The Product of the reaction of bromine with E-stilbene is optically inactive yet it has stereogenic centers. Why?

1 Answer
anor277
Dec 10, 2015

Because when a trans -olefin is brominated, the bromines are added from opposite sides; a meso (#RS#) form results.

Explanation:

When an olefin is brominated, the bromine atoms are presumed to add from opposite faces of the olefin (the first #Br# atom adds as an electrophile, and the second bromine atom from the opposite side as a #Br^-# ion (the positively charged bromonium ion intermediate necessitates this approach of #Br^-#). Given the original trans geometry, this addition from opposite faces results in an #RS# compound, a meso isomer, which are not optically active. Why not? Because meso compounds still have a mirror plane of symmetry.

On the other hand, when the cis olefin is brominated, this attack from opposite faces leads to #R*R# or #S*S# isomers, which are non-superposable mirror images, i.e. enantiomers.

You should do the bromination with simple models in order to satisfy yourself that this explanation accounts for the observed stereochemistry.

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