Electrophilic addition of halogens

The reaction of either the enol or the enolate anion (Equations 17-2 or 17-3) with Br2 resembles the first step in the electrophilic addition of halogens to carbon-carbon multiple bonds (Section 10-3A). However, the second step,... 

For electrophilic additions of halogens to alkenes, not only is the reaction rate strongly solvent-dependent [79-81] [cf. Eq. (5-29) in Section 5.3.2), but the stereochemical course may also be affected by the polarity of the medium [79, 386-388], For example, the stereoselectivity of bromine addition to cis- and trans -stilbene according to Eq. (5-140) has been found to be solvent-dependent, as shown in Table 5-23 [79, 386],... 

A reaction pathway involving heterolytic cleavage of the B—B bond might be expected to yield trans addition products if intermediates such as a coordinated boronium ion (VI) or a coordinatively stabilized carbonium ion (VII) were involved or nonspecific addition if a carbonium ion intermediate such as (VIII) were produced. The formally similar electrophilic addition of halogen to simple olefins is, of course, well known to lead to... 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

Electrophilic addition of halogen passes through either a three-membered cyclic cation (49) or an open vinyl cation (50). The difference between these cations will not be a distinct one because asymmetric three-membered intermediates (51) are quite well possible. If in 51 one of the two bonds is weak, the intermediate resembles an open cation, e.g. 50. 

Scheme 5.1-15 Synthesis of vicinal dihaloalkanes and dihaloalkenes by electrophilic addition of halogens to double and triple bonds in various ionic liquids.

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