Chapter 6: Reactions of Alkenes: Addition Reactions

Halohydration of Alkenes

Summary

• Overall transformation :  C=C to HO-C-C-X
• Reagent : the halogen (e.g. Br₂) in water, or the hypohalous acid, HOX.
• Electrophile : "X+"
• Reaction proceeds via cyclic halonium ion (compare with halogenation)
• The alternative nucleophile, water opens the halonium ion (instead of the halide 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 σ bonds form in separate steps.

• Halogenation of alkenes
• Hydration of alkenes

MECHANISM FOR REACTION OF ALKENES WITH Br2 / H2O
Step 1: Same first step as for the reaction of Br2/CH2Cl2.  The π 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 opens the cyclic bromonium ion to give overall trans addition.
Step 3: An acid / base reaction converts the oxonium into the alcohol.

© Dr. Ian Hunt, Department of Chemistry