Microwave Synthesis of Phenacetin - Lab Report Example

Name: Lab Instructor: Date: Experiment 3: Microwave Synthesis of Phenacetin Objective This experiment was carried out with the aim of learning about an aspect of green chemistry; using microwave heating to synthesize phenacetin instead of the conventional heating method. Introduction Microwave-accelerated synthesis reactions have produced amazing results that have made chemists to be interested in developing this method as an alternative to synthesis of different substances (Dicks). Microwave radiation is efficiently converted to superheat at ambient pressure, hence, achieving enormous accelerations in reaction time. Superheating in a closed reaction vessel under high pressure can reduce a reaction that takes several minutes under conventional conditions to a few minutes (Strauss and Trainor). The method is also environmentally clean. In this experiment, phenacetin was synthesized using p-phenetidine and acetic anhydride in a microwave at a temperature of 90oC. Below is the chemical equation of the reaction. Synthesis of phenacetin Experimental Procedures Synthesis of Phenacetin: A magnetic stirring bar was inserted into a quartz reaction vessel before filling the vessel with 10 mL of reverse osmosis water and 1mL of 4-ethoxyaniline. Using a syringe, 1.5mL of acetic anhydride was transferred into the quartz reaction vessel. The reaction vessel was then sealed securely and placed in the stand on the reaction rotor before executing the microwave program. After the program was complete, the reaction was allowed to cool to about 50oC and then removed from the microwave and placed in the fumehood. The reaction vessel was then removed from the rotor, and with care, the pressure relieving valve cap and the white Teflon seal were removed, followed by the small magnetic stirrer. The contents of the reaction vessel were then poured into a clean 50mL beaker and the solution allowed to slowly cool at room temperature to form solid crystals. After the flask attained room temperature, it was placed in an ice bath for about 12 minutes. The vacuum filtration apparatus and the 125 mL flask were then set up using the small Buchner funnel with a disk of filter paper before collecting the crystals through vacuum filtration. The crystals collected were then transferred to a pre-weighed watch glass, oven dried at 80oC for about 10 minutes and the watch glass and its contents weighed to obtain the crude yield. Recrystallization of the crude, impure phenacetin: The crude, impure phenacetin sample obtained after the procedure above was placed into a wide neck 50mL conical flask and 15 mL of hot methanol, along with a boiling stick were then added to the sample. The mixture was then heated on a hot-plate until it gently started to boil. After ensuring that all the sample has dissolved, 5 mL of hot water was added to the solution. The solution was then boiled again for about 5 minutes, before removing the conical flask from the hotplate. The solution was then allowed to cool at room temperature to form crystals of phenacetin. Once the crystals appeared, the conical flask was cooled in an ice bath for about 10 minutes to form more crystals. The mixture was then filtered through vacuum by swirling the flask and then transferring the solution containing the phenacetin crystals into a Hirsch funnel. The crystals were then left to dry by leaving them on the filtration apparatus for about 10 minutes. Finally, the crystals were then transferred to a pre-weighed watch glass and then oven dried at 80oC for 10 minutes before determining the weight of the final phenacetin yield. The figure below shows a picture of the reaction vessel placed on the rotor. Figure 1: A picture taken during the experiment, showing the reaction vessel on the rotor. Results After synthesizing phenacetin in the first part of the procedure, the mass measurement results obtained were recorded in table 1 below: Table 1: Mass of crude phenacetin product before recrystallization Mass of glass 34.60 g Mass of glass + crystals product 34.86 g Mass of crystals product 0.26 g Observations: The crystals of the crude product are odourless and white in colour. Crude yield: The limiting reagent for this reaction is 4-ethoxyaniline; Density of 4-ethoxyaniline = 1.063g/cm3 1 mL of 4-ethoxyaniline = 1.063 g Molecular weight of 4-ethoxyaniline e = 137.18 g/ mol Moles of 4-ethoxyaniline in the reaction = Molecular weight of phenacetin = 179.22 g/ mol Mass of expected (theoretical) yield = (0.0129 mol 179.22 g/ mol) = 2.31 g % crude product yield = ( 100 %) = 11.26 % After recrystallization, the mass measurements were carried out and the results recorded in table 2 below. Table 2: Mass of pure phenacetin after recrystallization Mass of glass 34.60 g Mass of glass + crystals product 34.74 g Mass of crystals product 0.14 g Melting point of pure phenacetin product 138 Observations: The purified product is odorless with fine white crystals. The product was observed to melt at a temperature of 138. This compares to the literature value of melting range of pure phenacetin, which is 137oC - 138oC. Therefore, the product obtained is highly pure. Pure product yield: % yield of pure, recrystallized product = ( 100 %) = 6.06 % % Recovery = The figure below show the IR spectrum for starting material and the pure material. Figure 2: The FT-IR spectrum for the starting material In the figure 2 above, the spectrum for the starting material has a peak of 97.09% at 4000 cm-1. This is comparable to the pure substance and shows the high purity of the starting material used in this experiment. Figure 3: The FT-IR spectrum for the pure material Figure 4: Ideal FT-IR spectrum of phenacetin The FT-IR spectra in figure 3 above is a close resemble of the one in the literature, shown in figure 4 above. Maximum transmittance occurs at wavenumbers above 1500 cm-1, though the transmittance in the experiment is higher compared to the ideal spectrum. Combining this IR results and the melting point of the product, it can then be characterized as highly pure. The green aspects in this experiment This lab demonstrates some important principles of green chemistry, which is being introduced in chemistry experiments to complement the existing conventional experiments that may be not friendly to human health and the environment. In the conventional heating method for synthesis of phenacetin, for instance the Williamson ether synthesis, acetaminophen is converted to phenacetin by reflux (Leadbeater). During this process other products like potassium iodide and potassium hydrogen carbonate, and also used sodium sulphate waste. Although 4-ethoxyaniline is toxic and harmful to human health, the reaction conditions are in this experiment are mild. The reaction medium used is mainly water. Water and ethanol are used as the re-crystallization solvent. All the materials used in the synthesis reaction are incorporated into the final product, preventing the formation waste. The substance produced in this method has little toxicity to human health and the environment (Dunn, Wells and Williams). Suggestions to improve the experiment In order to increase the product yield, neutralization of the liquid that remains after filtration can be done (Hanson). The product can also be characterized by Raman spectroscopy and NMR analysis. Conclusion The experiment is based on some principles of green chemistry, hence, making it a convenient complement to the conventional method. The reaction yielded 6.06 % of pure phenacetin, which melted at a temperature of 138oC. The possible impurities in this product are ethanol and the re-crystallization solvent. The product yield is lower than the expected theoretical yield and this may be due to loss during transfer between glassware and spills. The low yield could have been also due to the decolorization and filtration processes, which tend to decrease the yield. This experiment illustrates a simple green method for synthesis of phenacetin, and therefore, the objective of this experiment has been successfully achieved. References Read More