Summary.

• Overall transformation :  C=C to HO-C-C-X
• Reagents : X₂ / H₂O or HOX (hypohalous acid)
• Reaction proceeds via cyclic halonium ion (compare with halogenation)
• In the presence of an alternative nucleophile, water opens the halonium ion.
• Regioselectivity : X reacts as the electrophile so the C-O bond forms at the more stable cation center.
• Stereoselectivity : anti since the two new s bonds form in separate steps.

MECHANISM FOR REACTION OF ALKENES WITH Br2 / H2O
Step 1: Same first step as for the reaction of Br2/CH2Cl2. 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 water molecule from the side away from the bromonium center in an SN2 like fashion opens the cyclic bromonium ion to give overall trans addition.
Step 3: An acid / base reaction converts the oxonium into the alcohol.

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