University of Wisconsin Chemistry 116 Preparation and Characterization of Aspirin and Some Flavoring Esters * Esters are an important class of organic compounds commonly prepared via a condensation reaction between a carboxylic acid and an alcohol in the presence of a dehydrating agent, often H 2 SO 4. Esters are responsible for many of the pleasant aromas associated with various fruits, and as such are used in perfumes and flavorings. Some esters also have useful physiological effects. The best known example is the analgesic ( pain killing ) and anti-pyretic ( feverreducing ) drug acetylsalicylic acid, commonly known by its trade name aspirin. Aspirin is prepared from salicylic acid (see below). Salicylic acid contains both the organic acid and the alcohol functional groups and is capable of two different esterification reactions, depending on which functional group reacts. To make acetylsalicylic acid, the -OH group of the salicylic acid reacts with acetic acid to produce the ester and water. This reaction, like most esterification reactions, is an equilibrium process that does not go to completion and will not produce 100% yield unless the water is removed during the reaction. It is also not a very fast reaction. If acetic anhydride is used instead of acetic acid, the reaction takes place much more quickly and the yield can be near 100%. You will use acetic anhydride in your aspirin synthesis. O C OH salicylic acid CH 3 OH C O + O C O CH 3 acetic anhydride O C O OH + CH 3 OH O acetic acid O CH 3 acetylsalicylic acid 1
If the ester is made from the carboxylic acid group of the salicylic acid by reacting with methanol, the product is methyl salicylate. You can also make methyl salicylate and some other flavoring esters in this laboratory activity. Synthesis of Aspirin Weigh approximately 1 gram of salicylic acid into a 25 ml Erlenmeyer flask. Add 3.5 ml of acetic anhydride to the test tube. This reagent is kept in the fume hood in the laboratory and is outfitted with a pipet dispenser (called a repipet ) which has been pre-set to deliver exactly 3.5 ml. Once the acetic anhydride has been added do not point the open end of flask at yourself or anyone else. Cautiously add 3 to 4 drops of concentrated (18M) sulfuric acid to the flask, and then mix the contents thoroughly using a glass rod. If the solid does not dissolve, add 1 to 2 more drops of concentrated sulfuric acid and stir well. If the solid still does not dissolve, discard the mixture and begin again with a clean DRY flask. Allow the reaction to take place for about 10 minutes. In the meantime, chill approximately 50 ml of deionized water in an ice water bath. After 10 minutes, place the flask in an ice water bath for about 5 minutes, and then SLOWLY add 5 ml of cold deionized water (< 0.5 ml at a time, stir with a glass rod after each addition). The addition of water will destroy any unreacted acetic anhydride, releasing heat in the process. Although the amount of acetic anhydride is reduced greatly after the reaction, you should still use caution. Return the flask to the ice water bath for about 15 minutes. After this time, you may see the white crystals of the solid aspirin. If you don t see a significant amount of white solid, stir the solution and scratch the inside of the flask with a glass rod to induce precipitation. Continue to cool for an additional 15 minutes after precipitation begins. Separate the crystals from the liquid using a Büchner filtration apparatus (Figure 1), washing the aspirin out of flask as thoroughly as possible with small portions of the ice-cold deionized water. Wash your product with an additional small portion of the ice-cold deionized water and draw air through it for several minutes. 2
Figure 1. Buchner filtration apparatus When the aspirin crystals are dry, weigh them and record the mass. Assuming the reaction goes to completion, determine the percent yield for your synthesis. Salicylic acid (MW = 138 g/mol) Acetic anhydride (MW = 102 g/mol, density = 1.08 g/ml) Acetylsalicylic acid (MW = 180.16 g/mol) Recrystalization The crude products obtained from most preparations of organic compounds are contaminated with unreacted starting materials and substances from side-reactions. These can often be eliminated by a simple process known as RECRYSTALIZATION. Save about 100 mg of the crude product for a TLC test. Dissolve the rest of your crude product in a minimum volume of ethanol in a 125 ml Erlenmeyer flask, warming the alcohol in a water bath to affect dissolution. This should take 10-30 ml of ethanol depending on the amount of aspirin. If any solid material remains undissolved, filter the solution to remove the solid materials Add 25 ml of warm water (about 50 o C) to the clear alcohol solution. If any crystals appear at this point, heat the contents of the flask until they dissolve. Set the flask aside to cool, observing it carefully. When crystals start to form, cool the flask by surrounding it with cold water. The crystallization process will then go to completion. Collect the crystals by vacuum filtration. Allow the crystals to dry and save your sample of the aspirin for further analysis. TLC Analysis You will run a TLC analysis on your recrystalized product, your crude product, the salicylic acid starting material, and a sample of a commercial aspirin tablet. Dissolve just a few mg of each into 0.1 ml of ethanol. You will need to grind the commercial acid tablet with a mortar and pestle before dissolving. Mark a 7 cm x 2 cm piece of Silica/UV paper and spot the paper with each solution. The UV paper is used so that the spots can be seen under ultraviolet illumination. Place about 5 ml of the 80/10/10 petroleum ether/ethanol/ethyl acetate solution in the wide mouth developing 3
