2. Synthesis of Aspirin

Transcription

1 This is a two-part laboratory experiment. In part one, you will synthesize (make) the active ingredient in aspirin through a reaction involving a catalyst. The resulting product will then be purified through a method called recrystallization. Prelaboratory Assignment Read the lab guide, and complete the prelab assignment in Chem21. The prelab assignment is due at the beginning of lab on the day your lab meets. 2.1 Introduction The salicylate ion is credited with the pain relieving effect in aspirin. The ion is easily formed from salicylic acid and is a chemical relative to salicin, which is found naturally in the bark of the willow tree. Since salicylic acid is internally corrosive and the sodium salicylate salt has an objectionable sweet taste, acetylsalicylic acid was synthesized (by a German chemist working for the Bayer firm) and found to be non-corrosive and palatable. Later named aspirin, this compound is stable under the acidic conditions found in the stomach but is converted to the salicylate ion under the more basic conditions found in the intestines. Aspirin is usually prepared by reacting salicylic acid with acetic anhydride in the presence of a catalyst. The general class of reaction is called ester formation. In the reaction scheme below, the chemical structures of the compounds in the reaction are shown. The hexagonal, ring-looking structures are called benzene rings and each point of the ring represents a carbon atom. If there is no other bond coming off the structure, you can assume there is a hydrogen atom attached to the carbon. The ring in the center means that every other bond is a double bond. 1

2 H H + H 3 C CH 3 H 3 C H + H CH 3 salicylic acid acetic anhydride acetylsalicylic acid (aspirin) acetic acid C 7 H 6 3 C 4 H 6 3 C 2 H 4 2 C 9 H 8 4 Due to incomplete reaction or side-reactions that often occur, chemical reactions rarely lead to the formation of pure products. Therefore, products must be purified after the reaction is over. Well-packed crystal structures form when many similarly-structured molecules pack together. However, if there are impurities in the collection of molecules, they tend to disrupt the crystal packing process. (See Figure 2.1.) Recrystallization is a process used to purify chemicals that takes advantage of this packing process. When a compound precipitates slowly out of a solution, if the majority of the molecules are identical (hopefully the product), then they will pack well together, and any impurities will be excluded. In other words, the impurities will tend to stay in solution since they don t fit well in the crystal structure. The newly purified product can then be filtered to remove it from the impurities left in the solution. Figure 2.1: The image on the left represents a well-packed crystal structure for a pure compound. The image on the right represents a poorly-packed crystal structure for a compound containing an impurity. 2.2 Precautionary Measures Acetic anhydride is a dangerous chemical. The vapors will irritate the eyes and nose. The liquid can cause burns on the skin. Use the chemicals in your hood area 2

3 and if you spill any, wash with large amounts of water. Sulfuric acid is a strong acid that will burn your skin or put holes in your clothes if you spill some on yourself. If you spill any (even a drop) clean it up immediately using lots of water. If you spill any on your skin, rinse the area immediately with lots of cool water for several minutes and notify your instructor right away. Gloves and goggles should be worn at all times during this experiment. Use caution when working with a hot plate. Don t let the cord come in contact with the hot surface. 2.3 Experiment Set up your laboratory notebook as you have for the previous lab. In this experiment, there isn t much quantitative data (numbers). However, your qualitative data (observations) is very important. You should describe each step of the procedure and any associated observations. Synthesis Fill a beaker about half-full of water, and begin heating. The temperature should be about C. It s okay if the temperature fluctuates a little. Meanwhile, accurately weigh out approximately 1 g of salicylic acid. (Record the mass of salicylic acid used and the appearance of the compound.) Transfer this powder to a medium-sized, dry test tube, and add 2 ml of acetic anhydride to the test tube. Stir the contents of the test tube carefully with a glass rod for a couple of minutes. (The solid won t dissolve completely.) When you are ready, notify your instructor to add two drops of the catalyst, concentrated sulfuric acid, to the test tube. Heat the reaction in the warm water bath for 15 minutes, stirring occasionally. Now that the acetylsalicylic acid has been synthesized, it must be isolated from the solution. Fill a beaker about half-full with an ice water bath (a mixture of ice and water in a 400 ml beaker). Allow the test tube solution to cool for 5 minutes in the ice water bath, stirring occasionally. Then, add a mixture of about 5 g of ice and 5 ml of cold water directly to the test tube containing your reaction mixture, and continue cooling for 10 minutes in the ice water bath. Watch for crystal formation. You may need to swirl the test tubes and/or stir the solution with a glass rod to cause crystals to form. Set up a Büchner funnel with some filter paper to filter out the crystals. (Your instructor will demonstrate the use of a Büchner funnel.) Then, use a rubber policeman (the rubber part that fits on the end of the stirring rod) to transfer all the crystals to the filter paper. Wash the reaction test tube with two-5 ml portions of ice cold water, pouring the 3

4 liquid over the crystals on the filter paper to help wash them. Record your initials on a small watch glass, and transfer a small amount (about the size of the tip of your spatula) of your crude (slightly impure) product from the funnel to the watch glass. Set this aside to be dried and tested next week. The supernant liquid, the liquid in the bottom of the filter flask, should be added to the waste container. Leave your filter apparatus set up to use in the next part of the experiment. Purification Add about 5 ml of ethanol to a labeled test tube. To another labeled test tube, add about 5 ml of DI water. Put both tubes into the hot water bath to warm up. Use a rubber policeman to transfer the remaining crude aspiring product into a third test tube and add about 2.5 ml of the warm ethanol. Then, add this tube to the hot water bath and bring the solution in the test tube to a boil. When the ethanol-aspirin mixture starts to boil remove it from the heat and add the warm water directly to the test tube. If a precipitate forms, heat the test tube until everything has re-dissolved. (It may be necessary to add 2 additional ml of warm ethanol, but use as little additional solvent as possible.) nce all the aspirin has dissolved, let the solution cool slowly to room temperature for about 10 minutes. (Don t rush this step or your product will recrystallize too quickly and will contain impurities!) Finally, place the test tube in an ice water bath to complete crystallization. You may need to swirl the test tube contents or stir with a glass rod to cause crystals to form. If crystals still don t form, try scratching the inside surface of the test tube with the stirring rod. While the crystals are forming, obtain a watch glass and a new piece of filter paper. In pencil, write your initials and the date on the filter paper. Then, weigh the paper and watch glass together. (Record the mass!) With the new filter paper, filter the crystals using a Büchner funnel apparatus and ice cold water as a rinse. When you ve collected all your crystals, gently transfer the filter paper back to the watch glass. Don t lose any crystals! Place the labeled filter paper/watch glasses on a sheet of plain paper that has your name, your partner s name, and lab section on it. Then, give the paper and watch glasses to your instructor to store until next week. 4

5 2.4 Hazardous Waste and Clean-up Hazardous Waste The chemicals used in this experiment are dangerous and toxic. Save your waste in a large beaker until the end of the lab. Then, pour your waste in the in the hood at the front of the lab Clean-up Dry your glassware and return it to your station. Then, wipe down your benchtop with a wet paper towel and dry it. Have your instructor check your station before your leave. BEFRE YU LEAVE LAB: Tear out the carbon-copy pages of your notebook. Make sure your name, your partner s name and your section number are on each page. Staple these pages together and turn them in to your instructor Lab Assignment For this experiment, your grade will be based on the observations you recorded in your lab notebook. You will see a completed Lab 2 Assignment in Chem21, and the grade for your observations will be entered by your TA. You do not need to enter anything in Chem21 for this experiment. 5