Question

Write the reaction for the bromination of cis-stilbene and draw all the stereoisometric products. What is the relationship between the products?

Answer 1

Since, the cis and trans isomer of stilbene have different geometries, it follows that upon reaction with bromine they give rise to steriomeric bromonium ions and, eventually, products that differ only by their steriochemistry.

     The product of this reaction, 1,2-dibromo-1,2-diphenylethane, has two steriogenic centres, which normally leads one to predict four possible sterioisomers. However, there are only three possible isomers in the case of 1,2-dibromo-1,2-diphenylethane since one, the so-called meso isomer, is superimposable on its mirror image. The meso isomer does contain steriogenic centers, but since it is superimposable on its mirror image, it does not rotate the plane of polarized light. The other isomeric product is a recemic mixture. A recemic mixture contain an 50:50 mixture of the two molecules that are non-superimposable mirror images of each other.