A+Greener+Bromination+of+Stilbene


 * Introduction**

In this lab, the bromination of E- stilbene into 1,2-dibromo-1,2-diphenylethane (dibromostilbene) was performed (Figure 1). The bromination of an alkene, a reaction where bromine is added across a double bond, is an important reaction in organic chemistry (Figure 2). Environmental b romine (Br 2 ) is hazardous to work with because it forms hydrobromic acid in water. This can be problematic when it comes in contact with mucus membranes in the respiratory system. H owever, some dangers of working with Br 2 can be mitigated when it is generated in situ. This is done by adding hydrogen peroxide to hydrobromic acid in a reaction flask (figure 1). By generating Br 2 in situ and using a more benign solvent, ethanol, a greener bromination of stilbene will be performed.

This reaction works because Br 2 is polarizable. This means that the electrons that make up the bond between the two bromine atoms can move back and forth between the atoms. This movement causes partial negative and partial positive charges on the bromine atoms. When one of the atoms has a partial positive charge, the electrons making up the pi bond in the alkene act as a nucleophile and grab the partial positively charged bromine atom, transferring it to the trans-stilbene. The electrons that had formed the sigma bond in the Br 2 transfer onto the remaining bromine atom and create a bromide ion. The negatively charged bromide ion then attacks the carbocation formed with the initial addition of bromine to the alkene, yielding 1,2-dibromo-1,2-diphenylethane (dibromostilbene).



Figure 1



Figure 2

**Procedure**

The procedure for this experiment can be found [|here].

Relevant Compounds:

 * Name:** E-Stilbene
 * Cas #:** 103-30-0
 * Molecular Formula:** C 14 H 12


 * Name:** Ethanol
 * Cas #:** 64-17-5
 * Molecular Formula:** C 2 H 6 O


 * Name:** Hydrobromic acid
 * Cas #:** 10035-10-6
 * Molecular Formula:** HBr


 * Name:** Hydrogen Peroxide
 * Cas #:** 7722-84-1
 * Molecular Formula:** H 2 O 2


 * Name:** Sodium Bicarbonate
 * Cas #:** 144-55-8
 * Molecular Formula:** NaHCO 3

**Name:** 1,2-Dibromo-1,2-Diphenylethane **Cas #:** 13440-24-9 **Molecular Formula:** C 14 H 12 Br 2

**Data**

Reaction:
.523g +/- .001g of E-Stilbene was added to a 100 ml round bottom flask with a magnetic stir bar. The E-Stilbene was noted as being a white granular substance whose individual grains preferred to be close to one another. Approximately 10 ml of ethanol was added to the 100 ml round bottom flask. In order to aid in the dissolution of the solid, the mixture was stirred while heating it to reflux. This process took approximately thirty minutes. At this time, the solid had not completely dissolved; however, it was clear that no more of it would dissolve.

At this point, 1.2 mL of concentrated aqueous hydrobromic acid was added to the mixture via a pasteur pipette. As expected, this caused a significant amount of the E-stilbene to precipitate out of the solution. The solution was allowed to continue heating at reflux with the stir bar active for 10 minutes. This allowed the majority of the E-stilbene that had precipitated out with the addition of the hydrobromic acid to dissolve back into the solution. However, there were still visible signs of solid E-stilbene in the solution.

0.8 mL of 30% hydrogen peroxide was measured via a pasteur pipette and added drop wise to the mixture. Small bubbles were immediately visible as CO 2 was formed and released from the mixture. As more drops were added, the color changed from a milky white into a very rich orange color as Br 2 began to form. The mixture continued to be heated at reflux while the stir bar was active so that the addition of Br 2 could be made across the pi bond. After 3 minutes, there was a minor but noticeable lightening of the color of the mixture. Over the next minute, there was a rapid lightening of the color until it reached a cloudy white color.

Workup:
The flask was removed from the hot water bath and allowed to cool to room temperature which took approximately twenty minutes. Aqueous Sodium Bicarbonate (NaHCO 3 ) was added to the solution in order to bring the pH level to a value between 5 and 7. The pH was tested via pH paper 4 times throughout the addition of 7 mL of NaHCO 3 until the desired pH was met. The flask was then place Placed into an ice bath for five minutes. The precipitate was then collected via vacuum filtration and then sandwiched between two pieces of filter paper in order to draw out moisture.

Characterization:
The recovered product was a dirty white in color. From the initial weight of .523 g of E-stilbene, .705 g of product was recovered. Two capillary tubes of product were collected and put into a meltemp with a LabQuest thermometer in order to determine the melting point of the product. The first sample began to sweat at 219⁰ C and was fully melted at 229⁰C. The second sample began to sweat at 218⁰C and was fully melted at 232⁰C.

Analysis:

 * Percent Yield:**

very nice. organized, clear calculations and good attention to sig figs.


 * Product Purity:**



**Conclusion**

In this lab, a greener bromination of stilbene was performed by generating Br 2 in situ and using a more benign solvent. The recovered product was 0.705g (2.07 x 10 -3 mol) and percent yield was calculated to be 71.4%. This shows that for every thousand molecules of starting material, 714 molecules of the target molecule was produced. The expected yield for this experiment was reported at about 90%. The lower yield could be due to some errors or limitations in the lab. Did you happen to think at all about the stereochemistry of the product you formed? That's the ONE thing that I think you might want to add into the discussion in your report.

**Error**

One possible source of error during this lab occurred while testing the pH. While testing the pH, a strip was dropped into the reaction mixture. The strip remained in the mixture for approximately 10 seconds. During that time, a pink color leaked from the pH testing strip into the reaction mixture. Whoops! This possibly led to an increase of impurities in the final product and could be a cause for the lower than expected melting points. Another possible source of error during this lab occurred while filtering out the impurities. The initial funnel had a convex surface on the bottom instead of a concave bottom. This caused some of the mixture to pour down around the sides of the filter paper. This loss of product could account for the lower than expected %yield.

Very nice report. A pleasure to read!

References:
Doxsee, K. M.; Hutchison, J. E. Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments, Print 2004; pp 125-128.