Electrophilic Addition of Br2 to Alkenes

Halonium ion (Section 7.2) A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of Br2 to alkenes. 

The result is no surprise if we think hrst of the formation of the bromonium ion that is opened with inversion in an Sn2 reaction. Here is the mechanism drawn flat , which is all we need to explain the stereochemistry of the product. The fact that this reaction (like other similar ones) gives a single diastereoisomer is one of the best pieces of evidence that electrophilic additions of Br2 to alkenes proceed through a bromonium ion. 

Low-temperature experiments have since confirmed the existence of bromonium ions as the intermediate in electrophilic additions of Br2 to alkenes, including additions to wmcyclic alkenes such as the one in Model 1. 

The reagent used to form the bromonium ion here is not bromine, and may be new to you. It is called N-bromosuccinimide, or NBS for short. Unlike the noxious brown liquid bromine, NBS is an easily handled crystalline solid, and is perfect for electrophilic addition of bromine to alkenes when the bromonium ion is not intended to be opened by Br-. It works by providing a very small concentration of Br2 in solution a small amount of HBr is enough to get the reaction going, and thereafter every addition reaction produces another molecules of HBr which liberates more Br2 from NBS. In a sense, NBS is a source of Br+>. 

Electrophilic additions of Brs" to alkenes and alkynes have been carried out [48-50] both in [BMIM][Br] and in other ionic liquids bearing non-nucleophilic anions (Scheme 5.1-16). The reaction is always completely anti-stereospecific, independent of alkene or alkyne structure. It follows a second-order rate law, suggesting a concerted mechanism of the type reported for Brs" addition in aprotic molecular solvents, involving a product- and rate-determining nucleophilic attack by bromide on the alkene or alkyne-Br2 jt-complex initially formed. 

Fig. 5.4. Enthalpy profile for the electrophilic addition of Br2 (reactions proceeding towards the left) and for the electrophilic substitution by Br2 (reactions proceeding towards the right) of cyclohexene (top) and of benzene (bottom). Altogether, the facts presented here are likely to be prototypical of the chemoselectivity of all electrophilic reactions on alkenes versus benzenoid aromatic compounds. In detail, though, this need not be true both in the alkene and the aromatic compound AWSubstitution as well as AWadditio depend on the electrophile, which is why an electrophilic dependency can in principle also be expected for AAH = AWsubstitution - A//a(1(l t. ol. ...

In the first step of the oxymercuration mechanism, the electrophilic mercury of mercuric acetate adds to the double bond. (Two of mercury s 5d electrons are shown.) Because carbocation rearrangements do not occur, we can conclude that the product of the addition reaction is a cyclic mercurinium ion rather than a carbocation. The reaction is analogous to the addition of Br2 to an alkene to form a cyclic bromonium ion. 

Based on what we ve seen thus far, a possible mechanism for the reaction of bromine with alkenes might involve attack by the tr electron pair of the alkene on Br2, breaking the Br-Br bond and displacing Br ion. The net result would be electrophilic addition of Br+ to the alkene, giving a carbocation that could undergo further reaction with Br to yield the dibromo addition product ... 

An explanation for the observed stereochemistry of alkene addition came in 1937 with the suggestion that the reaction occurs through an intermediate bromonium ion (R2Br" ), formed hy electrophilic addition of Br" " to the alkene. (Similarly, a chloronium ion contains a positively charged, divalent chlorine, R2C1. ) The bromonium ion is formed in a single step hy interaction of the alkene with Br2 and simultaneous loss of Br (Figure 8.1). 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Electrophilic addition of Cl., and In tn nlkenes is similar in mechanism to the electrophilic addition of Br0.46 The rate of chlorination in acetic acid is second-order, first-order each in olefin and in chlorine.47 Predominantly anti addition to alkyl-substituted double bonds occurs, indicating that a chloronium ion is formed.48 Further evidence for the chloronium ion is that addition of hypo-chlorous acid to double bonds is not entirely regiospecific. For example, addition to propene gives 91 percent of the Markownikoff product 29, and 9 percent of the anti-Markownikoff product, 30. Phenyl-substituted alkenes give a mixture of syn and anti adducts with Cl2 as they do with Br2.49... 

We expect the reactions complementary to equations (1) and (2), namely electrophilic attacks, to be faster for alkenes than for alkynes. Thus, reactivity ratios (/-ii and rj2) for corresponding alkynes and alkenes (PhC CH, PhCH=CH and BuC CH, BuCH=CH2) in radical copolymerizations favour the alkene over the alkyne . Electrophilic additions of Br, CI2, ArSCl and H3O+ to alkenes are usually much faster than those to alkynes . However, A (C=C)/A (C=C) can vary from 10 to < 1 for the different electrophilic processes and by 10 for one process (Br2 addition) when the solvent is changed from HjO to HOAc . This unexpected trend in reactivity continues undiminished in the rates of acid-catalysed hydration... 

However, the bond joining the two halogen atoms is relatively weak (see the bond dissociation energies listed in Table 3.1) and, therefore, easily broken. When the tt electrons of the alkene approach a molecule of Br2 or CI2, one of the halogen atoms accepts the electrons and releases the shared electrons to the other halogen atom. Therefore, in an electrophilic addition reaction, Br2 behaves as if it were Br" " and Br , and CI2 behaves as if it were Cl and CF. 

The mechanism for the addition of oxygen to a double bond to form an epoxide is analogous to the mechanism described in Section 4.7 for the addition of bromine to a double bond to form a cyclic bromonium ion. In one case the electrophile is oxygen, and in the other it is bromine. So the reaction of an alkene with a peroxyacid, like the reaction of an alkene with Br2, is an electrophilic addition reaction. 

In behaviour that is typical of a 1,3-dipolar cycloaddition reaction, OSO4 reacts almost as well with electron-poor as with electron-rich alkenes. OSO4 simply chooses to attack the alk-ene HOMO or its LUMO, depending on which gives the best interaction. This is quite different from the electrophilic addition of m-CPBA or Br2 to alkenes. 

One possible mechanism for reactions of Bra" with alkenes involves the dissociation of Bra to Br2 and Br ion before reaction of Bra with the alkene. ° Alternatively, Bra" might act as an electrophile and add directly to the alkene. Bellucci and co-workers used the stopped-flow technique to study the kinetics of addition of bromine to cyclohexene in 1,2-dichloroethane solution. With molecular bromine the kinetics were overall third order, first order in cyclohexene and second order in Bra. 

Electrophilic addition of the halogens (CI2 and Br2) to alkenes occurs in a similar manner. AlAough the mechanism is not identical to that for acids, the end results are the same. For example, when a molecule of Br2 approaches the pi bond of an alkene,... 

Yet another example of an electrophilic addition is the reaction of alkenes with the hypohalous acids HO—Cl or HO-Br to yield 1,2-halo alcohols, called halohydrins. Halohydrin formation doesn t take place by direct reaction of an alkene with HOBr or HOC1, however. Rather, the addition is done indirectly by reaction of the alkene with either Br2 or Cl2 in the presence of water. 

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