In the Classroom
Personalized Combined Organic Spectroscopy Problems— Online and in the Lab Marjorie Kandel* and Peter J. Tonge Department of Chemistry, State University of New York, Stony Brook, NY, 11794-3400; *
[email protected] Unknown identification problems that use combinations of infrared, proton and carbon nuclear magnetic resonance, and mass spectroscopy (or selections from these) are a good way for students to learn the power and the limitations of the individual spectroscopies and how they complement each other. In our second-semester majors’ organic course at Stony Brook, in addition to weekly homework problems on individual spectroscopy topics, we prepare for each student or small group two personalized combined problems, one online and one in the lab. There are many good combined problems available in texts and on the Internet; a few are given in the reference section (1–4). However, consulting even multiple sources, an instructor would be limited to the relatively small number of those problems suitable for his or her class. By using a vast library such as the Japanese National Institute of Materials and Chemical Research Spectroscopic Database (5), the instructor can create a different problem for each student but still have all problems be of comparable difficulty. An online assignment is also practical logistically, since there is no need to duplicate spectra or distribute paper copies. The technology of the Internet offers a new way to personalize students’ experiences. Traditionally, the same personalized approach is embodied in the familiar laboratory unknown identification experiment, which remains an ideal vehicle for teaching spectroscopy. The Online Combined Spectroscopy Problem We assign each student a different “primary” compound and five “companion” compounds. The student is asked to view spectra of all the compounds and to identify spectroscopic features that distinguish among them. The instructions for the assignment are given in the next section. We construct our problems from SDBS, the Japanese National Institute of Materials and Chemical Research Spectroscopic Database. As of March 2001, this database contained 30,300 compounds with the following number of spectra: IR H NMR 13 C NMR MS 1
47,300 13,700 11,800 20,500
To create a problem, we start with a search by molecular formula. For example, the formula C8H8O2 corresponds to 24 compounds in the database. Display of the list shows which of these are represented by all four spectra of interest to us: IR, 1H NMR, 13C NMR, and MS. In the example, 1,4-benzodioxane, benzyl formate, methyl benzoate, paratoluic acid, and 11 other compounds meet our criterion. We
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choose several combinations of these 15 as primary compounds and companions. Other molecular formulas generate other combinations. Instructions for the SDBS Spectroscopy Assignment You have been assigned the primary compound number SDBSNO ______ and the five companion compounds numbered _______ , _______ , _______ , _______ , _______. 1. Access the primary compound on SDBS, the Japanese National Institute of Materials and Chemical Research Spectroscopic Database, at . 2. Draw the structure of the compound (given on SDBS). 3. Print the four spectra: IR, 1H NMR, 13C NMR, MS. 4. For each companion compound: a. Access the compound on SDBS. b. Draw the structure. c. Identify structural features that would be expected to distinguish spectroscopically the companion compound from your primary compound. For example, the companion compound might have a carbonyl group whereas the primary compound does not. d. View the spectra of the companion compound. Choose one distinguishing spectroscopic feature, and write a brief explanation of how that feature would be inconsistent with the structure of your primary compound. You are expected to choose a feature that would convincingly differentiate the companion from the primary compound. For this reason, it is most likely that you will need to use several different spectroscopies for your different compounds. See the examples below. Example: The IR of compound #2428 1,4-benzodioxane shows no peak in the carbonyl region 1680–1740 cm᎑1. My compound #1725 benzyl formate has a C=O group, which absorbs strongly at 1725 cm᎑1. Example: The 1H NMR of compound #1448 para-toluic acid has a methyl group at δ = 2.375. My compound does not have a methyl group and shows no peaks in that region. Example: The MS of compound #725 methyl benzoate shows the base peak at 105, which is 31 mass units from the molecular ion peak at 136. This corresponds to loss of a methoxy group. My compound does not have any methoxy groups and shows only a small peak at 105.
