odifan ahol isedthe -m. moeeoateseemacgbteaatonh CH rea to恤 12-7yme ercuration:A Special and re0的 tateaooeaceh 55 Vicinal haloethers can be produced if an alcohol is used as the solvent, rather than water. Halonium ion opening can be regioselective. Mixed additions to double bonds can be regioselective: The nucleophile attacks the more highly substituted carbon of the bromonium ion, because it is more positively polarized. Electrophilic additions of unsymmetric reagents add in a Markovnikov-like fashion: The electrophilic unit becomes attached to the less substituted carbon of the double bond. Mixtures of products are formed only when the two carbons are not sufficiently differentiated. Reagents of the type A-B, in which A acts as the electrophile, A+, and B the nucleophile, B- , can undergo stereo- and regiospecific addition reactions to alkenes: Oxymercuration-Demercuration: A Special Electrophilic Addition 12-7 The electrophilic addition of a mercuric salt to an alkene is called mercuration. The product formed is known as an alkylmercury derivative. A reaction sequence known as “oxymercuration-demercuration” is a useful alternative to acid-catalyzed hydration: Oxymercuration is anti stereospecific and regioselective. The alcohol obtained from oxymercuration-demercuration is the same as that obtained from Markovnikov hydration, however, since no carbocation is involved in the reaction mechanism, rearrangements of the transition state do not occur