Bromination+of+Stilbene

**Background:**
This experiment uses safer, greener solvents and reagents to generate a bromine through bromination. This is done by oxidizing hydrobromic acid and hydrogen peroxide to brominate (E)-Stilbene into //meso//-1,2-Dibromo-1,2-diphenylethane. I appreciate your attention to the stereochemistry.

**Intro:**
This experiment consists the addition of Br to a stilbene molecule better to say "bromine" or use Br2, since you are adding two bromine atoms. To do this, your stilbene must be put into a solution (in this case, ethanol was used). Once in the solution, the addition of HBr and hydrogen peroxide will produce the essential Br2 needed to open up the pi bond and two bromine atoms that are added to the stilbene molecule in the trans position. To measure the success of the experiment, take the melting point of the new molecule and compare with its true value.

For full credit, show the reaction drawn out every time you do a synthesis lab.

**Procedure:**
To begin 0.5g of (E) stilbene is added to 10ml of ethanol into a round bottom flask. The flask is then connected to a condensing tube with cool water running through it. A magnetic stir rod is placed into the flask with the mixture. Using a hotplate and a water bath the mixture is stirred and heated until the stilbene is close to dissolved in the solvent. At this point 1.2ml of HBr is added to the solution using a Pasteur pipette. After stirring and heating the solution thoroughly 0.8ml of 30% hydrogen peroxide is added to the mix. At this point, the solution turned bright orange and was stirred and heated for 20 minutes. During this 20 minute period the color begins to fade away until the solution is clear with a white precipitate formed at the bottom. The flask is taken out off the heat and cooled to room temp. 5ml of NaHCO3 is added slowly until the pH is close to 5. The flask is then placed in an ice bath for an addition 10 min. The solid precipitate is then collected using vacuum filtration. The percent recovery is then calculated and the melting point of the final product is tested.

Data/Results:
Starting: 0.5g (E) - Stilbene sig figs here: measurement of mass should be to the nearest mg. End Mass: 1.08 grams. //meso//-1,2-Dibromo-1,2-diphenylethane

Sweat Point: 207 °C Melting Point 232 °C

Molar mass of stilbene: 180 g/mol Molar mass of //meso//-1,2-Dibromo-1,2-diphenylethane: 340 0.5 g x 180 g/mol = 0.00278 mol 1.08 g x 340 g/mol = 0.00317 mol Theoretical yeild should be 0.00278 mol of //meso//-1,2-Dibromo-1,2-diphenylethane 340 g/mol x .00278 mol = 0.945 g Percent yeild = (1.08 g / 0.945) x 100 = 114%
 * Percent Yield**:

always include general observations (color changes, description of product, etc.) in your data section.

Discussion/Conlusion
The purpose of this experiment was to show the formation of 1,2-dibromo -1,2-diphenylethane from (E) stilbene through green chemistry with safer solvents and reagents. During the reaction process, two bromine atoms are added to the two carbons bonded in between the two benzene rings after breaking the double bond between the carbons. Starting with 0.5 g of stilbene, a final value of 1.08 g of //meso //-1,2-dibromo-1,2-diphenylethane was found. This final weight was higher then what was expected (according to the percent yield found), however the experiment was considered successful after taking the melting point. The literature value of the melting point for //meso //-1,2-dibromo-1,2-diphenylethane is 241°C In order to find the percent yield, molecular masses were needed since //meso //-1,2-dibromo-1,2-diphenylethane is much heavier then stilbene. 0.5 g of stilbene equaled out to 0.00277 mol, so 0.00277 mol of //meso //-1,2-dibromo-1,2-diphenylethane had to be found(this would become the theoretical yield of the mass gained from a perfectly executed experiment). The theoretical yield was calculated out to be 0.945 g of //meso //-1,2-dibromo-1,2-diphenylethane, making the percent yield to be 114%. This number is so high for a variety of reasons/errors. During the experiment while melting the stilbene, not all of it had dissolved by the time the hydrobromic acid and hydrogen peroxide were added. This means that not all of the stilbene had bromine added to it. Also, when taking the final measurement, it was noted the substance was not entirely dry. Despite the efforts of making it as dry as possible during filtration, not all of the excess liquid was removed, causing our product to be slightly damp. This probably added quite a bit of weight to the final product, therefore giving such a high percent yield

No need to explain the calculations at this level; I'd rather hear more about the reaction itself. A pretty good lab, but if you respond to my comments I expect you'll score higher next time.