Post lab dehydrobromination PDF

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Youngjung Choi Benjamin Hong 11/28/17

8. Discussion The main purpose of this lab was to make an alkyne by a double dehydrohalogenation.

Two-step

dehydrobromination

reaction

synthesizes

diphenylacetylene from meso stilbene dibromide. Therefore, meso stilbene dibromide mixed with potassium hydroxide to get rid of bromine and hydrogen to form diphenylacetylene. Hydrogen and a halogen are removed from a molecule to make a pi bond. A strong base causes elimination rather than a substitution. Therefore E2 mechanism will perform. In order to increase the success of the reaction, basic condition and high temperature are required. This reaction needs two reactions in order to form a triple bond. From double bond, a temperature around 200°C is preceded to form the triple bond. The triethylene glycol is a high boiling point solvent that it can withstand during high-temperature condition. The rotation of bond occurs during the heating process. The orientation of molecule happens during elimination reaction and affects the reaction. The E2 reaction prefers anti-periplanar conformation. During transition state of this mechanism, the halogen and hydrogen are 180 degrees from the other. The –OH group of the potassium hydroxide attracts the leaving group (hydrogen and the bromine), and it is eliminated from the molecule. Formation of a C-C double bond is formed from removal of the hydrogen and bromine, and it is favorable elimination reaction. The syn periplanar is unfavorable conformation due to the eclipsed conformation. It has high energy formation and difficult to react with. Formation of the CC double bond prevents atoms to rotate freely and locked the molecule into an synperiplanar position.

To overcome the inimical orientation of the intermediate

mechanism, excessive heat is added. The heat trigger second step of the elimination

Youngjung Choi Benjamin Hong 11/28/17

and forms diphenylacetylene. The high temperature is very hard to reach up on the hot plate because of the heating capacity. Therefore, the conical vial must not be placed directly on the hot plate and surround vial with aluminum foil to get enough heat for this reaction.

Youngjung Choi Benjamin Hong 11/28/17

9. Data and Results Analysis The melting point of this final product was around 51 to 53 degrees Celsius. The true melting point of diphenylacetylene is around 62 degrees Celsius that it was slightly lower than the actual melting point. The uncalibrated thermometer can affect the result. Especially, the presence of impurities in a product can change melting point to up or down from the actual melting point. In order to prevent the error, additional recrystallization can take place, but it can reduce the percent yield or lost massive product. The percent yield of this experiment was lower than an expectation. 0.3g * (1/340.07g meso stilbene) = 0.00088mol meso stilbene dibromide. 0.00088mol * 178.22g diphenyl=0.157g of diphenylacetylene. Percent yield= (0.075g/0.157) *100 = 48%. During vacuum filtration, not scraping all of the crudes definitely lower the percent yield. Also, some of the product was left behind in beaker or glassware during recrystallization. Still, 48% percent yield was a very reasonable result and successfully performed double dehydrobromination. 10. Conclusion This reaction starts with the meso-stilbene dibromide into a triacetylene glycol. Two pellets of potassium hydroxide into solution makes reaction favorable and faster. The temperature is turned to the maximum heat to get two-step mechanisms. The vacuum filtration and recrystallization minimize the chances of oiling out and get solid diphenylacetylene. Overall reaction performed very well, but the melting point was lower than the actual melting point because of impurities. Also, 48% percent yield was a decent yield....