Here is the general chemical formula of the halogen addition reaction:
The atom is electrophilic at this time and attacks the negatively charged, high energy pi bond portion of the alkene's carbon-carbon double bond. It forms for an instant a single sigma bond to both of the carbon atoms involved. The bonding of bromine is special in this intermediate, due to its relatively large size compared to carbon, the bromide ion is capable to latching onto both carbons which once formed the π-bond, making a three-membered ring. The bromide ion acquires a positive charge, which it shares slightly with the two carbon atoms. At this moment the halogen ion is called a "bromonium ion" or "chloronium ion", respectively.
When the first bromine atom attacks the carbon-carbon pi-bond, it leaves behind one of its electrons with the other bromine that it was bonded to in Br2. That other atom is now a negative bromide anion and is attracted to the slight positive charge on the carbon atoms. It is blocked from nucleophilic attack on one side of the carbon chain by the first bromine atom and can only attack on the other side. As it attacks and forms a bond with one of the carbons, the bond between the first bromine atom and the other carbon atoms breaks, leaving each carbon atom with a halogen substituent.
In this way the two halogens add in an anti addition fashion, and when the alkene is part of a cycle the dibromide adopts the trans configuration. For maximum overlap of the C-Br σ* antibonding molecular orbital (the LUMO, shown to the right in red) and the nucleophile (X−) lone pair (the HOMO, shown to the right below in green), X− must attack the bromonium ion from behind, at carbon.
This reaction mechamism was proposed by Roberts and Kimball in 1937 . With it they explained the observed trans-additions in brominations of maleic acid and fumaric acid. Maleic acid with a cis-double bond forms the dibromide as a mixture of enantiomers:
but both by anti-addition. The reaction is even stereoselective in alkenes with two bulky tert-butyl groups in a cis-position as in the compound cis-di-tert-butylethylene . Despite the steric repulsion present in the chloronium ion, the only product formed is the anti-adduct.
For reactions taking place through this mechanism no stereoselectivity is expected and indeed not found.
Roberts and Kimball in 1937 already accounted for the fact that brominations with the maleate ion resulted in cis-addition driven by repulsion between the negatively charged carboxylic acid anions being stronger than halonium ion formation. In alkenes such as anetholes and stilbenes the substituents are able to stabilize the carbocation by donating electrons at the expense of the halonium ion .
Halonium ions can be identified by means of NMR spectroscopy. In 1967 the group of George A. Olah obtained NMR spectra of tetramethylethylenebromonium ions by dissolving 2,3-dibromo-2,3-dimethylbutane in magic acid at -60°C . The spectrum for the corresponding fluorine compound on the other hand was consistent with a rapidly equilibrating pair of β-fluorocarbocations.