of the π bond of the alkene. The postulated existence of a cyclic bromonium ion explains the stereochemical outcome of the halogenation. For steric reasons, nucleophilic attack of the bromine anion at the rigid three-membered ring can only occur from the side opposite to the bromine atom in the ring. The ring opening reaction follows a mechanism. In symmetrically-substituted alkenes no preference of attack at a particular ring carbon atom is observed (and it is not expected) resulting in a racemic compound as the final product.
The bromination and chlorination of alkenes is usually carried out in non-nucleophilic solvents such as CCl4. If the reaction is carried out in a nucleophilic solvent, such as H2O or an alcohol, the attacking species in the nucleophilic ring opening of the cyclic intermediate is the solvent molecule rather than the halide ion. In the case of water as the solvent, the result of the reaction is a halohydrin (R2XC-C(OH)R2). In the halohydrin reaction, the reagents, HOCl or HOBr, are often generated in situ by reaction of water with chlorine or bromine, respectively. Under specific reaction conditions, HOI also adds to double bonds.