Experiment 2 - lab report 2019 PDF

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1 Tiffany Rutledge CH 237-021 Lisa Fealy 2 February 2020 Stereochemistry of The Addition of Br2 to Trans-Cinnamic Acid Introduction: The problem in this experiment was that there are three possible mechanisms for the addition of Br 2 across the double bond in trans-cinnamic acid, including bromonium ion (anti-addition), carbocation (not stereoselective), and concerted (syn addition) mechanisms. Due to the multiple possible mechanisms, there are multiple possible products for this reaction. The purpose of this experiment was to determine which mechanism occurred through measuring the melting points of the product formed in the reaction. In order to approach this, the expected products must be determined for each mechanism. The bromonium ion mechanism would result in an anti-addition yielding erythro-2,3-dibromo-3-phenylpropanic acid enantiomers, the carbocation mechanism would cause no stereoselectivity resulting in a mixture of the erythro- and threo- products, and the concerted mechanism would result in syn addition yielding thero2,3-dibromo-3-phenylpropanoic acid. To carry out this experiment, a solution of bromine in acetic acid was added to a solution of trans-cinnamic acid which is then separated to dibromide by vacuum filtration. Then the crude product was purified by recrystallization and the melting point range of the product was measured. Finally, to determine the product formed, the melting point range is then compared to that of thero-2,3-dibromo-3-phenylpropanoic acid and erythro-2,3-dibromo-3-phenylpropanic acid.

2 Results:

2,3-dibromo-3-phenylpropanoic acid

Actual Yield

Percent Yield

Melting Point Range

0.00405 moles

(0.00405/0.01)*100

200 °C- 205 °C

= 40.58%

Discussion: The goal of this experiment was to determine the product obtained from the addition of bromine to trans-cinnamic acid by measuring the products melting point. The possible products are thero-2,3dibromo-3-phenylpropanoic acid which would be attained by the concerted mechanism, erythro-2,3dibromo-3-phenylpropanic acid attained by the bromonium ion mechanism, or a mixture of the two by the carbocation mechanism. The three Bond partners of the Bond Triplex chemical specialties company assume that the reaction was undergoing the bromonium ion addition mechanism to form the erythro-2,3dibromo-3-phenylpropanic acid product. This experiment was run in order to make their production more effective by determining the final stereochemistry of the product formed. In order to achieve this, combine 10 mmol of trans-cinnamic acid and 6 mL of acetic acid to an Erlenmeyer flask. Then, 10 mL of a 1.0 M solution of bromine in acetic acid was put into a separatory funnel and slowly added to the Erlenmeyer flask while it was continuously stirred with a stir bar. Following the addition of the solution of bromine in acetic acid a drop of cyclohexene was added to turn the solution from orange to light yellow, then placing it in an ice bath for fifteen minutes. The product was then collected by vacuum filtration and washed with ice cold water, then purifying the product from recrystallization with 50% aqueous ethanol. Finally, weighing the purified product and measuring its melting point. The weight of my isolated yield was 1.25 grams or 0.00405 moles and the measured melting point range was 200 °C- 205 °C. The percent yield obtained was only 40.58%, which is less than half of the theoretical yield. The low obtained yield from the experiment could have been due to an error in the recrystallization process, in which all of the yield may have not been fully retrieved from the flask or not all obtained from the funnel. In addition, the solution could have not been in the ice bath for enough time.

3 In the future to avoid similar errors, one should ensure all the product is fully removed from the flask and the funnel by using the microspatula to scrape all the product out. Based upon the melting point range of the product achieved from the experiment, the product was likely erythro-2,3-dibromo-3-phenylpropanic acid. The book states that the melting point of erythro-2,3dibromo-3-phenylpropanic acid was 204 °C and the melting point of threo-2,3-dibromo-3-phenylpropanic acid was 95 °C. The experimentally obtained melting point range was 200°C-205°C, which is closer to that of erythro-2,3-dibromo-3-phenylpropanic acid. Confirming the Bond Partners hypothesis that the mechanism that occurred was the bromonium ion mechanism resulting in an anti-addition. Conclusion: In conclusion, the results of the experiment show that the addition of Br 2 to trans-cinnamic acid under goes the bromonium ion mechanism undergoing an anti-addition yielding erythro-2,3-dibromo-3phenylpropanic acid enantiomers. As the measured melting point range of the product was 200°C-205°C, which fits the melting point of 204 °C for erythro-2,3-dibromo-3-phenylpropanic acid given in the book. Suggesting that the hypothesis given by the Bond partners was correct in that the bromine addition reactions proceeded by the bromonium ion mechanism. Questions: 1. Bromonium ion mechanism for the addition of bromine to trans-cinnamic acid

2. Products formed if you started with cis-cinnamic acid

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3. The purpose of adding cyclohexene at the end of the reaction was so that it would react with the excess bromine left in the solution....