chamber. Add few drops of acetic acid to make the medium acidic. This should protonate all of the acids. Develop the TLC plate and mark the spots lightly with a pencil while illuminating the paper with the 254 nm Hg-UV lamp. Calculate the R f for each component that you see. Can you get an idea of the purity of your sample from the TLC data? Spectroscopic Analysis of Aspirin Although some of the details of the spectroscopic methods that you will use to analyze your aspirin are beyond the scope of this course, it should be interesting to see how modern instrumental methods of analysis provide information about the identity and the purity of a reaction product. For your purified product, you will obtain a proton nuclear magnetic resonance (NMR) spectrum, a vibrational spectrum using an Fourier transform infrared spectrometer (FTIR) equipped with an attenuated total reflection (ATR) crystal, and a gas chromatogram and mass spectrum using a gas chromatography mass spectrometer (GCMS). These instruments are located in room 2330 chemistry. Dr. McClain, the instrumental laboratory director, will help you run your samples on these instruments. Prepare your samples for each analysis as follows: NMR Dissolve 50 mg of your aspirin in 1 ml of the deuterated chloroform/tms solvent in a small sample vial. Its OK if your solution is a little cloudy; if it is cloudy filter it through a small plug of glass wool into the NMR tube. FTIR About 10 mg of the solid sample will be used for this analysis. GCMS Dissolve 10 mg of your aspirin in 0.5 ml of dichloromethane in a small sample vial An NMR spectrum, FTIR-ATR spectrum, and a GCMS chromatogram and spectrum of pure acetylsalicylic acid are attached to the end of this handout. These data were taken on the same instruments that you will be using. After your experiment, you can compare these spectra to the spectra from your sample. Preparation of Some Flavoring Esters If you have time after the aspirin synthesis, or during waiting periods, you can synthesize some common esters. The general procedure for preparing flavoring esters is as follows: Place approximately 2 g (or 2 ml if the substance is a liquid) of the carboxylic acid and 2 ml of the alcohol in a large test tube. Add 5-7 drops of concentrated (18 M) sulfuric acid, mix the solution well with a glass stirring rod and then place the test tube in a hot water bath (~ 80 C) for 5-10 minutes. For test tube #1 the water bath temperature should not exceed 65 C. Remove the test tube from the hot water bath and cautiously pour the mixture into about 15 ml of saturated sodium bicarbonate contained in a small beaker. The sodium bicarbonate will destroy any unreacted acid. 4
Observe the aroma produced by the product of each of the esterification reactions. Draw the structure of the esters produced and the balanced equations for the esterification and the acid/sodium bicarbonate reactions. Carry out the following reactions using the procedure above. Be sure to record your observations. Safety and Disposal The 18 M H 2 SO 4 and glacial acetic acid are both corrosive compounds. Flush skin that has been exposed to these reagents with water for at least 15 minutes. Remove any clothing that is contaminated with these corrosive solutions. If blistering occurs, seek advice from a medical professional. Many organic compounds, in particular glacial acetic acid, are irritating to the respiratory system. Do not take a big sniff of these materials!! The chlorinated hydrocarbons are known carcinogens so you will work with these in the hood. Gloves must be worn when handling sulfuric acid, acetic acid or acetic anhydride and when cleaning up any spill that occurs during this laboratory period. 5
Acetic anhydride reacts vigorously with water in acid solutions. Make sure that test tubes are dry and that concentrated acid is added in the first step of the reaction. Dilute acids, which are mostly water, will cause unwanted vigorous reactions, so make sure that you do not add acetic anhydride to a dilute acid. All organic compounds used in this experiment are flammable. Use a water bath warmed on a hot plate (NOT a Bunsen burner) to heat organic solutions. All aqueous chemicals may be disposed of in the sink. 18M H 2 SO 4, glacial acetic acid and acetic anhydride should be diluted and disposed of slowly by adding excess water to the sink. Solid aspirin and any solid esters must be disposed of in the Solid Chemical Waste jars. Organic liquids (liquid esters, alcohols, TLC mobile phases) must be placed in the organic waste carboy. Halogenated organic liquids (chloroform and dichloromethane) must be placed in the carboy designated for halogen waste. Used filter paper should go in the regular trash. Used TLC plates should go in the appropriately labeled container. Laboratory Report For this experiment, you will submit a written report. Your TAs will provide you with details of its format. * This experiment was written by Jacqueline L. Scott and revised by G. Bain and C. C. Wilkinson. Portions of the procedure were adapted from Laboratory Experiments for Chemistry 108, by G. E. Dirreen, Stipes Publishing Co., 1985. The spectroscopic analysis and TLC tests were added by J.Laaser, L.Buchanan and R. McClain, January 2009. Copyright 2009 by the Department of Chemistry, University of Wisconsin-Madison.. 6