The Addition of Bromine to 1,2-Diphenylethene

Abstract

We investigated the reaction of (Z)-1,2-diphenylethene (cis-stilbene) with various brominating reagents and solvents following directions in standard organic chemistry manuals. We were particularly interested in learning which combination of brominating reagent and solvent gave the best yield of (d,l)-1,2-dibromo-1,2-diphenylethane without the formation of significant amounts of meso-1,2-dibromo-1,2-diphenylethane, which is essentially the sole product from the reaction of bromine with (E)-1,2-diphenylethene (trans-stilbene). Based on the results from the standard preparatory methods, some permutations of solvent and brominating reagent were tried. Using pyridinium bromide perbromide in either glacial acetic acid or dichloromethane gave high (about 80%) yields of the d,l product from (Z)-1,2-diphenylethene. Bromine in glacial acetic acid gave 66% of the d,l product and 20% of the meso product from (Z)-1,2-diphenylethene. However, bromine in dichloromethane gave about equal amounts of the two dibromo products. The various combinations tried could be used in an introductory organic chemistry laboratory to illustrate how changing solvents or reagents affects product formation. For an advanced organic chemistry laboratory, the students could carry out the various combinations and then be asked to provide an explanation for the results.