Summary.

• Overall transformation :  C=C to X-C-C-X
• Reagent : normally the halogen (e.g. Br₂) in an inert solvent like methylene chloride, CH₂Cl₂.
• Regioselectivity : not relevant since both new bonds are the same, C-X.
• Reaction proceeds via cyclic halonium ion.
• Stereoselectivity : anti since the two C-X bonds form in separate steps one from Br₂ the bromide ion Br^-.

MECHANISM FOR REACTION OF ALKENES WITH HALOGENS
Step 1: The p electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cationic cyclic bromonium ion as an intermediate.
Step 2: Attack of the nucleophilic bromide from the side away from the bromonium center in an SN2 like fashion opens the cyclic bromonium ion to give overall trans addition.

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