Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Addition Reactions Involving Alkenes and Alkynes
8.4 Addition of Halogens to Alkenes (Halogenation of Alkenes)
Chlorine and bromine readily add across the carbon—carbon double bonds of alkenes. The reaction of alkenes with bromine is often used as a test for the presence of carbon—carbon double bond functionality in compounds. The reddish bromine solution turns colorless in the presence of alkenes due to the reaction of both reagents. Since bromine (Br2) is a large polarizable molecule (molecular weight is 160 amu), it is possible that at any instant the molecule is polarized. That is, a partial positive charge resides at one end of this large molecule and a partial negative charge resides at the other end. Since the pi (π) electrons of the double bond of alkenes are nucleophilic, a reaction of the nucleophilic alkene and the partial positive end of the polarized bromine molecule will occur as shown in Reaction (8-43) to form a bromonium ion and a bromide anion.
(8-43)
The bromonium ion intermediate is relatively stable since the positive charge is distributed throughout the three atoms, including the very large bromine atom, which is located just above the carbon—carbon bond. Owing to the unusual stability of the bromonium ion, it will not rearrange like some carbocations. Even though the bromonium ion is relatively stable, it is reactive and will react with a nucleophile since it is electrophilic. Since the bromide anion that was generated in Reaction (8-43) is also nucleophilic, it is possible for it to attack the bromonium ion to give a neutral product, as shown in Reaction (8-44).
(8-44)
There is one very important observation that should be made about this last step of the reaction mechanism; the nucleophilic bromide anion attacks the electrophilic bromonium ion opposite to the side that has the bromine of the bromonium ion. The very large bromine atom occupies most of the space of one side of the bromonium ion, and as a result, the nucleophilic bromide anion adds to the opposite side. Hence, the stereochemistry of the product that results is a trans product. As pointed out earlier, the bromonium ion does not rearrange, and as a result, there are typically no unexpected products observed for the bromination of alkene addition reactions. Shown in Reaction (8-45) is the addition of bromine to a 4-methyl-2-pentene.
(8-45)
The trans-addition product is given in Reaction (8-45) showing two different representations, the dashed-wedge and Newman projection. This reaction represents an ideal application of the use of the stereochemistry concepts and representations that were learned in earlier chapters. A similar type of reaction of alkenes with chlorine is expected, but since chlorine is not as large as the bromine molecule, the chloronium ion is not as stable as the bromonium ion. As a result, the stereochemical outcome (trans isomer) is not as pronounced, compared to the reaction of alkenes with the bromine. Since a specific stereoisomer is formed from either a trans or cis alkene, this type of reaction is described as a stereospecific reaction. A stereospecific reaction is one in which the stereochemistry of the reactant completely dictates the stereochemistry of the product.
Problem 8.7
Give the major product that results from the addition of bromine to the following alkenes, indicate stereochemistry where appropriate.
