Synthesis of Aspirin: Understanding Limiting Reagents and Percent Yield, Lecture notes of Medicine

Download Synthesis of Aspirin: Understanding Limiting Reagents and Percent Yield and more Lecture notes Medicine in PDF only on Docsity!  E x p e r i m e n t 5 73  Synthesis of Aspirin Lecture and Lab Skills Emphasized • Synthesizing an organic substance. • Understanding and applying the concept of limiting reagents. • Determining percent yield. • Learning how to perform a vacuum filtration. • Understanding and performing recrystallization. In the Lab • Students will work in pairs. • Parts must be completed in order. • Record your procedure and original data in your lab notebook along with your calculations. • Report data collected and subsequent calculations to www.chem21labs.com. • All equipment should be returned to the correct location after use. Waste • Salicylic acid should be disposed of in the solid waste container. • Acetic anhydride should be disposed of in the organic waste container. • Filtrates (liquids) can be washed down the drain with excess water. • Solid product and filter paper can be disposed of in the solid waste container. • Purity test solution should be disposed of in the aqueous waste container.  74  E x p e r i m e n t 5 • Synthesis of Aspirin Expt. 5  →  H3PO4 Safety • Gloves and safety goggles are mandatory when anyone is performing an experiment in the lab. • Work in the hood when indicated in the procedure. • Wear long pants, closed-toed shoes, and shirts with sleeves. Clothing is expected to reduce the exposure of bare skin to potential chemical splashes. • Always wash your hands before leaving the laboratory. Additional information can be found at http://genchemlab.wordpress.com/5-aspirin. Salicylic acid has the same analgesic properties as aspirin and was used for many years as a headache medicine. However, salicylic acid is more acidic than aspirin, and is especially irritating to the mouth and stomach. Some people find even aspirin too acidic and prefer to use aspirin substitutes like acetaminophen and ibuprofen. Aspirin is produced from a reaction between acetic anhydride and salicylic acid. The structures of the re- actants and products are shown in Figure 5.1. Examine them. What is changing about the salicylic acid? Being able to identify what is reacting helps organic chemists determine how a reaction occurs. acetic anhydride salicylic acid aspirin acetic acid F.W. = 102.04 g/mol F.W. = 138.12 g/mol F.W. = 180.15 g/mol F.W. = 60.05 g/mol density = 1.082 g/mL M.P. = 135 °C (CH3CO)2O HOC6H4COOH CH3CO2C6H4CO2H CH3COOH Figure 5.1. Synthesis of aspirin. A local pharmaceutical company is looking at forming a partnership with your research company. As part of their research on your company, they have asked your supervisor to look at researcher performance and have asked that you synthesize a simple substance and dem- onstrate its purity. Synthesis of Aspirin Aspirin is the single most manufactured drug in the world. Aspirin’s chemical name is acetylsalicylic acid, and it is synthesized from the reaction of acetic anhydride with salicylic acid in the presence of phos- phoric acid as a catalyst. The by-product is acetic acid (Figure 5.1). 77  E x p e r i m e n t 5 • Synthesis of Aspirin  Expt. 5 5. Add 15 mL of water and cool the 100 mL beaker in a larger beaker with ice water. Stir with a stirring rod. Allow 5 minutes for the aspirin to crystallize from the solution. In the meantime, put a wash bottle filled with distilled water in ice water to cool. You will need cold water in the filtration step to follow. 6. Set up a Büchner filtration system as shown in Fig- ure 5.4. Place a filter paper in the funnel, and wet the paper thoroughly with distilled water. Turn the water on to allow the paper to be sucked onto the funnel. In order to ensure a good seal, you may want to add a small amount of water into the funnel. 7. Collect the crystals by pouring the solution onto the filter paper. Make sure that the water is fully open for maximum suction filtration. Do not disturb the crystals on the filter paper or you will break the filtration vacuum. Wash any product in the beaker into the funnel with cold distilled water; rinse your stirring rod as well. Remember, any product you lose here will affect your percent yield. ©Hayden-McNeil, LLC Figure 5.3. Hot water bath. Filter paper To aspirator on water faucet Filtration flask Büchner funnel Filtrate Separation Technique: Suction Filtration © H ayden-M cN eil, L L C Figure 5.4. Separation technique: Suction filtration. 8. Allow the filtration to continue until you see no more water droplets fall from the funnel. Turn the water off. Insert a spatula between the edge of the paper and the side of the funnel and lift the paper from the funnel. Transfer the product to a 100 mL beaker. Scrape all the product from the filter paper with a spatula. Work carefully as you don’t want to lose any product in this step. Note the color and texture of your crystals in your data table. Recrystallization of Aspirin 9. Add about 3 mL of ethanol to the crystals. Warm the solution in a hot water bath. If all the crystals do not dissolve, add another 1–2 mL of ethanol. You want to use as little ethanol as possible in the recrystallization step. 10. Pour 15 mL of warm water into the alcohol solution. Cover the beaker with a watch glass and set aside to cool. (The crystallization process can be sped up by putting the beaker in an ice bath.) Allow 5 minutes for the crystals to crystallize from the solution.  78  E x p e r i m e n t 5 • Synthesis of Aspirin Expt. 5 11. Record the mass of your clean, dry filter paper and filter the crystals using suction filtration as before. While the filtration is in progress, determine the mass of a dry, clean watch glass. 12. When the filtration is complete, remove the filter paper and place the paper and crystals on the watch glass. Label your watch glass before putting it in the oven and dry your aspirin in the oven at 110°C for 10–15 minutes. 13. When your product is dry, remove your sample from the oven and allow it to cool. Weigh the watch glass with the aspirin product and record the value in your data table. Record the color and texture of the recrystallized aspirin. Testing the Purity of Aspirin 14. Dissolve a small spatula point of your aspirin in about 50 mL of warm water. Transfer 1 mL of this solution to a test tube. Add 3 drops of the iron re- agent (Fe(NO3)3). Shake well for 2 minutes. Record all observations. Data Analysis Make sure to show all of your calculations in your lab notebook as a record of how you completed your calcu- lations. Don’t forget to include your units and correct number of significant figures! Then, go onto Chem21 and report your results. 1. What was the mass of aspirin synthesized? 2. What is the maximum mass of aspirin possible if salicylic acid is the limiting reagent? 3. What is the maximum mass of aspirin possible if acetic anhydride is the limiting reagent? 4. What is the theoretical yield of aspirin? 5. What is the percent yield of aspirin? 6. What happened when the iron nitrate solution was added to the aspirin? Make sure to respond with both an observation and the interpretation of your result.