Formal Lab Write up #2 - Grade: A- PDF

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Chemistry 322 Sec. 11 Organic Chemistry Lab 1 Cody Harvey Experiment #11 Formal Lab Report Due December 10th, 2018

Title: Green Chemistry Bromination Abstract: For this lab experiment the preparation of (E)-dibromostilbene was performed. Bromine will be generated through oxidizing hydrobromic acid with peroxide. Adding bromine to (E)-stilbene will result in the product formation (E)-dibromostilbene. This SN2 type reaction will occur when the more positively charged bromine attaches to the alkene attaching the two benzene rings found in the (E)-stilbene molecule. To determine if the desired product was obtained, infrared spectroscopy (IR) was conducted and analyzed.

Introduction: The substitution reaction that forms (E)-dibromostilbene can be performed with multiple reagents. Bromine in a chlorinated solvent will also produce the desired product. However, this reagent is carcinogenic. Pyridinium tribromide is another reagent that is used for this bromination reaction. Pyridinium tibromide is favored over bromine, because it can be weighed more easily. Pyridinium bromide is also a safer reagent when compared to bromine and a chlorinated solvent. Using pyridinium tribromide results not only in the production of (E)-dibromostilbene, but also a significant amount of pyridinium bromide as waste (1). For this experiment, a “green” approach was used. Green chemistry is the production of chemical products that reduce or eliminate the use and generation of hazardous substances (2). Therefore, bromine will be generated through oxidizing hydrobromic acid with peroxide. The mechanism in this reaction involves the alkene between the benzene rings of (E)-dibromostilbene acting as the nucleophile, and the bromine acting as the electrophile. As the nucleophile and electrophile move close together, the bond attached to the bromine becomes polarized. This causes one of the bromines to have a partially positive charge and the other a partially negative charge. The separation of these partial charges causes the more positive

bromine to form a cyclic bond with the alkene. Then the negatively charged bromine attaches on the opposite side of the plane to one of the carbons in the chain, resulting in the formation of a sigma bond. This new bond causes the cyclic bonds to break apart. As a result, this SN2 reaction forms a final product that will always have anti-stereochemistry and be stereospecific. An illustration of this mechanism is shown below in Figure 1.

Figure 1: (E)-stilbene to dibromostilbene mechanism

Once the preparation of (E)-dibromostilbene was performed, an IR spectroscopy of the product was run. IR Spectroscopy measures the vibrations of atoms within a molecule to determine functional groups. IR spectroscopy is an effective tool in analyzing the structure of organic substances to determine compound characteristics and purity. Brominating alkenes is a reaction also known as the halogenation of alkenes. Halogenation can occur with all halogen atoms, however bromine and chlorine are the most common due to their reaction stability. The addition of chlorine to ethylene results in the formation of polyvinyl chloride or PVC. PVC is common organic polymer used in plumbing. The polymer is desired for its resistance to corrosion and decomposition (3).

Experimental: The green preparation of (E)-dibromostilbene was performed. In order to achieve this, a reflux apparatus was assembled over a hot plate. A conical vial with a spin vane was attached to a condenser with rubber tubing leading to the sink for both inlet and outlet water flow. A sand bath was used to evenly distribute the heat and stabilize the vial during heating. A depiction of this reflux apparatus setup can be seen in Figure 2.

Figure 2: Reflux setup for the preparation of (E) - dibromostilbene.

Once the reflux apparatus was assembled .1302 g of (E)-stilbene was dissolved in 3 mL of ethanol in a conical vial with a stir bar. This mixture was heated to reflux until the solid (E)-stilbene was dissolved into solution. Then 0.3 mL of HBr was added to the refluxing mixture using a pipette. The solution was continuously heated and 0.2 mL of 30% H2O2 was added with a pipette. The solution’s color changed to a yellow hue. The solution was heated and stirred until the color returned to clear. Once this was achieved the vial was removed from the reflux apparatus and cooled to room temperature. Sodium bicarbonate was then added dropwise (approximately 10 drops) to the solution until the pH was observed to be 7 using litmus paper.

After the solution reached room temperature and the desired pH was observed, it was placed into an ice bath to precipitate the product. The product was then collected using vacuum filtration. The product and filter paper was rinsed dropwise with deionized water during the filtration process. Once the product was thoroughly rinsed and filtered it was transferred to a watch glass and dried. The dried precipitate was then added to a craigtube where it was recrystallized using approximately 9 mL of xylene. The recrystallization process entailed dissolving the precipitate in xylene during heating and then cooling the solution in order to precipitate the products. The precipitated crystals were then isolated through vacuum filtration and dried. The recrystallization process is carried out to produce a purified final product. The final product was then collected for melting point and IR spectra analysis.

Results: A successful green preparation of (E)-dibromostilbene was performed. The final mass of the product collected was 0.1015 g. The percent yield was accurately determined to be 77.5%. The experimental melting point of the product was determined to be 237.1 ˚C to 238 ˚C. Once percent yield and melting point was determined, the product sample was submitted for IR spectroscopy analysis. The IR spectra for this sample can be seen in Figure 3.

Figure 3: IR Spectra of (E)-dibromostilbene

Discussion: Analyzing the IR spectra data shows a distinct peak at 3028 (cm-1), indicating a C-H bond stretch within an aromatic ring. This is consistent with the molecular structure of the desired product, (E)-dibromostilbene. Within the fingerprint region of the IR spectra there are peaks in the 700-500 (cm-1) region likely indicating a C-Br bond. The absence of a peak in the 1610-1680 (cm-1) region indicates that there is no C=C bond stretching. This is also a positive indication that the C=C bond in (E)-stilbene was broken in order to form (E)-dibromostilbene. The 77.5% recovery of (E)-dibromostilbene was an impressive yield. The percent recovery was found by first calculating the theoretical yield base on the limiting reagent, which

was determined to be HBr, and then comparing it to the experimental yield. The observed melting point of the product was found to be 237.1 ˚C -238 ˚C. The actual melting point of (E)-dibromostilbene is 241 ˚C (4). The actual and experimental melting point of (E)-dibromostilbene are within a 1.5% error. The melting point analysis was a positive indication that a pure production of the desired product was achieved.

Conclusion: A successful preparation of (E)-dibromostilbene was performed. The final product had a 77.5% recovery yield and the observed melting point was within 1.5% of the actual melting point. A higher yield could have been achieved by conducting a more efficient filtration method. Human error led to some of the product crystals being discarded as waste during the vacuum filtration process. In order to produce the desired product in the safest and most efficient way, a green approach was taken. This involved generating the reagent of Bromine through oxidizing hydrobromic acid with peroxide. The usefulness of IR Spectrometry and a better understanding of SN2 reactions mechanism was gained from this experiment. For further experimental changes, the effectiveness of different reagents could be analyzed. The reagent of bromine formed by oxidizing hydrobromic acid with peroxide, bromine in a chlorinated solvent, and Pyridinium bromide could be compared to one another. Percent yields of the product would have to be determined for each individual reagent and then compared. This would determine the most effective reagent.

References: (1) Muathen, H. (2004). Pyridinium Dichlorobromate: A New Stable Brominating Agent for Aromatic Compounds. Synthesis, 2002(02). doi:10.1055/s-2002-19794 (2) “Principles Of Green Chemistry And Green Engineering.” Green Chemistry and Engineering, 2014, pp. 21–42., doi:10.1002/9781118720011.ch2. (3) Wigal, Carl. (1998). Chemical Education Resources: Bromating Alkenes. (4) Fieser, Louis F.; Journal of Chemical Education, 1954, V31, P291-7...