Journal of Chemical Education • Vol. 78 No. 9 September 2001 • JChemEd.chem.wisc.edu
In the Classroom
The In-Lab Combined Spectroscopy Project Working individually in lab the first semester, students have already had the experience of identifying an unknown monofunctional liquid compound by standard methods: boiling point, IR spectrum, solubility and chemical tests, and derivative. At the beginning of the second semester, NMR and mass spectroscopies are introduced, and students are given the online problem to help them develop the interpretive skills needed for the challenging in-lab combined spectroscopy project. For this project, we choose as unknowns compounds with unusual carbon skeletons, multifunctionality, or other properties that make their spectroscopic interpretation challenging. Multifunctional compounds can give nonstandard IR spectra. Halides and sulfur compounds give distinctive MS patterns. The presence of an even number of nitrogen atoms in a neutral compound is elusive; the element is revealed by neither basicity nor an odd molecular ion in the MS. Particularly interesting are some β-diketones, which show the keto form in one spectrum and the enol form in another, or mixtures of these forms, depending on phase and solvent. The project runs the entire semester, concurrently with synthetic and mechanistic experiments. Students work in small groups, each group being assigned an unknown compound to purify and identify. Spectra are obtained in the order IR, 1 H NMR, 13C NMR, and MS, as the students successively learn the corresponding techniques of sample preparation and interpretation. At several checkpoints, the spectra are submitted so that instructors can give feedback about impurities (commonly acetone or crystallizing solvent) or other potential problems. At Stony Brook, we have offered a variety of cross-course lab experiments, including freshman–organic and organic– inorganic collaborations (6 ). The combined spectroscopy project brings together the undergraduate organic lab students and first-year graduate students taking an instrumentation course. After the undergraduates do the wet chemistry (purification, analysis of purity, determination of chemical properties, and identification of a suitable NMR solvent), groups meet to discuss results and plan strategy. By this time the undergraduates have learned how to use the lab IR instrument and the graduate students are familiar with the departmental NMR. In the process of obtaining the spectra, each cohort demonstrates its instrument to the other. Other than this general plan, we have no formal requirements for group participation; as may be expected, members of some groups collaborate extensively, and some individuals work alone. Instructors from both courses are available for consultation. At present, without MS capability, we provide these spectra from our files or from online sources such as SDBS and the National Institute of Standards and Technology
Webbook (5, 7 ). These sites are of course also useful for spectroscopies other than MS and in situations where access to instrumentation is limited. Student Response On the Spring 2000 end-of-semester evaluation, students in the undergraduate course answered the following question about the online problem and the unknown experiment: On a scale of 1 (helpful) to 9 (useless), did these assignments help you learn spectroscopy? There was a wide range of ratings for the online problem, but the average score was in the middle, 3.9. The rating for the unknown was high, 2.2. Comparing the two, our students felt the unknown experiment taught them more spectroscopy than did the online problem, in spite of the fact that fewer spectra—only 4, compared to 24—were viewed. Students put much more time and effort into the unknown experiment. The unknown was a puzzle that had to be solved, whereas the online problem was merely a situation that had to be explained. One comment on the course evaluation stated, “It was very interesting to try to analyze spectra that you created.” Perhaps the reason students found the unknown experiment so instructive was that it caused them to deal with real-life considerations of sample purity and choice of sample preparation method. In summary, interesting literature and lab problems both enrich the curriculum, although students particularly value the hands-on experience. Literature Cited 1. Silverstein. R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; Wiley: New York, 1998; pp 301–475. 2. Field, L. D.; Sternhell, S.; Kalman, J. R. Organic Structures from Spectra, 2nd ed.; Wiley: Chichester, UK, 1995. 3. Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 3rd ed.; Harcourt: Fort Worth, TX, 2001; pp 466–525. 4. WebSpectra: Problems in NMR and IR Spectroscopy; Department of Chemistry and Biochemistry, UCLA: Los Angeles, http:// www.chem.ucla.edu/~webspectra/ (accessed May 2001). 5. SDBS: Integrated Spectral Data Base System for Organic Compounds; National Institute of Advanced Industrial Science and Technology: Tsukuba, Ibaraki, Japan; http://www.aist.go.jp/RIODB/ SDBS/menu-e.html (accessed May 2001). 6. Kandel, M. J. Chem. Educ. 1994, 71, 513. 7. NIST Chemistry Webbook; National Institute of Standards and Technology: Gaithersburg, MD; http://webbook.nist.gov/chemistry/ (accessed May 2001).
JChemEd.chem.wisc.edu • Vol. 78 No. 9 September 2001 • Journal of Chemical Education
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