Week+2-+Bromination+of+Stilbene

=**Bromination of Stilbene**=

=**Introduction:**=

The purpose of this lab is to synthesize 1,2-dibromo-1,2-diphenylethane (dibromostilbene) from stilbene. (E)-Stilbene ((E)-1,2-Diphenylethene), is a hydrocarbon. It consists of a trans ethene double bond substituted. On each of the carbons of the double bond is a phenyl group. Stilbene can exist as two potential isomers, the one described previously and the other (Z)-stilbene. This isomer however is less stable due to steric hinderance. Elemental Bromine (Br2) is an extremely acidic and a corrosive brown fuming liquid, if it were to come in contact with a person’s skin or get into their respiratory tract it would be very problematic. This is why the reaction (bromine?)  is generated //in situ//(formed in a reaction flask) allowing it to be a greener way for the handling and usage of bromine. The reagents will be refluxed in the reaction flask. It is also  a green lab due to the use of ethanol which works well with the SN1 reaction seeing as how it is polar protic. The reflux process is boiling and allowing the reagents to recondense keeping the vapors //in situ //. The hydrobromic acid will be oxidized with hydrogen peroxide to produce the bromination of (E)-stilbene to dibromostilbene. Below is the mechanism for this reaction. The melting point of dibromostilbene is 241°C. It has chiral carbons but is a meso compound due to it having an internal mirror plane and occurs in an anti conformation only allowing for the trans formation. Yes! I am happy to see you noticed the stereochemistry.

Source: [],

=**Procedure:**=

Reaction

1. Prepare the reflux apparatus:

To assemble the reflux apparatus (Picture 1 below is an example of how it should look), use a macroscale glassware kit. Attach a 100 mL round-bottom flask to a water cooled condenser using a plastic clip. The bottom tube on the condenser should have a latex tube attached to it which comes from the faucet. A latex tube should be attached to top of it allowing the incoming water to drain in the sink. The top of the condenser will remain open. Use a ring stand and clamps to support the apparatus vertically with the flask at the bottom. When that is set up prepare a hot water bath in a crystallization dish on a stirrer/hot plate. Place the stirrer/hot plate on a jack, this is so if the substance in the flask needs to be removed from the heat quickly, it can be done. The round-bottom flask should be immersed roughly ¾ of the way in the crystallization dish.

2. In the round-bottom flask place a magnetic stir bar, 0.5 g of (E)-stilbene, and 10 mL of ethanol. Clamp the flask to the condenser and begin stirring and heating. Make sure cold water is running through the condenser. Continue to heat and stir till the majority of the solid has dissolved.

3. Measure out 1.2 mL of concentrated aqueous hydrobromic acid. Carefully removing the flask from the condenser and slowly add the hydrobromic acid, then returning the flask to the condenser. The stilbene will probably precipitate. The continuation of heating and stirring will cause the majority of the solid to redissolve. (If some remains undissolved still continue with the next step.)

4. Measure out 0.8 mL of 30% hydrogen peroxide. Carefully remove the flask and add the hydrogen peroxide dropwise. The color of the mixture will change to a yellow-orange color. Connect the flask back to the condenser and continue to heat and stir. The mixture should slowly lighten, eventually becoming a cloudy white. This may take 20 minutes or so.

Work up

<span style="font-family: Arial,sans-serif; font-size: 10pt;">5. <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">When it has reached a cloudy white color remove the flask from the crystallization dish allowing to cool at room temperature. Check the pH with litmus paper. The solution needs to have a pH between 5 and 7. To obtain this level of pH carefully add NaHCO3 to the solution.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">6. <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">Place the flask in an ice bath to further cool the solution bringing more product out. By vacuum filtration, collect the solid that formed, continuing to draw air through the product till it is dry. When dry record the mass to determine the % yield. Then collect some of the product in a capillary tube and gather its melting point using a Mel-Temp apparatus.

Source: //<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">Green Organic Chemistry- Strategies, Tools, and Laboratory Experiments // (A link is listed in Work Cited)

**what the macroscale glassware apparatus should resemble**

how the mixture looks before vacuum filtration to separate the precipitate

=**Data/Analysis:**=

I like the way you laid out this table, sort of chronologically.
 * **<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">Substance Additions ** || **<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">Observations ** ||
 * <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">0.507 g of (E)-stilbene and 10.0 mL of ethanol || <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">No reaction. As the two mixed the solution became white. ||
 * <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">1.2 mL of concentrated aqueous hydrobromic acid || <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">White precipitate formed, with continued heating/stirring some but not all precipitate redissolved ||
 * <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">0.8 mL of 30% hydrogen peroxide || <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">The color immediately turned dark yellow-orange, after continued stirring mixture turned a cloudy white ||
 * <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">121 drops of NaHCO3 || <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">The addition of multiple drops of NaHCO3 at one time caused the solution to bubble. ||

Melting Point of stilbene: wet at 124.1 C and completely melted by 126.7 C

Melting Point of final precipitated product: wet at 235.3 C and completely melted by 236.9 C (documented melting point of dibromostilbene is 241 C)


 * Melting Points were measured using a Mel-Temp and Lab Quest device.

Initial mass of stilbene: .507g x 1mol/180.24g = 0.003mol stilbene sig figs! YIKES! You lost a few important digits!

Final mass of precipitated solid (dibromostilbene): 1.191g x 1mol/340.04g = .004 mol dibromosilbene same error, repeated. Time to get those sig figs rules under control!

Percent yield= experimental yield/theoretical yield x 100 = .004mol/.003mol x 100 = 133% yield

=**Conclusion:**= The typical percent yield for this synthesis is around 90%, according to the procedure. The actual percent yield reported was 133% in this case. The purity of the final product could be in question. However, the purity could be fine and the source of this specific error could be due to insufficient dry time for the precipitate, resulting in a higher mass. Also, the melting point of the final precipitate was lower than the reported melting point of dibromostilbene and that could be due to the same error discussed in the previous example. If the sample being tested in the Mel-Temp was not sufficiently dry, the melting point of the sample would not be accurate. There were a few additional sources of potential error along the way, though the severity of such errors are unknown. The tip of the glass pipette broke off while transferring the hydrobromic acid, making the 1.2 mL measurement inaccurate. In addition, after the hydrobromic acid was added, only slightly more than half of the precipitate redissolved before adding the hydrogen peroxide**.** If your product was contaminated with a bunch of other stuff (was very impure), you would expect very specific changes in melting point. However, your melting point data is BEAUTIFUL. So that doesn't make sense. I wish you had been a bit more specific in thinking through the consequences of some of the errors you suggest.

= = =**Work Cited:**=

"(E)-Stilbene." //Wikipedia, the Free Encyclopedia//. Web. 26 Jan. 2012. <http://en.wikipedia.org/wiki/(E)-Stilbene>.

//<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">Green Organic Chemistry- Strategies, Tools, and Laboratory Experiments //<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;"> by K.M. Doxsee and J.E. Hutchinson, Thompson Brooks/Cole, 2004; pp 125-128. Retrieved from GEMS Online Database: []

Higginbotham, Carol. Ph.D. “Bromination of Stilbene” CH-335: Organic Chemistry Course. COCC/UO: Blackboard, Winter 2012. Web. 26 Jan 2012.

"Overview." //<span style="font-family: Calibri,sans-serif;">GEMs Home //. Web. 26 Jan. 2012. <http://greenchem.uoregon.edu/Pages/Overview.php?CategoryIDString=>.