8.3 : هلجنة الألكينات

The addition of bromine or chlorine to an alkene produces a 1,2-dihalide product. Halogenation of alkenes is typically performed in non-nucleophilic solvents, like methylene chloride, chloroform, or carbon tetrachloride.

Consider the bromination of cyclopentene to obtain trans-1,2-dibromocyclopentane. The reaction is initiated by the polarization of molecular bromine in the proximity of the electron-rich π bond of an alkene. The electrophilic bromine adds across the alkene double bond generating a bromonium ion.  

This cyclic intermediate is more stable than a carbocation, as it contains more covalent bonds, and all the atoms have a filled octet.

However, the ring strain and a positive charge on the electronegative bromine make the bromonium ion susceptible to the nucleophilic attack by the bromide ion. The leaving group is the bromine in the intermediate and it remains tethered by a bond to the other carbon.

Due to the steric overcrowding by the cyclic bromonium ion and non-availability of bonding orbitals, the nucleophile approaches the antibonding orbital, pointing opposite to the carbon–bromine bond, leading to the anti addition. Thus, only trans-1,2-dibromocyclopentane is obtained as a product and not cis-1,2-dibromocyclopentane.

Furthermore, as both of the carbons of a bromonium ion are accessible to the nucleophile, the product is a racemic mixture.

The different stereoisomers of the starting alkene give different stereoisomers of the product. The bromination of cis-2-butene produces a pair of enantiomers, while another stereoisomer, trans-2-butene, forms a meso compound. Therefore, the halogenation of alkenes is a stereospecific reaction.

In a simple qualitative test for the presence of olefinic-double bonds, when bromine is added to an alkene, the deep red-colored bromine solution turns colorless, as the bromine adds across the double bond. Alternatively, a solution of bromine in an alkane retains a deep red color. 

Although the bromination and chlorination of alkenes are common, the addition of fluorine and iodine are not synthetically useful. Fluorine reacts too vigorously,  and the iodination products decompose back to the alkene and iodine quickly.