Electrophilic Addition of Bromine to Alkenes

One of the classic reactions of organic chemistry is the addition of bromine to an alkene. We will first summarize the evidence that led to the textbook mechanism (shown in Figme 9.2), and then we will survey experimental data that reveal the complexities underlying this simple mechanistic representation. 

The intermediate is a bromonium ion, a three-membered ring containing a bromine atom bearing a formal charge of +1. The structure is also called a a complex, because the valence bond representation implies a full or partial r bond between the olefinic carbon atoms and the bromine atom.  

The attachment of the two bromine atoms to the carbon-carbon double bond is anti. This stereochemistry results because a bromide ion attacks the back side of one of the bromonium ion C-Br bonds in a 

Schmid, G. H. Garratt, D. G. in Patai, S., Ed. The Chemistry of Double-bonded Functional Groups, Supplement A, Part 2 John Wiley Sons London, 1977 pp. 725-912. 

The results of ab initio calculations (reference 75) suggested that it might be more correctly described as a strong jt-complex. 

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Ionic reactions of neutral substrates can show large solvent dependence, due to the differential solvent stabilization of the ionic intermediates and their associated dipolar transition states (Reichardt, 1988). This is the case for the electrophilic addition of bromine to alkenes (Ruasse, 1990, 1992 Ruasse et al., 1991) and the bromination of phenol (Tee and Bennett, 1988a), both of which have Grunwald-Winstein m values approximately equal to 1 so that the reactions are very much slower in media less polar than water. Such processes, therefore, would be expected to be retarded or even inhibited by CDs for two reasons (a) the formation of complexes with the CD lowers the free concentrations of the reactants and (b) slower reaction within the microenvironment of the less polar CD cavity (if it were sterically possible). 

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 addition of bromine to alkenes similarly involves a ],2-trcms addition. 

The electrophilic addition of bromine to alkenes is an oxidation. The starting alkene is equivalent in oxidation level to an alcohol, but the product has two carbons at the alcohol oxidation level—the elimination reactions of dibromides to give alkynes that you met in Chapter 17 (p. 398) should convince you of this. There are a number of other oxidants containing electrophilic oxygen atoms that react with nucleophilic alkenes to produce epoxides (oxiranes). You can view epoxides as the oxygen analogues of bromonium ions, but unlike most bromonium ions they are quite stable. 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

We shall give particular attention here to the addition of bromine to alkenes because this reaction is carried out very conveniently in the laboratory and illustrates a number of important points about electrophilic addition reactions. Much of what follows applies to addition of the other halogens, except fluorine. 

Reaction (a) is an electrophilic addition of bromine to an alkene the appropriate reagent is bromine in carbon tetrachloride. 

Higher alkynes can be synthesised from alkenes through a two-step process which involves the electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base(Following fig.). The second stage involves the loss of two molecules of hydrogen bromide and so two equivalents of base are required. 

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],... 

Electrophilic addition of acids to alkenes involves two steps, the first being attachment of hydrogen ion to form the carbonium ion. What is the mechanism of the addition of chlorine and bromine ... 

Cations react rapidly and indiscriminately with nucleophiles. Thus if a reaction which is suspected to go via a cationic intermediate is carried out in the presence of an added nucleophile, and an adduct containing the new nucleophile is obtained, this provides evidence for a cationic intermediate which is trapped by the added nucleophile. For example, the addition of bromine to alkenes is thought to go via a cationic intermediate 4 (reaction 5.11). If chloride or nitrite ions are added to the reaction mixture, the chloride or nitrite adducts 5 and 6 are obtained, even though the chloride and nitrite ions do not react with ethene (or 1,2-dibromoethane) directly at a rate that would account for the amount of these products in the reaction mixture. This provides strong evidence for the intermediate bromoethyl cation 4, which will be trapped by the added chloride or nitrite ions (reactions 5.12 and 5.13). The structure of bromoalkyl cations is discussed further in the section on electrophilic addition. 

The regioselectivity of electrophilic addition of HBr to alkenes is controlled by the tendency of a proton to add to the double bond to produce the more stable carbocation. Under free-radical conditions the regioselectivity is governed by addition of a bromine atom to give the more stable alkyl radical. 

Possible role for electrophilic properties of solvent in addition of bromine to alkenes. 

Addition of bronrine to an alkene is much more complicated than the simple representation in Figure 9.2 would suggest. The classical bromonium ion description of electrophilic addition of bromine to an alkene is useful only as a beginning point to describe the mechanistic options. The structure of the intermediate, the kinetics of the reaction, and both the stereochemistry and the regiochemistry of the products are all complex functions of the nature and concentration of tiie brominating agent, the solvent, any added nucleophiles, and the structure of the alkene. 

It has been proposed that there is a close analogy between the SnI reaction and the electrophilic addition of bromine to an alkene. Compare and contrast the two reactions in terms of (a) gross mechanistic features (especially charge development), (b) substituent effects, and (c) solvent effects. 

The absence of rearrangement and the anti stereochemistry of the addition product must be accommodated in a proposed mechanism for the addition of a halogen to an alkene involving a carbocation intermediate. The first step of the reaction mechanism is the electrophilic addition of bromine to the Jt bond to give a three-membered ring called a tydic bromonium ion. 

Based on what we ve seen thus far, a possible mechanism for the reaction of bromine with alkenes might involve electrophilic addition of Br+ to the alkene, giving a carbocation that could undergo further reaction with Br- to yield the dibromo addition product. 

Displacement Mechanisms. In these reactions the organic substrate uses its electrons to cause displacement on an electrophilic oxidizing agent. One example is the addition of bromine to an alkene (15-37). 

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