In the Laboratory
Developing Critical Thinking Skills: The “Sabotaged” Synthesis of Methyl p-Bromobenzoate
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Eric J. Mahan* and Mary Alice Nading Department of Chemistry, University of Hartford, West Hartford, CT 06117; *
[email protected] Some of the weakest portions of student laboratory reports in organic chemistry are the discussion and conclusion sections. Students are usually quite adept at presenting and reporting the procedure followed during the experiment and the resulting data, but the critical thinking that is required for detailed analysis and explanation of experimental results is usually one of the more difficult laboratory-associated skills for them to acquire. In order to further develop this ability, a puzzle type laboratory was designed to engage students actively in investigating the outcome of a synthetic organic experiment. While a number of puzzle-solving experiments have been reported (1–4), these generally feature a known solution that is determined from several choices that are provided. In this experiment, however, a known reaction is used to convert the reactants, one of which is ultimately found to be an unknown, into a product that can be characterized. Students then use these results from the laboratory, known melting point values from chemical reference materials, and spectroscopic information supplied by the instructor to deduce the identity of their particular unknown reactant. In working to solve this puzzle, students enhance their critical thinking skills and develop their ability to analyze experimental data and draw conclusions. This type of experimental approach is similar to one utilized by Pickering (5) in a puzzle-oriented organic laboratory course as well as a number of other discovery-based organic laboratory experiments that have been developed more recently (6–11). Inspiration for this experiment was derived from the recent reality television program “The Mole”. During the prelaboratory lecture, the students are presented with the premise that someone in the class is working against them. Acting as a mole, the instructor or one of their classmates may have sabotaged the experiment so that the expected result will not be obtained. Students expect most experiments to work, and as a result they may struggle when asked to analyze what went wrong. The sabotage in this exercise demonstrates that not all experiments occur as planned and it mimics the unexpected outcomes or potential side reactions that can be encountered during organic synthesis. The class is informed that their new goal is to complete the procedure, determine whether the experiment has been tampered with, and, if so, how it was altered. Finally, using the results compiled from the entire class, they will also discern the identity of the mole. During the process of discovering this information, students are able to strengthen the critical thinking skills that will be further required in the remainder of the course. Choice of Reaction Since the main objective of this exercise was to improve critical thinking skills, students needed ample time to analyze the results of the experiment and attempt to identify the unknown reactant. The selection of a reaction that could be performed quickly with a reasonable yield was vital. Fischer
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esterification was utilized since this reaction is a common component of the second-semester organic laboratory course and can be found in many organic laboratory manuals (12). In fact, we have successfully implemented the parallel combinatorial esterification experiment of Birney and Starnes (13) in our course for several years. While the basic procedure of the Birney–Starnes experiment could be adapted, some modifications were required to fit the purposes of this project. In the combinatorial synthesis, the ester products are all volatile liquids since the goal is to analyze the odor of the resulting compounds. For the mole experiment, however, a solid product would be preferable since the melting point would provide a quick method to determine whether the correct material was synthesized. Finally, and most importantly, simple changes to either of the carboxylic acid or alcohol starting materials for the Fischer esterification would lead to different products and would cause the desired sabotage to the experiment. The specific reaction that fit all of these criteria for the target product of the students was the synthesis of methyl pbromobenzoate. This ester has a low melting point (81 ⬚C) that can quickly and easily be used to assess whether or not the product has been formed. The starting materials, methanol and p-bromobenzoic acid, are inexpensive and pose few safety concerns, which is also highly desirable for an undergraduate laboratory. Through simple changes in either of the starting materials, this reaction can easily be sabotaged to provide different products that are readily detectable through their physical state or melting point. If the alcohol is changed to ethanol or the acid changed to the o-bromo isomer, liquid products are formed, which immediately suggests a problem to the students. Changing to the p-chloro or p-hydroxy acid, meanwhile, generates solid esters that have vastly different melting points (44 ⬚C and 131 ⬚C, respectively) from the target pbromo ester. Another option that was utilized was to provide sodium hydroxide in place of the sulfuric acid catalyst. In this case, the sodium salt of the acid is isolated instead during the workup. Both the unusually high melting point and the formation of the solid at a different point in the procedure indicate to the student that something is amiss. Identifying the Problem Once the students have completed the experiment and found that it has deviated from the expected outcome, they are then left with the task of determining what caused the change to occur. Since sabotage has been alleged, the focus of their investigation is the reactants and reagents that were used during the procedure. Appropriate infrared (IR) and mass spectrometry (MS) data are provided to the students so that they may analyze the starting materials. Since a change in the alcohol would not affect the functional group, an IR of the alcohol is not provided. The mass spectrum, however,
