Electrophilic addition mechanism consists of two steps

Before constructing the mechanism let us summarize conditions for this reaction. We will use Br₂ in our example for halogenation of ethylene.

Step 1: In the first step of the addition the Br-Br bond polarizes, heterolytic cleavage occurs and Br with the positive charge forms a intermediate cycle with the double bond.

Step 2: In the second step, bromide anion attacks any carbon of the bridged bromonium ion from the back side of the cycle. Cycle opens up and two halogens are in the position anti.