Trans-Cinnamic Acid Results

Yield Data

The reaction of 0.139 grams of trans-cinnamic acid, 0.8 mL of glacial acetic acid, and 1.0 mL of 1.0 M of Bromine in acetic acid was recrystallized to form a solid, clear, crystal- like product. The melting was taken from the recrystallized product to determine what had been obtained. The melting point was found to be 130.5°C-133.7°C. Concluding that the product formed from the addition of bromine was a mixture of the erythro-2, 3-dibromo-3-phenylpropanoic acid, which has a known melting point of 204°C, and threo-2,3-dibromo-3-phenylpropanoic acid, which has a known melting point of 95°C. Meaning the recrystallized product was a racemic mixture. I obtained .204 grams of the racemic mixture product, meaning my percent yield is 66.23%.

Percent yield= experimental yield (the amount obtained) = 0.204 X 100= 66.23%
Theoretical yield (.308 g) .308

Discussions

The melting point of the product is half way between the melting points of the two possible products, concluding the final product is a racemic mixture of the erythro and threo -2,3-dibromo-3-phenylpropanoic acid.
The melting point range is less than five degrees apart, meaning there were not many impurities in the final product. The H’ NMR of the starting material, trans cinnamic acid, is almost the exact same as the he H’ NMR of the final product. The only difference was the extra signal around 5 ppm. The reason behind the extra signal is the addition of the Bromine to the trans-cinnamic acid.
The H’ NMR matched my predictions almost exactly, meaning the product is what was suppose to be obtained. All of the signals (b.c.e.d.f) were where they were projected- except for signal a. Signal a was the projection for the hydrogen in the carboxylic acid. I did not see a signal in the projected region, which was 9.5-10.1. The cause of the signal being missing may be because there is still water present in the mixture, or trans-cinnamic acid in the solid product that was formed. (see attached paper for viewing of
signals) There was a major human error that occurred during the lab process. During the recrystallization process, the flask containing the mixture tipped over in the cold-water bath. As a result, some of the mixture was lost. Although, the experiment was not affected, only the percent yield. The melting point and H’ NMR could still be obtained with the product that remained after the spill. A proposed mechanism can be made based on this reaction (see separate sheet of paper). The mechanism does not match the reaction we learned in class because it Br2 and HOAc instead of BH3 and H2O2 for the conditions. If this experiment were repeated, precaution would be taken more highly. The experiment was successful in producing a low impurity product, but the carrying out of the procedure was poor. Next time, I will be careful not to lose any of my product. Also, I forgot to take a crude unknown. Next time, I will be sure not to forget to take one. I waited a while to go back and take the melting point and H’NMR, therefore the product was very dry, making it hard to put it in a melting point capillary tube. To reduce impurities, I also need to allow my mixture to dry longer in the recrystallization stage.