Chemical Education Today
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Infrared Spectroscopy in the General Chemistry Lab by Margaret A. Hill
Infrared spectroscopy is an effective laboratory tool at the introductory chemistry level because it complements lecture discussions on bonding and molecular structure. Additionally, the availability of high-quality, student-operable IR spectrometers makes it feasible to include hands-on experiences with infrared spectroscopy for first-year students. In the fall of 1995 funds from the NSF-sponsored Instrumentation and Laboratory Improvement (ILI) program were used to purchase Nicolet Impact 410 IR spectrometers for use in our two-semester general chemistry sequence for science majors. After several years of experimenting with a variety of IR-based labs in our general chemistry course, we have found a number of lab experiences that most successfully engage students at the critical thinking level appropriate for firstyear chemistry students. Over the two-semester general chemistry sequence, three labs involve the use of IR. These labs build upon each other in terms of difficulty and level of understanding of IR and its applications. Identification of Household Polymers by IR Students’ first exposure to IR occurs just after covalent bonding and molecular structure topics have been covered in lecture. In our curriculum this occurs near the end of the first-semester course. This lab is designed to enable students to collect infrared spectral data from three polymers and use the spectral information to identify their structures; a similarly designed lab was earlier reported (1). The polymers we use are polyethylene, polytetrafluoroethylene, and polystyrene because their IR spectra are so distinct and they provide a reasonable problem for students at this level. Prelab discussion begins with a comparison of the molecular structures of the three polymers. Students are asked to identify similarities and differences in the types of bonds present in the structures. General background information about the basis of infrared spectroscopy is given. Specific vibrational modes of C–H and C–F bonds are discussed and the corresponding infrared peaks are noted. Students are shown how a correlation chart represents a summary of spectral information and are given a greatly simplified correlation chart to support their work in lab. In lab, student pairs are given one representative sample of each type of polymer shown. The unknowns are in the form of easily recognizable products or materials. All samples are sufficiently transparent for good spectral analysis except the plumber’s tape, which is too opaque to use directly. As a substitute for student preparation of this polymer, we give students commercially prepared samples of polytetrafluoroethylene. Alternatively, students can be given a copy of the spectrum of this polymer. Students do not know the chemical identities of their samples and must answer this question by running and analyzing the three infrared spectra. They refer to a correlation 26
polyethylene Handiwrap, freezer bags, food storage bags polystyrene business envelope windows, lids from yogurt containers, weigh boats polytetrafluoroethylene plumber’s tape
chart that contains the information they need to assign the major peaks present in the spectra. Each spectrum has characteristic peaks that are readily distinguishable from others, so students are able to deduce the polymeric identities of the plastic samples. By working through such a set, students learn how to use infrared spectroscopy to identify bond types in unknowns. In the postlab segment of lab, students are given infrared data, molecular weight, and empirical formulas for hypothetical unknowns and must deduce the chemical structures. At this level, very simple compounds such as methanol, dichloroethylene, and dibromoethane are appropriately challenging. Unknown Structural Characterization by IR We run a second IR-based laboratory during the middle of the second semester course just after a series of labs that utilize visible spectroscopy as a quantitative analytical tool. Since this IR lab is a qualitative one, it reinforces understanding of the usefulness of spectroscopic tools for both qualitative and quantitative purposes. Pedagogically, this lab is similar to the household polymer lab described above. To identify three unknowns, students use infrared spectroscopy. They are given the three molecular structures and a correlation chart to aid in identifying the IR peaks. However, students also learn how to make potassium bromide pellets of organic solids, and so the lab operates at a higher technical level than the polymer lab. The three unknowns are fluorene, 9fluorenone, and 9-fluorenecarboxylic acid .
fluorene
O 9-fluorenone
C O
OH
9-fluorenecarboxylic acid
Journal of Chemical Education • Vol. 78 No. 1 January 2001 • JChemEd.chem.wisc.edu
Chemical Education Today edited by
Susan H. Hixson National Science Foundation Arlington, VA 22230
Richard F. Jones Sinclair Community College Dayton, OH 45402-1460
The functional group differences in these compounds are readily apparent to general chemistry level students. The IR spectra are nice to compare because they are similar except for the few functional-group differences that are very apparent to novice students. It is difficult to disperse the solids sufficiently in the potassium bromide, so typical student spectra show some light scattering. Otherwise, these compounds provide a good unknown identification problem using infrared data. We are able to accommodate groups of up to 44 students by using 12 sets of potassium bromide pellet-making stations. Each station includes a hand press, anvil and die set for pellet formation,1 and mortar and pestle for mixing. We use an inexpensive, disposable substitute for the metal pellet disk that comes with this set; craft foam2 of 2 mm thickness can be cut into 2.5 × 2.5-cm squares and a 0.5-cm hole can be punched out of the middle with a paper hole punch. Students weigh their potassium bromide and sample, then move to a station to mix their samples, transfer the mixture to the foam pellet holder on a metal anvil, sandwich it underneath a second metal anvil, and form the pellet using the hand press. The pellet in its foam square fits into a sample holder that slides into the sample chamber for data collection. A typical student can easily generate a very good spectrum using this procedure. Another simple pellet-making setup suitable for undergraduates has been described by Kalberg and Ogren (2). In the postlab segment of this lab, students are given problems to solve in which collections of data for unknowns including molecular weight, empirical formulas, and infrared data are given. Students must solve the structures of the unknowns. At this point in their studies, they are able to solve unknowns such as acetone, acetic acid, and benzene. Organic Acid Unknown This is our last lab exercise for the two-semester general chemistry course. It represents a capstone experience for students in which they use several lab techniques to deduce the identity of an unknown and then prepare a formal written report to communicate their findings. This gives students an opportunity to see how one problem can be solved from a
combination of experimental approaches. Student pairs are given the final three weeks of the semester to identify an organic acid from a list of 14 possibilities.3 (We actually only supply a limited set of the 14 possible acids owing to the difficulty in distinguishing pKa values for some diprotic acids.) Students narrow down the list of 14 by determining pKa values, and molecular weight (by freezing point depression and titration against standardized base), and finally by running infrared spectra of known acids to compare with a spectrum of their unknown. We provide bona fide samples of all 14 acids so that students can run infrared spectra on any that they suspect of being their unknown. We steer students toward effective strategies for approaching this problem by pointing out that running 14 infrared spectra is not a good use of time, and so the IR work is best left until the end when the list of possibilities is much smaller. By seeing how infrared data can be used in conjunction with other results, students gain a better appreciation for the relative value of various lab techniques. The value of the infrared results becomes very clear at the end, when many students discover that their infrared results allow them to be quite certain of the identity of their acid. Notes 1. Table press and die set purchased from SpectraTech, Inc., Shelton, CT 06484, Catalog # 0016-130. 2. Two-millimeter-thick craft foam (Darice, Inc.) can be purchased at craft stores and discount department stores. 3. The organic acids used are benzoic, p-chlorobenzoic, cinnamic, fumaric, o-iodobenzoic, itaconic, maleic, malonic, pnitrobenzoic, oxalic, salicylic, succinic, tartaric, and p-toluic.
Literature Cited 1. Webb, J.; Rasmussen, M.; Selinger, B. J. Chem. Educ. 1977, 54, 303. 2. Kalberg, C.; Ogren, P. J. J. Chem. Educ. 2000, 77, 391.
Margaret A. Hill is in the Department of Chemistry, Central Michigan University, Mt. Pleasant, MI 48859;
[email protected].
JChemEd.chem.wisc.edu • Vol. 78 No. 1 January 2001 • Journal of Chemical Education
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