Formation of a bromonium ion intermediate by electrophilic addition of Br + to an alkene.
Alkene p electrons attack bromine, pushing out bromide ion and leaving a bromo carbocation .This is heterolytic cleavage of bond in which shared pair of electrons are taken by bromide ion (anion) and other bromine atom is left with a positive charge (cation).
The neighboring bromo substituent stabilizes the positive charge on the carbon by using two of its electrons to overlap the vacant carbon p orbital, giving a three membered-ring bromonium intermediate .
Bromide ion attacks on the carbon of three membered ring from the opposite side of the ring (anti addition) because ring side is completely blocked by bromonium ion .
Unstable three membered ring opens up to give a neutral dibromoproduct with anti geometry .