COMPARISON OF THE MECHANISMS OF THE BROMINATION AND OXYMERCURATION REACTIONS OF ALKENES

An NMR method is used to measure the formation constants for complexation of mercuric chloride and silver triflate with a series of alkenes. The trends for mercury complexation parallel the argentation constants. These data are used in a discussion of the mechanism of both the bromination and oxymercuration of alkenes. Arguments are presented that the two electrophilic addition reactions have different rate-limiting steps. In the bromination reaction the rate-determining step is the formation of a bromonium ion intermediate while oxymercuration proceeds by rate-limiting attack by solvent on a mercuronium ion intermediate in agreement with the accepted mechanism.