LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Separation of the Components of a Commercial Analgesic Tablet: A Two-Week Sequence Comparing Purification by Two-Base Extraction and Column Chromatography Kevin D. Revell* Department of Chemistry, Murray State University, Murray, Kentucky 42071, United States
bS Supporting Information ABSTRACT: A new laboratory experiment is described in which students compare two benchtop separation methods to isolate the three active components of the commercial analgesic Excedrin. In the two-week sequence, aspirin, acetaminophen, and caffeine are separated using either a two-base liquid liquid extraction or silica column chromatography. Students then evaluate the two separation methods based on percent recoveries and on purity, as determined by thin-layer chromatography. This procedure introduces three important techniques using a single theme. The experiment has been designed to minimize cost of chemicals and hazardous waste generation, while challenging students to critically evaluate the strengths and weaknesses of different purification techniques. KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Acids/Bases, Chromatography, Drugs/Pharmaceuticals, Separation Science, Thin Layer Chromatography
T
he isolation of chemical compounds is a key component of any undergraduate organic laboratory course. In addition to recrystallization, distillation, and liquid liquid extraction, chromatographic techniques such as thin-layer and column chromatography are of particular value to synthetic chemists. Although several published experiments introduce preparative (column) chromatography,1 3 the cost of materials and the volume of waste produced make it difficult to teach this skill in an introductory laboratory. A two-week laboratory sequence emphasizing chromatographic techniques has been developed and implemented that involves the isolation of the three active components of the commercial analgesic Excedrin. Students are able to evaluate the utility of chromatographic techniques within the framework of other purification methods. In the first week, purification is accomplished by liquid liquid extraction, using aqueous bases of different strengths to selectively extract the acidic components. In the second week, students purify the materials by column chromatography. The materials isolated by each technique are analyzed and compared using thinlayer chromatography (TLC). The students then evaluate and compare the efficacy of the two techniques in terms of recoveries and purity of materials obtained. These experiments have been tested and modified over three semesters and have been conducted by approximately 150 students. Excedrin has been used in teaching labs to introduce a multitude of techniques, including TLC,4 HPLC,5,6 capillary electrophoresis,7 solid-phase extraction,8 and GC and LC MS.9,10 However, the use of two-base extraction11 and column chromatography for the isolation of the three analgesic components appears to be Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
unique. This experiment is a valuable addition to the curriculum because it introduces the students to three techniques and challenges them to critically evaluate the advantages and disadvantages of each method. Furthermore, the materials are inexpensive and only a small amount of hazardous waste is produced.
’ BACKGROUND A single tablet of extra-strength Excedrin contains three active compounds: aspirin (250 mg), acetaminophen (250 mg), and caffeine (65 mg) (Figure 1). In addition, Excedrin contains a starchy material that is used to bind the active components together. The three analgesic components differ in terms of their acidity, with aspirin being the most acidic (pKa 3.5), acetaminophen second (pKa 9.9), and caffeine is effectively not acidic. The differences in polarity of these three compounds produce good separations on TLC, and a number of published procedures exist for the separation of these analgesic components by TLC.1 4,12,13 ’ EXPERIMENT Week 1: Separation of the Excedrin Components by TwoBase Extraction
The students crush one tablet of Excedrin (or the generic equivalent) using a mortar and pestle, then add diethyl ether (20 mL) to the powdered material (note: ethyl acetate may be substituted for diethyl ether; see the technical notes in the Published: July 27, 2011 1413
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Figure 1. The three active components in extra-strength Excedrin.
Supporting Information). The starchy material is insoluble in organic solvents; however, a good portion of the active components will dissolve in ether. The ether is decanted or filtered through a conical filter into a separatory funnel. The solution is then extracted successively with 1 M aqueous K2HPO4 (10 mL) and 1 M aqueous KOH (10 mL). Because of the differences in basicity, aspirin is extracted into the phosphate solution, and acetaminophen is extracted into the hydroxide solution. Each of these solutions is then acidified using 6 M aqueous HCl, causing the conjugate acid to precipitate. Each component may then be collected by filtration and air-dried. After extraction, the ether layer contains primarily caffeine, which can be collected by evaporating the ether layer using a rotary evaporator or by traditional vacuum distillation. Following isolation, students determine the masses and calculate the percent recoveries for each component. Typically, the recoveries of aspirin and acetaminophen improve if the acidic solutions are allowed to stand; thus, students store these solutions in their lab drawers until the next period, when they complete the filtration and conduct the TLC analysis described below. Week 2: Separation of the Excedrin Components by Chromatography
Part A: Thin-Layer Chromatography. In the first part of week 2, students compare the purified components of Excedrin (stock solutions of the purified components are provided by the instructor) with the mixture obtained when a tablet is crushed, dissolved in ether, and filtered. The students obtain good results by TLC when they use small (2 5 cm) glass-backed plates with a UV indicator, use a 30 mL beaker with a watch-glass lid as a developing chamber, use commercial micropipets, and make very small spots on the plates. The students develop the plates using a solution of 1:2 hexanes/ethyl acetate with 1% acetic acid as eluent. Three distinct spots having Rf values of 0.6 (aspirin), 0.3 (acetaminophen), and 0.1 (caffeine) are observed for the mixture under these conditions (visualized under UV light). Part B: Column Chromatography. In the second part of the week 2 experiment, students prepare an ether solution containing the active components of the two tablets, using a procedure identical to part A. This solution is placed in a small beaker, and a small volume (0.5 mL) of silica gel is added. The solution is stirred as the ether is evaporated under a gentle stream of air and slight warming, resulting in a white solid containing the three active components adsorbed onto silica. This solid is loaded onto a small polypropylene column containing 3 g of silica. The silica is then flushed with solvent to elute the three products. Although all three products can be eluted with a single solvent mixture, successive addition of 1:1 hexanes/ethyl acetate (30 mL), 1:2 hexanes/ethyl acetate (30 mL), and acetone (30 mL) elutes all three components quickly and with good separation. When 15 mL capacity test tubes are used, the three components can be collected and resolved using only 6 tubes. The contents of each tube are determined using TLC. The students used rotary
Figure 2. TLC of Excedrin components after separation, showing aspirin (Rf = 0.6), acetaminophen (Rf = 0.3) and caffeine (Rf = 0.1). Eluted with 33:66:1 hexanes:ethyl acetate:acetic acid. (A) Results from week 1: (1) acidified phosphate extracts, (2) acidified hydroxide extracts, and (3) organic layer. (B) Results from week 2: column eluent, tubes 1 6.
