The addition of bromine to alkene is a stereospecific reaction. For example, the addition of bromine with cyclopentane gives, trans -1,2-dibromocyclopentane, an anti -addition product (Scheme 4).

Scheme 4

4.6.3 Halohydrin Formation

Halohydrin can be obtained when the halogenation of an alkene is carried out in aqueous solution. If the halogen is bromine, it is called a bromohydrin, and if chlorine, it is called as a chlorohydrin. For example, the reaction of 2-methylpropene with bromine in the presence of water gives bromohydrin along with a vicinal dibromide as a minor product (Scheme 5).

Scheme 5

The first step is the same as that the above described halogen addition reaction. In the second step, water acts as the nucleophile attacking the carbon atom of the bromonium ion to give the target bromohydrin (Scheme 6).

Scheme 6

If the alkene is unsymmetrical, the bromine ends up on the carbon atom with the greater number of hydrogen atoms. The more highly substituted carbon atom bears the greater positive charge. Consequently, water attacks this carbon atom preferentially. But symmetrical alkenes usually give a racemic mixture (Scheme 7).

Scheme 7

4.6.4 Oxymercuration–Demercuration

Oxymercuration-demercuration is another method for converting alkenes to alcohols. Many alkenes do not easily undergo hydration in aqueous acid. Some alkenes are nearly insoluble in aqueous acid, and others undergo side reactions such as rearrangement, polymerization. Thus, the use of oxymercuration–demercuration process has two advantages over acid-catalyzed addition: (i) it does not require acidic conditions and (ii) no carbocation intermediate is invovled, so that rearrangements do not occur. For example, the oxymercuration of propene with mercuric acetate gives the organomercurial alcohol, which is reduced to 2-propanol, called demercuration, by sodium borohydride (Scheme 8).

Scheme 8

Regarding the mechanism, the first step, the oxymercuration involves an electrophilic attack on the double bond by the positively charged mercury species to give mercurinium ion, an organometallic cation containing a three-membered ring. In the second step, water attacks the mercurinium ion to give an organomercurial alcohol. A subsequent reaction with sodium borohydride removes the mercuric acetate fragment with a hydrogen atom to give the alcohol (Scheme 9).

Scheme 9