Discovery-Based Purification of Excedrin

Introduction

The separation and purification of complex mixtures are tasks that organic chemists perform on a regular basis. As a result, many experiments have been designed for the undergraduate organic chemistry laboratory to simulate this task [1–7]. During these experiments, students are introduced to skills that include liquid–liquid extractions, solid–liquid extractions, recrystallization, and chromatography. As these skills are developed, it is hoped that students begin to understand chemical concepts, such as solubility and acid–base reactions, which codify their experiences through the development of logical thought processes.

One such experiment involves the separation of benzoic acid, aniline, and dimethoxybenzene using acid, base, and organic solvent extractions [8, 9]. Students obtain excellent recovery yields and each compound is readily purified. In our experience, there are two negative results from this type of experiment. First, students exhibit minimal interest in the separation of the basal components, as they appear to have no relevance to their lives. Second, student learning is often impeded because students simply follow written laboratory procedures without mentally engaging in or understanding the concepts behind the skills.

In an effort to make the experiment relevant to students’ interests and address student learning, we report a discovery-based laboratory project for the separation and purification of a poisoned Excedrin mixture [10, 11]. This laboratory project produces several excellent learning outcomes. First, by investigating the purification of Excedrin, students engage in applying desired laboratory techniques to the separation of an interesting medically related system. Second, by introducing a discovery-based approach [12], students learn how to ask questions about chemical solubility and use this information to determine their own laboratory procedure. Third, students become active participants in their own learning, understand why they are conducting the laboratory experiment, learn how to work in groups, and retain the developed skills. Finally, students take pride in their experiment and have fun.

Experimental

The two-week laboratory project is the fifth project in the first semester of organic chemistry at Whittier College. In previous experiments, students are introduced to TLC, NMR, GC, melting point determination, recrystallization, liquid–solid separation procedures and other basic laboratory techniques. Laboratory sections are limited to 20 students. For this project, students are allowed to work in collaborative groups; however, each individual is responsible for the separation of their own mixture. Each laboratory period begins with a one-hour discussion section during which students are engaged in discussion questions using a peer-lead team-oriented learning model [13].

Students are presented with a scenario in which their pain medication (the only item they brought with them on a reality game show) has been ground up and poisoned (with p-dimethoxybenzene). Because they have developed a significant headache, it would behoove them to purify their medicine in order to cure their headache. Students are presented with 1.0 g of an equal mixture of aspirin, caffeine, acetaminophen, and p-dimethoxybenzene (a prepared mixture analogous to commercially available Excedrin with the exclusion of the medicinal binder). Students are required to (1) identify the components of the mixture, (2) determine the solubility characteristics of the pure substances in a variety of solvents, (3) develop an appropriate separation protocol, (4) use the developed procedure to separate each component, and (5) assess the success of their protocol through the characterization and possible further purification of each component [14].

As this is a discovery-based laboratory, students are required to design their own procedures during the discussion section. With the reminder that reproducibility is an important aspect of science, students typically divide the first two tasks among the entire class, ensuring that two or three groups of students conduct each task. By the completion of the discussion section, students should have tangible procedures for accomplishing the first two tasks.

In order to establish the identity of the mixture, students are allowed to use a variety of techniques. Thin-layer chromatography (TLC), using 4:4:1 hexane:dichloromethane:acetone as the eluting solvent, demonstrates the presence of all four components. GC–MS allows for the identification and quantification of all components except aspirin, which does not elute cleanly from the column. NMR analysis shows a complex yet identifiable mixture. Melting point

determination is used to demonstrate the unique melting properties of a complex mixture.

In order to determine solubility characteristics, students are provided with pure samples of each component and several different solvents (hexane, ether, dichloromethane, acetone, methanol, water, 3 M HCl, and 3M NaOH). Students are required to determine solubility data for the substances in various solvents. A typical procedure involves placing a known amount of substance (~0.01 g) in a test tube followed by the addition of a sequential amount of solvent. The amount of solvent required to dissolve the substance is used to determine the degree of solubility; students typically characterize solubility in terms of soluble, partially soluble, or insoluble.

Upon compilation of all data, students devise a protocol for the separation of the mixture and defend their protocols with other students. There are several different protocols that can be used to effect separation (see Figure 1 for one possible route). Typical procedures take advantage of the acidic properties, the differential solubility of aspirin and acetaminophen, the basic properties of caffeine, and/or the nonpolar properties of dimethoxybenzene. Students use the neutralization of the soluble aqueous solutions followed by either filtration or extraction to remove the components from the aqueous solution. Students are allowed to use any protocol they desire; however, egregious errors are discussed with the student and corrected. Approximately 35% of the students have fundamental flaws in their protocol that require discussion and rethinking before being are allowed to proceed. At the completion of the laboratory discussion section, student protocols are approved by the laboratory instructor.

The separation portion of the laboratory typically takes a solid four hours of laboratory time (1 week of laboratory). Students learn that separations often require multiple sequential extractions and that there are many pathways for losing material. Upon separation of each component, students assess the quality of their separation using TLC, GC, and/or NMR. If the component is overtly impure, purification using recrystallization or sublimation is used. Students obtain the mass of their recovered components and calculate a percent recovery.

