A Dibromination Dibromance Part I

This year we performed a dihalogenation reaction that is featured in the reactions of alkenes: a topic  typically covered during first semester Organic Chemistry. This reaction has been a popular undergraduate experiment for decades. I did quite a bit of background research in setting this one up. The two most popular substrates are trans-cinnamic acid and trans-stilbene. Both give solid products when dibrominated. The dibromination reaction can be studied from the point of view of stereochemistry of addition. For example, the dibromination of cinnamic acid has to possibility of creating two pairs of enantiomers while the same reaction with stilbene has the possibility of creating a meso compound and a pair of enantiomers. As far as I am aware only the meso-1,2-dibromo-1,2-diphenylethane is available commercially for a reasonable price to use as a standard. Chemspider gives the systematic name of “meso-dibromostilbene “ as [(1R,2S)-1,2-Dibromo-2-phenylethyl]benzene. I would prefer to call it meso-1,2-dibromo-1,2-diphenylethane. Cis-stilbene is commercially available but is more than a 100x more expensive than trans-stilbene. Cis-cinnamic acid is not readily available. An interesting study published in the Journal of Chemical Education, The Addition of Bromine to 1,2-Diphenylethene, was done comparing the dibromo products of trans and cis-stilbene. Generally, cis-stilbene was more likely to give significant amounts of both isomers compared with the trans-stilbene which produces an excess of a single diastereomer. Bromination of cis-stilbene with elemental bromine in dichloromethane gave almost equal amounts of the two diastereomers! The authors remark that since crystallization is the routine method of recovering the dibrominated product, the R,R/S,S diastereomer is simply not recovered.


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