Brominating Alkenes

The purpose of this lab will be to brominate trans-stilbene by using acetic acid and pyrimidium tribromide by refluxing the mixture. To test the purity of product I will take the melting point which should be around 240 ºC along with a flame test which should be green-blue. CHEMICAL PROPERTIES TABLE

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REACTION MECHANISM

In this lab we used multiple techniques to complete and verify the bromination of trans-stilbene. First technique we used was to reflux the mixture; this contains having the mixture boiling so it will evaporate while the condenser will recondense the solvent which is boiling off so the solvent may stay in the reaction.

Next we used the melting point of the product to verify which product was produced along with the purity of the compound. Lastly we did a flame test using copper which will change to a green-blue flame in the presence of a halogen.

PROCEDURE, DATA, OBSERVATIONS
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RESULTS
Compound or Object
Mass (g)
M.P. Measured (ºC)
M.P. Theoretical
(ºC)
Percent Yield
(%)
Meso-1,2-dibromo-1,2-diphenylethane
0.167
236.9-237.9
238.0

Trans-stilbene
0.105
Pyridinium acid
0.203
acetic acid
2.0 mL
Filter paper
0.073
Watchglass
57.252
Flame Test w/ copper only
Orange
Flame Test w/ copper & product
Green

Percent Yield = (0.167 g) / (g) X 100% =

For this lab I had a percent yield of % for meso-1,2-dibromo-1,2-diphenylethane. I verified this product by my measured melting point range which was within 0.1 of theoretical melting point of 238 ºC and from the flame test, where the flame of the copper with the product turned green showing that a halogen was present. CONCLUSION

In conclusion, I was successful at brominating trans-stilbene with the use of pyridinium tribromide by using a reflux apparatus. When refluxing the mixture, the color was a yellow-orange while boiling, once I took this off the heat the mixture separated with the while solid settling at the bottom of the vial while the liquid top layer had a transparent yellow-orange color. Next I added DI water which immediately turned the mixture white and cloudy and from there slowly became a yellow-white color. After using a vacuum filtration and oven to separate and dry the product I yielded a mass of 0.167 g for a percent yield of %. This change in color is the trans-stilbene being brominated by the pyridinium tribromide.

The reaction is an electrophilic addition where the electrophile will react with an electron rich species known as the nucleophile to form an intermediate bromonium ion. The nucleophile will react with the electron deficient bromonium ion forming the vicinal dihalide which is a product from alkene halogenation. The bromides will bond on opposite sides of the double bond in the process of anti-addition resulting in the formation of a trans vicinal dihalide, which will produce a meso dibromide. For meso-1,2-dibromo-1,2-diphenylethane there will be two chiral carbons which will have an internal plane of symmetry and therefore meso will have superimposable mirror images. For this lab the possible sources of error could be not allowing the mixture to reflux completely or allowing the mixture to cool properly which may lead to a lower percent yield. It is possible there could also be loss of product on the sides of the Hirsch funnel used for the filtration along with product left in the conical vial. POST-LABORATORY QUESTIONS

1a. My measured melting point of the product for the fast ramp was 235.9 – 244.0 ºC and for the slow ramp was 236.9 – 237.9 ºC, therefore I produced meso-1,2-dibromo-1,2-diphenylethane. 1b.
1c. I do not have pre-lab assignment returned yet, therefore I am unsure of question 4. 2. Percent Yield = (0.167 g) / (g) X 100% = 3a. Silver nitrate was not used in this lab instead we used a flame test to verify the presence of a halogen, to complete this technique. 3b. I placed a copper wire into a flame and was shown an orange flame, when I added the produced product to end of copper wire and placed into the flame, this showed a green flame confirming the presence of the halogen Br. 4.