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Bromination of alkenes is stereospecific because the geometry of the starting alkene determines which diastereoisomer is obtained as the product. Bromination of Z– and E-2-butene in acetic acid produces a single diastereoisomer in each case, both of which are different from each other. Anti addition occurs in both cases.
The product of each reaction is shown in three different ways. Firstly with the new C-Br bonds in the plane of the paper, to highlight the inversion of configuration during the bromonium opening step. Secondly with the carbon chain in the plane of the paper to highlight the two different diastereoisomeric products, clearly showing that there has been anti-addition across the E double bond. Finally, the middle bond has been rotated 180° to give an (unrealistic) eclipsed conformation. This shows that the addition across the Z-butene is stereospecific and anti too, and also makes it clear that the product of the E-butene bromination is achiral, as you can see the plane of symmetry in the conformation.
S. E. Denmark, W. E. Kuester and M. T. Burk, Angew. Chemie Int. Ed., 2012, 51, 10938–10953.
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