Alkene Addition Bromination Essay

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Old Dominion University
Alkene Addition
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Alkene addition: Bromination of (E) Stilbene

In this lab we used the greener approach, which involves the addition of bromine across a double bond. When bromine reacts with E-stilbene (trans-1,2-diphenylethene), two new chiral carbons are created from the sp2 carbons, therefore 3 different dibrominated stereoisomers are possible: meso-(1R,2S), or the raceminc mixture-(1R,2R) or (1S,2S)-dibromo-1,2-diphenylethane (Gilbert, 2010). When the bromination ion intermediate proceeds through a stereospecific mechanism, then the meso dibromide is formed exclusively. The racemic dibromides are formed from the concerted syn addition if the mechanism proceeds
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The first stage involves the formation of a cyclic bromonium ion intermediate and the second stage is a nucleophilic attack by a bromide ion on the intermediate, with inversion occurring at the carbon atom attacked, to yield a vicinal dibromide. In this experiment, two melting points were taken from the obtained product in order to confirm if the identity of the final product was meso-1,2-dibromo-1,2-diphenylethane. The first melting point obtained was 242.2°C-246.4°C; the next melting point was measured to be 243.0°C – 247.1°C. These values are indicative of a meso compound, as the established melting range for a meso compound is 241°C - 243°C (Gilbert, 2010). The average between these two gave a MP of 242.6°C -246.7°C, this shows a range of 4.1°C which means the obtained product was not pure, this indicates either the presence of impurities or it could also be that the product has a small content of the racemic product in it. The product obtained was identifies as meso diabromostilbene, in this mechanism it is presumed that the bromine added across the alkene’s pi bond forming a bridged intermediate. It is important to note that if two different groups are present on one or both of the sp2 carbon atoms of the alkene linkage, chiral carbon centers are generated on bromination of these carbon atoms (Mayo, 2010). The product mass obtained after drying the product was 0.275g. After the proper calculations a percent yield of 72.9% was estimated. The