Halogen addition reaction

A halogen addition reaction is a simple organic reaction where a halogen molecule is added to the carbon-carbon double bond[?] of an alkene functional group.

Here is the general chemical formula of the halogen addition reaction:

C=C + X₂ in a nonreactive[?] solvent --> X-C=C-X

(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.