Oxymercuration-demercuration is usually preferred over acidic hydration to form Markovnikov alcohols from alkenes. The reaction begins with electrophilic approach onto the alkene by mercuric acetate to form a mercuric acetate carbocation derivative. The mercuric acetate moeity stabilizes the carbocation, so oxymercuration-demercuration is not nearly as susceptible to rearrangement as ordinary acid-catalized hydration. Following addition of mercuric acetate, hydration then occurs followed by a demercuration step with sodium borohydride.