Bromination+of+Stilbene


 * Bromination of Stilbene:**


 * Introduction:**

The addition reaction of Br2 is a very important reaction in organic chemistry resulting in a unique mechanism forming //trans// only products. The chemical industry uses this product in the production of polymers. In this synthesis, Bromine will attack across a double bond forming an epoxide ring it's a bromonium ion intermediate...yes it is cyclic, but it is not an epoxide. Epoxide requires an oxygen atom as the bridging atom. with Bromine carrying a positive formal charge. However, electronegativity of the halogen creates a δ+ charge on the carbon atoms in the epoxide ring. Which allows the negatively charged Bromine ion to attack the molecule from the most electropositive side of the epoxide ring. This final addition results in //trans// only products. The importance of this reaction to organic chemistry students leads to performing this reaction as a lab experiment regardless of the caustic nature of Br2. In the practice of green chemistry, lab will consist of making Br2 in situ with slightly less noxious compounds. Br2 will be added to (E)- stilbene after the reaction of HBr and H2O2 produce Br2 in the reaction vessel. This will be noted by the appearance of orange color from Br2 before the addition to (E)- stilbene creating 1,2-dibromo- 1,2- diphenylethane (dibromostilbene).

Procedure used: Experiment 3 from, Doxsee, K. M.; Hutchison, J. E. //Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments//, Print 2004; pp 125-128.
 * Procedure:**

The experiment can also be found here

(E)-stilbene: White crystals similar to table salt.
 * Data:**

95% ethanol: Clear liquid with a strong odor.

HBr: Translucent liquid with a slight orange tint to it.

H2O2: Translucent, colorless liquid.

NaHCO3: Translucent, colorless liquid.

0.506g (E)-stilbene (s) and 10.0mL of 95% ethanol (l) were added to a 100mL round bottom flask with a magnetic stirring rod. The water cooled reflux condenser was attached to the round bottom flask and the entire apparatus was placed in a hot water bath for heating. After approximately 15 minutes of heating, the solution still contained a small amount of un-dissolved solid.

As per the instructions, 1.2mL HBr was added to the mixture, which instantly became cloudy off-white; bubbles were observed. The solution was allowed to reflux until it showed no color. This change took 13 minutes from the addition of HBr.

At this time, 0.8mL H2O2 was added drop wise to the solution. The first few drops changed the clear solution into a penetrating yellow on contact. Subsequent drops turned the solution to a concentrated orange. During this process, bubbles were present.

Eight minutes after the H2O2 was added the color of the solution had changed to milky white with a slightly orange tint. An additional five minutes of cooking removed the remaining traces of orange, and the apparatus was removed from the hot bath.

After allowing the solution to cool to room temperature, the pH of the solution was tested and found to be 1.The addition of 3mL +130 drops of saturated NaHCO3 was added drop wise to neutralize the solution: final pH was 6.

The solution was placed in an ice bath to facilitate maximum precipitation of the solid. Once the temperature of the solution was equal to that of the ice bath, the precipitate and the solution was vacuum strained to separate the two.

After allowing the product to dry over the weekend, the dibromostilbene was weighed and the purity was found through the use of the Meltemp apparatus.

The final mass of the product was 0.660g.

Decomposition of the product occured at 221.3°C

Good job on the data section. There is a lot of information here but not excessive detail.


 * Analysis/Conclusion:**

To calculate percent yield of the product, the following equation was used:



First, the theoretical yield was calculated as follows:



Percent yield was calculated by using the formula above having inputted the figures specific to this experiment.



Yield calculation ok, sig figs ok too! Standard procedures for the bromination of alkenes create hazards both in the lab and to the environment with caustic agents such as liquid Br2 and chlorinated solvents. A greener approach to the bromination of stilbene (an alkene) in ethanol using hydrobromic acid and hydrogen peroxide can create a safer alternative that still typically has a yield around 90%. In the greener approach not only is ethanol used as a more benign solvent but a safer, solid bromination agent (pyridinium tribromide) replaces the caustic liquid agent. say what? You didn't use pyridinium tribromide. Despite a 69.1% yield in the above practiced experiment, errors in the filtration of product may have contributed to the reduction in yield typically gained using the greener approach. A good portion of product may have been too fine for the filter paper used during the vacuum filtration process and was lost as waste. This was indicated in the cloudy water present in the Hirsch filtration system. Another contribution could have also been the time lapse between when product was rendered and eventually weighed. There was a three day time lapse between when the experiment was performed and when the dried product was weighed. There could have been tampering that was unnoticed due to being in the lab without supervision. The decompisition temperature of 221.3 degrees C indicated a reduction in pure product from the above experiment as compared to the melting temperature of 241 degrees C for a product absent any impurities as reported in the lab procedure. However, in the educational setting it is not uncommon for students to experience a lower decomposition temperature than the reported melting temperature of dibromostilbene. This offers that the greener approach can not only yield a high amount of product, but can also present a highly purified result with reduced risk to the lab or outside environment.

This lab earned the following scores for: format (2/2) style (2/2) data (3/3) quality of result (1/1) quality of reported data (1/1) conclusion (1/2) error (1/1) post-lab Question (2/2) for a total of 13/14.

Nicely done.