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In the Laboratory
is very useful for determining whether methanol or a different alcohol was used for the reaction. In similar fashion, spectra of the carboxylic acids are also key pieces of evidence. The presence of the p-chloro or p-hydroxy derivatives is noticeable in the mass spectrum whereas the use of the o-bromo compound as the acid component can be detected in the IR spectrum. Obtaining a 1H NMR spectrum of the product may also be helpful to the students in their analysis. If time constraints, or other instrumentation issues, prevent the students from obtaining their own spectra, copies of 1H and 13 C NMR spectra could be included along with the other spectroscopic information provided by the instructor. While spectroscopy is less useful for verifying the acid catalyst, a simple litmus test can be used for analysis of this reagent. In the last phase of analysis, the suspected alteration to the experiment must be related to the results that were observed. Students can search chemistry reference materials to find the physical state and melting point for their suspected compound and compare this information to the experimental results. For example, if the mass spectrum indicates that the carboxylic acid was actually p-chlorobenzoic acid, the ester product should have a melting point of approximately 44 °C that would reflect this change in structure. Hazards Methanol and ether are toxic and flammable. Evaporation of these solvents should be done in a hood if possible to avoid exposure to the vapors. The sulfuric acid catalyst is toxic and corrosive so it should be handled with care. p-Bromobenzoic acid and the other acid derivatives suggested for use are irritants, so skin contact and other exposure should be avoided if possible. The target product, methyl p-bromobenzoate, and other derivatives are also irritants, so similar care should be taken with these compounds as well. Results and Discussion This experiment could be completed within a three-hour lab period and was conducted early in the second semester of the organic chemistry sequence. At this point, students had been exposed to laboratory techniques such as refluxing and extraction that were required in the procedure. Spectroscopic techniques for identifying organic compounds had been covered in lecture while students had also worked directly with the IR, NMR, and GCMS instruments in previous laboratory exercises. Although the mechanism for Fischer esterification had not yet been presented, the focus of the experiment was on analysis of the results, and a detailed understanding of how the process occurs was not required. The reaction was provided, and further discussion of the mechanistic aspects was approached later in the course at the appropriate time. Placement of the experiment early in the semester also allowed students to utilize the critical thinking skills that they developed while performing this exercise in all subsequent experiments, particularly the qualitative analysis sequence that is the capstone of many organic laboratory experiences. Students provided feedback through answers to postlab questions that asked for their opinions about the experiment. It was explicitly stated that these questions would not be graded based on content and that credit would be given simply for providing an answer to avoid leading the students into
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providing only positive comments. In general, students appreciated the uniqueness of the concept compared to other experiments and enjoyed the interaction with classmates as they attempted to determine the person who was the mole. They were enthusiastic about the experiment and understood the goal of learning to examine more closely all of the factors that influence an experiment, although this recognition was somewhat dependent on the focus of the prelab instruction. In a second lab section, not taught by the author, students expressed some confusion regarding the intent of the experiment. It is likely that the instructor for this section may not have placed the same emphasis on the fact that other outcomes for the experiment were possible. This resulted in confusion for some students when the expected product was not obtained. This issue can be avoided in the future by ensuring that the revised goal of the experiment is clearly conveyed to the students during prelab instructions. Conclusion Based on experimental results and student feedback, the experiment can be considered a success. The esterification proceeds smoothly and solid products were obtained in sufficient yield to allow for a melting point determination, whereas liquid or oil products were immediate indicators that the experiment was sabotaged. Although no quantitative assessment of the impact of the experiment was performed, students indicated in postlab questions that they appreciated the intent of the experiment. The vast majority of comments expressed that the exercise was a worthwhile use of lab time and that the experiment should be utilized in future classes. Acknowledgment The students of CH 231 who participated in this experiment during the spring 2004 semester provided insightful comments regarding its benefits. WSupplemental
Material Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Todd, D. J. Chem. Educ. 1992, 69, 584. Todd, D.; Pickering, M. J. Chem. Educ. 1988, 65, 1100–1102. Silversmith, E. F. J. Chem. Educ. 1991, 68, 688. Pickering, M. J. Chem. Educ. 1990, 67, 524–525. Pickering, M. J. Chem. Educ. 1991, 68, 232–234. Horowitz, G. J. Chem. Educ. 2003, 80, 1039–1041. Bendorf, H. D.; McDonald, C. E. J. Chem. Educ. 2003, 80, 1185–1186. Cabay, M. E.; Ettlie, B. J.; Tuite, A. J.; Welday, K. A.; Mohan, R. S. J. Chem. Educ. 2001, 78, 79–80. Centko, R. S.; Mohan, R. S. J. Chem. Educ. 2001, 78, 77–79. Moroz, J. S.; Pellino, J. L.; Field, K. W. J. Chem. Educ. 2003, 80, 1319–1321. Pelter, M. W.; Macudzinski, R. M.; Passarelli, M. E. J. Chem. Educ. 2000, 77, 1481. Williamson, K. L. Macroscale and Microscale Organic Experiments, 4th ed.; Houghton Mifflin: Boston, 2003. Birney, D. M.; Starnes, S. D. J. Chem. Educ. 1999, 76, 1560– 1561.
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