What reaction was conducted in this experiment and with what reagents? Was there a reason for this reaction beyond just synthesis of product(s)? What techniques were used to purify and identify the product(s) of the reaction?
The purpose of this experiment was to investigate the bromination of trans-cinnamic acid and determine what the isolated products tells us about the possible mechanism. The stereochemistry of the product results from either a syn or anti addition of Br2 to the alkene. Recrystallization using ethanol and water solvent mixture was used to purify the crude product and melting point was implemented in order to see which products were synthesized. The ...view middle of the document...
The Br–Br bond is cleaved heterolytically, forming the Br– ion. The nucleophilic attack of Br– opens the bromide ion ring via backside attack resulting in a new C–Br bond on the opposite side. Finally, the mechanism of the both syn and anti addition consists of the electrophilic addition a bromine atom (after the heterolytic cleavage of the Br–Br, forming Br–) by using the pair of electrons in the pi bond of the C=C. Its initial addition occurs from either side of the planar double bond. This results in a carbocation intermediate that is trigonal planar. The addition of the Br– ion can occur from either side of the carbocation face, forming two products of syn and anti addition. (The drawn mechanisms are displayed on the following page.)
What product was obtained? What yield and percent yield were obtained? What percent recovery was obtained from recrystallization? What melting point data were obtained?
The product obtained for the most part was for the anti addition; (2S, 3R- and 2R, 3S) 2,3-dibromo-3-phenylpropanoic acid. The percent yield was calculated by the following:
(0.103g trans-cinnamic acid)/×(1 mol trans-cinnamic acid)/(148.2 g trans-cinnamic acid)×(1 mol 2,3-dibromo-3-phenylpropanoic acid)/(1 mol trans-cinnamic acid)×(307.97 g)/(1 mol 2,3-dibromo-3-phenylpropanoic acid)= 0.214 g 2,3-dibromo-3-phenylpropanoic acid
The theoretical yield was 0.214 g of 2,3-dibromo-3-phenylpropanoic acid. Our actual yield for our recrystallized product was 0.040 g. To calculate percent yield the actual mass is divided by the theoretical mass and multiplied by 100%. So the percent yield was 0.040g/0.214×100%=18.7%.
The percent recovery obtained by the recrystallization was the mass of the pure product divided by the mass of the crude product (0.077g). So the percent recovery was 0.040g/0.077g×100%=51.9%.
The melting point data obtained was:
Compound Melting Point Range ᵒC
(2S, 3R- and 2R, 3S) 2,3-dibromo-3-phenylpropanoic acid (anti) 202-204 ᵒC
(2S, 3S- and 2R, 3R) 2,3-dibromo-3-phenylpropanoic acid (syn) 93.5-95 ᵒC
Experimental Pure Product 177-183 ᵒC
According to the data, it can be seen that there was anti product. The reason is due to the fact that the melting point of the acquired recrystallized product is lower than that of the pure anti product (202-204 ᵒC), but much higher than that of the pure syn product (93.5-95 ᵒC). Since the observed melting range is near the reported melting range from the given, it suggests that the product is of that enantiomer set. For this particular experiment the (2R,3S) and (2S, 3R) enantiomers of 2,3-dibromo-3-phenylproanoic acid are expected; indicating that the bromine addition occurred via an anti addition method. If bromine were to have added via syn addition, the melting point would have been 93.5-95 °C if pure and below that range if impure. The difference of several degrees between the theoretical and the experimental values may be the result of impurities and the possible...