Electrophilic additions to alkenes can be stereospecific

Although they really belong in Chapter 15 with other nucleophilic substitution reactions, we included the last few examples of epoxide-opening reactions here because they have many things in common with the reactions of bromonium ions. Now we are going to make the analogy work the other way by looking at the stereochemistry of the reactions of bromonium ions, and hence at the stereoselectivity of electrophilic additions to alkenes. We shall first remind you of an epoxide reaction from Chapter IS, where you saw this. 

Now we can move on to look at the stereochemistry of electrophilic addition to alkenes. 

When cyclohexene is treated with bromine in carbon tetrachloride, the racemic anti-, 2-6 -bromocyclohexane is obtained exclusively. 

The result is no surprise if we think first of the formation of the bromonium ion that is opened with inversion in an Sn2 reaction. 

Bromination of alkenes is stereospecific because the geometry of the starting alkene determines which product diastereoisomer is obtained. We couldn t demonstrate this with cyclohexene because only a Z double bond is possible in a six-membered ring. But bromination or chlorination of Z and -2-butene in acetic acid produces a single diastereoisomer in each case, and they are different from each other. Anti addition occurs in both cases—more evidence that a bromonium ion is the intermediate. 

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