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17.3: Stepwise Addition of Electrophiles to π Bonds with Formation of 3-Membered Ring Intermediates

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  • Sometimes the electrophilic part E of an electrophile E-Y approaches the C=C π-bond to simultaneously bond to both carbons and form a three membered ring intermediate with departure of Y as a leaving group. In a second step, Y can act as a nucleophile to perform backside attack on one of the carbons, prompting ring opening that results in net anti addition of E-Y to the C=C bond.


    The oxymercuration-demercuration reaction (addition of water in the presence of mercury salts), halogen addition, and the anti-hydroxylation (via epoxides) reaction are examples of this outcome.