Lab - Reactions of Aspirin and Other Analgesics

Pre-Lab 5 Reactions of Aspirin and other Analgesics WARNING: The pre-lab questions are due at the beginning of the lab. You will not be permitted to stay in the lab if this sheet is not completed. You may need to do some research to find all the answers for these compounds. 1. Find the melting point of aspirin. MP = _______ oC 2. Draw structures for each of the compounds in this reaction: + Salicylic acid + acetic anhydride  Aspirin Acetic acid 3. Predict the pH and draw a structure for the following analgesics: pH: Choose acid, Structure neutral, or base Aspirin X - See above Acetominophen Ibuprofen Buffered Aspirin X 0.15% Salicylic acid X - See above Caffeine Lab 5 Preparation of Acetylsalicylic acid and Properties of Analgesics Goal: Make a sample of acetylsalicylic acid (aspirin). Learn the physical and chemical reactions of a set of analgesics. Always follow the lab instructor’s directions for wearing goggles and gloves and for proper disposal of excess or used chemicals. 1. Preparation of Aspirin: All steps are done in a 125 mL Erlenmeyer flask under a fume hood. Mix 2.00g of salicylic acid (limiting reagent) with 5.0 mL acetic anhydride (excess). Add 10 drops of 85% phosphoric acid and stir. Heat the reaction mixture in a boiling water bath, stirring until the solid is dissolved. After the flask has cooled to room temperature, add 20 drops of distilled water. Add 50 mL cold water and cool the flask in an ice bath for at least 10 minutes while the product crystals form. Scratch the inside of the flask with a stirring rod to start crystallization. Collect the aspirin using a Buchner filtration system. Record the mass of the filter paper. Pour the contents of the reaction flask onto the filter paper in the Buchner funnel and turn on the vacuum line. The flask can be rinsed with cold water if any solid remains behind. Allow the air suction to dry the crystals. Place the filter paper with the aspirin on a watch glass and dry in the oven for 20 min. Weigh the filter paper and the aspirin then subtract the mass of the filter paper to determine the yield. Test and record the melting point of your product. Does your product match the MP of aspirin? MP of your sample = _______oC Sample yield = _________g Use the structures from the pre-lab for each of the compounds in this reaction and identify the functional groups that are involved in the reaction. Salicylic acid (s) + acetic anhydride (l)  Aspirin (s) + Acetic acid (aq) (Note that this reaction is reversible and when aspirin is stored too long it will start to break down into salicylic acid and acetate.) 2. pH: Make solutions of each of the solid analgesics by mixing a pinch of each (crushed) solid in 2 mL water. Use pH paper to test the pH of all the analgesics in the list. Save the samples for the iron(III) chloride test. The pH is a reversed log scale of the H+ concentration. The more protons a substance releases in water the more acidic and the lower the pH. pH = -log[H+] Organic acid groups are called carboxylic groups. They have the formula: COOH (or CO2H). The hydrogen shown in the formula is the one that can be donated making this group acidic. Complete the following reaction to show the loss of a proton from a carboxylic acid. The other product (carboxylate) has a delocalized pi system (resonance). Draw both Lewis structures to show how the negative charge is shared. (Note: The common name for the 2-carbon carboxylate is acetate.) -COOH  H+ + [  ] 3. Iron(III) chloride test for purity: Add 5 drops of 1% iron(III) chloride solution to each of the samples from the pH test and mix. The FeCl3 reacts with salicylic acid to give a purple color. The 0.15% salicylic acid represents the maximum allowed in commercial aspirin (by the expiration date). If the analgesic gives a deeper purple color, it would be considered impure. Test results for the analgesics: pH Iron(III) chloride test: Is the sample pure? Aspirin Acetominophen Ibuprofen Buffered Aspirin 0.15% Salicylic acid Acetylsalicylic acid (product from part 1) Compare the aspirin you made to the commercial products in terms of both the pH and the purity.