Addition of Bromine to Alkenes

Trans 1,2-bromination of an alkane proceeds wa a txomonium ion this cyclic intermediate is converted into the product by nucleophilic ring opening, this time by bromide. 

With molecular models, verify that an analogous nucleophilic attack at C(2) of 36 gives the same mesa compound. Confirm also that the configuration of 37 is (2R,3S). Show that cyclopentene (38) reacts similarly via a bromonium ion intermediate to give trans-1,2-dibromocyclopentane (39). 

Consistent with the mechanism proposed, Strating et al isolated a bromonium ion from addition of bromine to the hindered alkene 43 in fact the hindrance was such that the second stage, which would normally give the vicinal dibromoalkane product, did not occur. For an X-ray crystallographic structure determination of this bromonium ion, see Slebocka-Tilk et al4 

In the bromination of an alkene, one might ask can bromine approach the alkene sideways on, rather than from either the top or bottom Consider firstly the electron density of the double bond this is directed well away from, and perpendicular to, the plane of the molecule, as described in Chapter 1. 

From the congested diene 43, a stable bromonium ion has been isolated, and its structure determined. 

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Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

To account for this very high TRANS stereoselectivity, it has been suggested that addition proceeds via a cyclic bromonium radical (71), analogous to the cyclic bromonium cations involved in the polar addition of bromine to alkenes (p. 180) ... 

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 addition of bromine to alkenes is a rapid, exothermic reaction usually taking place at room temperature. In contrast to chlorination, the rate law in bromination depends on the solvent used. On passing from hydroxylic to nonpolar aprotic solvents, the overall second-order changes to a rate law that is first-order in alkene and second-order in bromine.226 Alkene-bromine complexes with varying compositions were shown to form under reaction conditions3,218,227 228(Scheme 6.5). At low bromine concentration in protic solvents the reaction proceeds via a 1 1 complex (23). A 1 2 alkene-bromine complex (25) is involved at high bromine concentration in nonprotic solvents. The ionic intermediates (24, 26) were shown to exist as contact ion pairs, solvent-separated ions, or dissociated ions. 

In one final example from the work carried out in our laboratories, it is interesting to consider the possibility of shape selection between chemically similar functional groups. This would be especially useful when catalytic sites within the pores of a zeolite were essential for reaction, but the selectivity could still be demonstrated even on uncatalysed reactions. We therefore considered the addition of bromine to alkenes. 

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. 

Bromine adds to 2-butene to form the v/c-dihalide, 2,3-dibro-mobutane- A student proposed the following two mechanisms for the addition of bromine to alkenes. 

The results of the experiments demonstrate that Mechanism B is correct. The addition of bromine to alkenes is ... 

Scheme 9.12 Products and stereochemistry in the addition of bromine to alkenes.

Fig. 22. Orbital symmetry correlation for the (forbidden) cis-addition of bromine to alkene.

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+>. 

Stereoselective bromiuation of olefins. Exclusive trans-addition of bromine to alkenes can be effected with this bromine-crown ether complex. Thus cis- and trans-jB-methylstyrene react with DBC-Brj to yield threo- and erylftro-l-phenyl-1,2-dibromopropane, respectively. Other brominating agents (Brj, PyBrj) are significantly less selective for this reaction. ... 

Making use of what you know about the addition of bromine to alkenes, what do you conclude about the stereochemistry of this elimination reaction Show the most likely mechanism for the elimination, including the part played by water (or acetylide ion). 

The bromonium ion has the same structure as that proposed two years earlier by Roberts and Kimball (Sec. 7.12) as an intermediate in the addition of bromine to alkenes. Here it is formed in a different way, but its reaction is the same, and so is the final product. 

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

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. 

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