Oxymercuration reaction

"In the laboratory, alkenes are often hydrated by the oxymercuration–demercuration procedure. … Alkene oxymercuration is closely analogous to halohydrin formation."

"Treatment of an alkene with mercuric acetate in aqueous THF results in the electrophilic addition of mercuric ion to the double bond to form an intermediate mercurium ion. Nucleophilic attack by H2O at the more substituted carbon yields a stable organomercury compound, which upon addition of NaBH, undergoes reduction. Replacement of the carbon-mercury bond by a carbon-hydrogen bond during the reduction step proceeds via a radical process. The overall reaction represents Markovnikov hydration of a double bond, which contrasts with the hydroboration-oxidation process."