Here is the general chemical formula of the halogen addition reaction:
(X represents the halogens bromine or chlorine, and in this case, nonreactive solvents could be CH2Cl2 or CCl4).
In the first step of the reaction, a bromine molecule approaches the electron-rich alkene carbon-carbon double bond[?]. The bromine atom closer to the bond takes on a partial positive charge as its electrons are repelled by the electrons of the double bond. 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, acquiring a positive charge, which it shares slightly with the two carbon atoms. (At this moment the halogen ion is called "bromonium[?]" or "chloronium[?]", respectively).
When the first bromine atom attacks the carbon-carbon pi-bond, it leaves behind one of its electrons with the other bronine that it was bonded to in Br2. That other atom is now a negative bromide[?] ion 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 with a halogen.
The two halogens always add anti[?] to each other, forming the trans[?] version of the molecule.
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