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Bromination of Trans-Cinnamic Acid

I recently did an experiment where the addition of bromine to trans-cinnamic acid gives a dibromide with two stereocenters (chiral centers). Thus, there were four stereoisomers possible (see figures on page 136 on attachment).

According to the melting points of my product, I ended up with the racemic mixture (2R,3S) and (2S,3R), erythro diastereomer (see attachment). EVERYTHING I have read points to the idea that this should be an anti addition, but the teaching assistant has just told me, no, this is a syn addition since we have ended up with the erythro diastereomer. How can this be? All the literature says that when a bromonium ion is formed in the bromination of an alkene, this is anti addition.

(1) Showing a lot of detail, please draw out the mechanism of the reaction of bromine with trans-cinnamic acid step-by-step for me. How does each bromine enter?
(2) Is it syn or anti addition or both?
(3) Isn't the erythro diastereomer a meso compound?


Solution Preview

a) See attachment for mechanism. To make things easier to follow the Br atoms are coloured. At the point when one Br atom (blue) is attached to double bond of the acid (e.i. bromonium ion is formed), one side of the ex-C=C double bond is hindered: the very one from which the blue Br atom attacked. ...

Solution Summary

Solution contains an illustration of the mechanism of the reaction of bromine with trans-cinnamic acid and an illustrated example of a meso combound, as well as full answer explanations. 220 words.