Student reports, in the form of student composed laboratory notebooks with experimental, data analysis, and discussion sections, focus on understanding the effectiveness of the separation protocol. Students explain their initial mixture-identification data with respect to their percent-recovery data. Their written discussions center on why their approach did or did not work. Student discussion in the laboratory notebooks is directed by using provided postlaboratory discussion questions that the students answers in the laboratory notebook.

Hazards. Standard precautions should be taken when working with acids and bases because of their corrosive nature. Acetone, methanol, ether, and hexane are all flammable liquids. Dichloromethane is a suspect carcinogenic material and should be handled with care. Caffeine, aspirin, and acetaminophen are all irritants.

Equipment. While not necessary for the completion of the laboratory, our departmental pedagogical approach to laboratory courses is to provide students with extensive hands-on experience with modern chemical instrumentation. For this experiment, we used the following instrumentation: Gas chromatography (Agilent 6890N) and gas chromatography–mass selective detectors (Agilent 6890N and 5973N) were used with HP-5 columns. NMR spectra were obtained on a JEOL ECX 300 MHz NMR. TLC analysis used Whatman Al SIL G/UV plates.

Student Results

The separation of poisoned Excedrin poses an experimental challenge to students and faculty. Of the multiple theoretically correct protocols, only a few allow for optimal separation of the poisoned Excedrin mixture. As a result, only 30% of the students were successful in obtaining all four components as pure substances. The remaining 70 % of the students have varied success, either obtaining three or fewer of their components as pure substances or obtaining most of their components as mixtures of two or more substances. Percent recoveries were typically around 40 to 50%. Student errors arise from improper separation protocols, insufficient extraction procedures, product loss during purification steps, and/or student carelessness. As a result, the purification of poisoned Excedrin is not an ideal project from an experimental perspective. We have modified the experiment by using poisoned Anacin (aspirin, caffeine, and dimethoxybenzene) in place of Excedrin. The removal of the acetaminophen simplifies the procedures substantially and results in greater student success while still maintaining the medicinal significance; however, because students do not struggle as much in the experiment, some important learning objectives are lost. Additional challenges include the need for two four-hour laboratory periods, which may not be feasible at all institutions, and because the experiment relies heavily on instrumentation and student collaboration, it may become unmanageable with larger classes.

Despite the experimental challenges, the student response to the experiment is outstanding as evident by student evaluations, end-of-the-year assessment, and student reflections in notebooks. First, there is a fundamental shift in student attitude towards the experiment. Because students are responsible for constructing their own protocols, they take pride in their efforts. In previous years when we used the “cookbook” protocols for separation, this was not the case. For example, when a student spilled her sample, she would simply explain that she were unable to continue because of the lack of material. With the discovery-based approach, the same student would demand to restart the protocol with a new sample. Second, students understand the implications for each of their purification steps. For example, acid–base reactions make sense to students. Not only do students understand the implications during this experiment, but students understand why acid or base washes are used during the workup of reactions in subsequent laboratory projects. Third, students focus on investigating the important questions of the laboratory instead of looking for the “correct” answer. Even though students are not overly successful from a percent recovery or separation standpoint, students focus on how they would improve the experiment if they could repeat the experiment. Fourth, students become self motivated active learners. Because the students have ownership of their experiment, they tend to help each other with their questions. As the laboratory instructor, I have noticed a significant decrease in the number of simple questions being asked, but perhaps more significantly, when students do ask questions, the questions are well developed and conceptual, as opposed to being procedural. This demonstrates that students are thinking about their ideas before asking questions, an indication that students are changing from experiment doers into effective researchers. Fifth, students learn that science does not always work as planned, a valuable lesson. Finally, students enjoy the laboratory project. This experiment can be experimentally daunting, especially if the outcomes of each step of the purification are not understood; however, because the first week forces students to think about why they are doing what they are doing, they understand what the experimental goals are and, therefore, view the experiment as an enjoyable challenge.

Conclusion

The discovery-based approach to the purification of poisoned Excedrin represents an experimentally challenging project for the organic chemistry laboratory. Despite this, the educational outcomes demonstrate that this project is well worth the challenges. Students demonstrate an increased interest in the medicinally related system and student learning is improved through demonstrated student thought, skill retention, enthusiasm, and ownership. By continuing to develop laboratory projects that help our students become actively involved in experimentation and learning, we will ensure that students learn the skills and thought processes necessary to succeed in the organic chemistry laboratory.

Acknowledgments. The Colleagues in the Department of Chemistry at Whittier College provided valuable assistance in the preparation of this experiment and manuscript. Jack Kampmeier provided valuable mentoring and suggestions for the improvement of this experiment. The work was supported by a Whittier College Faculty Development Grant. Instrumentation was provided through two grants from the Department of Defense.

Supplemental Material. Detailed student laboratory handouts for the purification of poisoned Excedrin and the purification of poisoned Anacin, student discussion questions, and instructor notes are available in a Zip file (http://dx.doi.org/10.1333/s00897040815a).

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14. A quantitative component can easily be added to the experiment. See the supporting information for more information.