Old Dominion University
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 through a carbocation.
To start this experiment, 0.200g of (E) Stilbene, white salt like compound with no specific aroma, were added to a round bottom flask weighting 17.73g. Then 4mL of cold acetic acid, clear liquid with vinegar smell, were also added to the flask and the mixture with a flea stir bar was placed in sand bath for around 12 minutes until the (E) Stilbene was completely dissolved. Pyridinium tribromide (0.394g), a bright red powder, was added to the mixture, resulting in a fast change in the coloration of the first mixture from clear to an orange/brown liquid. The new reaction was placed back to the sand bath and the dark orange color, which was expected to fade, changed colors from first a light yellow to a cloudish yellow color with a bright yellow layer on top. This reaction occurred in a period of 7.5 minutes at constant stirring. After the time indicated by instruction was completed, 4mL of cold DI water were added in order to increase the polarity and agitate the precipitate without dissolving the sample. After this addition the mixture turned completely white with notorious solid at the bottom. This solid formed in the flask was collected through vacuum filtration with a...
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