Bromination of (E)-Stilbene with Pyridinium Tribromide and Recrystallization of Stilbene Dibromide

Bromination of (E)-Stilbene with Pyridinium Tribromide and Recrystallization of Stilbene Dibromide In this experiment, 1,2-dibromo-1,2-diphenylethane (stilbene dibromide) was synthesized through the reaction between (E)-stilbene (trans-stilbene) and pyridinium tribromide, using ethanol as the solvent. The reaction between trans-stilbene and pyridinium tribromide occurred at a heated environment and the stilbene dibromide was obtained and purified through crystallization and vacuum filtration. Ultimately, the purity of the crude and pure stilbene dibromide was tested through melting point determination. Bromination of trans-stilbene and the formation of stilbene dibromide is a rapid and stereospecific reaction that involves the nucleophilic attack of an alkene to a bromine and a bromide ion attack on a cation intermediate formed through the nucleophilic attack. In general, tetrochloromethane or dichloromethane is used as the solvent with bromine to carry out the bromination reaction of alkene. However, because both of these solvents are suspected to be carcinogenic, an alcohol, ethanol, was utilized in this experiment as a safer alternative. Additionally, this reaction is typically conducted using elemental bromine, Br2, but because it is a highly corrosive substance, it was replaced by pyridinium tribromide, which is an orange solid that served as a source of elemental bromine generated in situ as shown in Figure 1, specifically for this experiment. Figure 1: In situ generation of bromine from pyridinium tribromide Since the bromination reaction of trans-stilbene is stereospecific, it is important to consider the stereochemistry of the products formed through the addition reaction. For example, there is a possibility that alkene bromination reaction will form chiral centers that will generate multiple products from just a single reactant. However, in this experiment, as shown in Figure 2, the presence of only two substituents on the olefinic carbon and the lack of a chiral center even after the bromination reaction yielded a single product. Figure 2: Bromination of (E)-stilbene In this experiment, the bromination reaction was dependent on both the amount of transstilbene and pyridinium tribromide—if either one was to run out, the reaction would not be continued. Through calculations shown in Figure 3, the limiting reagent was determined as transstilbene because 0.0055423 moles of trans-stilbene is expected to run out before the 0.00625539 moles of pyridinium tribromide. Using the limiting reagent, the theoretical yield of pure stilbene dibromide, 1.840 g, was calculated as shown in Figure 4. Eventually, the theoretical yield was compared to that of the actual yield to calculate the percent of stilbene dibromide collected through recrystallization and vacuumm filtration. As shown in Table 1, 1.101 g of stilbene dibromide was collected from 0.999 g of trans-stilbene to result in 59.837% recovery. The percent recovery of stilbene dibromide was relatively low due to the loss of mass in the process of removing impurities through recrystallization and vacuum filtration that ultimately remained in the mother liquor. 0.999 g of ( E ) stilbene × 1 mol of ( E ) stilbene =0.0055423 mol of ( E ) Stilbene 180.25 g of ( E ) stilbene 2.0006 g of tribromide× 1mol of tribromide =0.00625539 mol of tribromide 319.82 g of tribromide Figure 3. Calculations Determining Limiting Reagent 0.999 g of ( E ) Stilbene × 1 mol of ( E ) stilbene 1mol of dibromide 240.058 g of dibromide =1.840 g of dibro × × 1 mol of dibromide 180.25 g of ( E ) stilbene 1mol of (E ) stilbene Figure 4. Theoretical Yield Calculation for Stilbene Dibromide Crystallization is a method of separation centered on the reduced solubility of a compound in a certain solvent at a specific temperature or pressure. Crystallization methods are designed to produce a supersaturated solution that eventually forms crystals. In this experiment, ethanol was utilized as the solvent during recrystallization because the solubility of the desired compound increased with increasing temperature while the solubility of the compound decreased with the cooling of the solvent. In terms of purifying an impure sample of a compound, crystallized molecules have a greater affinity for molecules of its own kind than for the impurities, they leave behind the impurities initially existent in the compound in the mother liquor, to ultimately yield relatively pure crystals. In addition to recrystallization, vacuum filtration also helped in the process of removing impurities and collecting a pure sample of stilbene dibromide by separating the mother liquor from the solid product formed through recrystallization. In this process, methanol was used as the solvent to get rid of any pyridinium salts adsorbed through the bromination reaction, which ultimately transformed a crude sample of stilbene dibromide into a pure one. The purity of each sample was verifed and compared through the determination of melting point ranges. As shown in Table 1, the crude sample of stilbene dibromide had a melting point range of 233-236 °C, while the pure sample of stilbene dibromide had a melting point range of 236.9-238.7 °C. The melting point range of the crude sample was much lower than that of pure, indicating that the presence of impurities in the crude sample led to the reduction in the melting point range. On the other hand, the melting point range of the pure sample of dibromide fit perfectly within the literature value of the melting point range of stilbene dibromide as shown in Table 1, indicating that there were little to no impurities in the collected pure sample. Table 1: Percent Recovery and the Melting Point Ranges of Crude and Pure Stilbene Dibromide Initial Mass of (E)-stilbene (g) 0.999 Mass of Pyridinium Tribromide (g) 2.006 Calculated Theoretical yield (g) 1.840 Actual yield (g) 1.101 Percent Recovery (%) 59.837 Melting Point Range of Dibromide (Literature Value) (°C) 235-241 Melting Point Range of Crude Sample (°C) 233-236 Melting Point Range of Pure Sample (°C) 236.9-238.7