evaporators to concentrate their solutions and isolate the materials. The students then measure the masses and calculate the percent recoveries for each component. Part C: Comparison of the Two Preparative Techniques. In the last step, students use TLC to analyze the three components that were isolated from the two-base extraction (Figure 2A). They compare the purity of the materials obtained by extraction and by column chromatography (Figure 2B). Additionally, the students compare the yields obtained in each step.
’ HAZARDS Goggles should be worn at all times in the laboratory. This experiment involves strong acids and bases, which are corrosive. Diethyl ether, ethyl acetate, and hexanes are each flammable and should be handled in fume hoods. The students should especially exercise care when working with diethyl ether, which is highly volatile. If time and resources permit, the instructor may use ethyl acetate or dichloromethane in place of ether (see technical notes in Supporting Information). The MSDS should be consulted for all materials prior to conducting this experiment. ’ TYPICAL RESULTS Generally, students are able to obtain aspirin in very high recoveries (60 100%) in both procedures. Recoveries for acetaminophen are often substantially lower (0 10% for extraction, 10 20% for chromatography) because the acetaminophen often forms a fine solid that is difficult to filter. Caffeine typically is recovered in higher percentages in the chromatography (40%) than in the extraction (1 5%) (see technical notes in Supporting Information). ’ SUMMARY The experiment described involves the use of three important techniques (extraction, column chromatography, and TLC) to isolate and analyze the components of Excedrin. Although some capital equipment must be available to conduct the experiment, 1414
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the chemical components of the experiment are fairly inexpensive, and the amount of waste generated is modest.
’ ASSOCIATED CONTENT
bS
Supporting Information A list of required equipment and chemicals; a detailed experimental procedure; and notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The author gratefully acknowledges Beth Brubaker for her excellent management of the teaching laboratories at Murray State University and for her assistance in acquiring the elements needed to develop these experiments. The author also is grateful to Carl Woods and Daniel Johnson, who assisted with the TLC images above. Carl Woods also produced the artwork included in the student handout. ’ REFERENCES (1) Gilbert, J. C.; Martin, S. F. Experimental Organic Chemistry: A Miniscale and Microscale Approach, 4th ed.; Thompson Brooks/Cole: Belmont, CA, 2006; Chapter 6, pp 185 190. (2) Williamson, K. L. ; Masters, K. M. Macroscale and Microscale Organic Experiments, 6th ed.; Brooks/Cole: Belmont, CA, 2011; Chapter 1, p 184. (3) Schoffstall, A. M.; Gaddis, B. A.; Druelinger, M. L. Microscale and Miniscale Organic Chemistry Laboratory Experiments, 1st ed.; McGraw Hill: Boston, MA, 2000. (4) Martin, N. H. J. Chem. Educ. 1981, 58 (10), 818–819. (5) Beaver, R. W.; Bunch, J. E.; Jones, L. A. J. Chem. Educ. 1973, 60 (11), 1000–1001. (6) Ferguson, G. K. J. Chem. Educ. 1998, 75 (4), 467–469. (7) Thompson, L.; Veening, H.; Strein, T. J. Chem. Educ. 1997, 74 (9), 1117. (8) Williams, J. P.; West, K. J.; Erickson, K. L. J. Chem. Educ. 1992, 69 (8), 669–670. (9) Hill, D. W.; Mcsharry, B. T.; Trzupek, L. S. J. Chem. Educ. 1988, 65 (10), 907–910. (10) Fenk, C. J.; Hickman, N. M.; Fincke, M. A.; Motry, D. H.; Lavine, B. J. Chem. Educ. 2010, 87 (8), 838–841. (11) Gilbert, J. C.; Martin, S. F. Experimental Organic Chemistry: A Miniscale and Microscale Approach, 4th ed.; Thompson Brooks/Cole: Belmont, CA, 2006; p 158 for an experiment of this type using benzoic acid, naphthol and naphthalene. (12) Cawley, J. J. J. Chem. Educ. 1995, 72 (3), 272–273. (13) Elder, J. W. J. Chem. Educ. 1995, 72 (11), 1049.
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