Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride. Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate. A bromonium ion is more stable than the analogous carbocation, as it has more covalent bonds and all the atoms have filled octets. In the second step, the nucleophile, a bromide ion, attacks one of the carbon atoms in the bridged bromonium ion. Due to the non-availability of bonding orbitals and steric crowding, the nucleophile approaches the antibonding orbitals, pointing opposite to the carbon-bromine bond. This accounts for the anti addition. Thus, the addition of two bromine atoms takes place from the opposite faces of the double bond in cyclopentene to yield trans-1,2-dibromocyclopentane. The configuration of the starting alkene decides the stereochemical outcome for halogenation reactions. For example, the addition across cis-2-butene generates a pair of enantiomers, while addition across trans-2-butene produces a meso compound. Therefore, the halogenation of alkenes is a diastereospecific reaction.