Question

Bromination of Stilbene

1. Analyze your product in terms of purity (based on your experimental melting point range). Stilbene is highly sterically hindered and is therefore not nearly as stable as transstilbene. cis-Stilbene has a melting point of 5-6°C and trans-stilbene has a melting point of 124-125°C. Reacting either of these compounds with Br2 should produce 1,2-dibromo- 1,2-diphenylethane. This product has four possible structures. Two of them are actually the same (meso structures) and the other two contain two chiral centers, which are optically active and mirror images of each other. The meso structure has a melting point of 238-243

Experimental melting point is 224-235C.

Analyses based on the melting point and its relation ship to stereochemistry.

2.Analysis of theoretical atom economy and theoretical atom economy

Theoretical =90%

Experimental=37

Answer 1

1. The more stable an isomer is, the higher would be its stability and thus higher melting point is observed for trans-stilbene than cis-stilbene isomer. Bromination of cis-stilbene yields a racemic mixture of products and trans-stilbene yields meso product. Due to the symmetry in the molecule, meso isomer of dibromo-product has higher melting point.

2. Expeirmental the yield of the reaction is lower than theoretical and thus the atom economy value also is lower.