The Generality of Electrophilic Addition

Since we start with achiral starting materials, all products (if chiral) are racemates (Section 5-7). If the first step of addition of bromine to the double bond had given a carbocation, the bromide ion released in this process could have attacked the positively charged carbon atom from either side, resulting in both syn and anti addition products, which is not observed. 

Ring opening of bromonium ion by backside attack leads to anti addition products. 

In Summary Halogens add as electrophiles to alkenes, producing vicinal dihalides. The reaction begins with the formation of a bridged halonium ion. This intermediate is opened stereospecifically by the halide ion displaced in the initial step to give overall anti addition to the double bond. In subsequent sections, we shall see that other stereochemical outcomes are possible, depending on the electrophile. 

Halogens are just one of many electrophile-nucleophile combinations that add to alkene double bonds. In this section we begin a survey of some of the most important of these processes, beginning with addition of halogens (the electrophile source) in the presence of water (the nucleophile). The products, 2-haloalcohols, commonly known as halohydrins, are widely used in a number of industrial and synthetic applications. In particular, they are important intermediates for the synthesis of oxacyclopropanes (epoxides. Section 9-9). 

The creation of a bromonium ion in alkene brominations suggests that, in the presence of other nucleophiles, competition might be observed in the trapping of the intermediate. For example, bromination of cyclopentene in water as solvent gives the vicinal bromoalcohol (common name, bromohydrin). In this case, the bromonium ion is attacked by water, which is present in large excess. The net transformation is the anti addition of Br and OH to the double bond. The other product formed is HBr. The corresponding chloroalcohols (chlorohydrins) can be made from chlorine in water through a chloronium ion intermediate. 

---