In the Laboratory
A Guided Inquiry Approach to NMR Spectroscopy Laura E. Parmentier, George C. Lisensky, and Brock Spencer Department of Chemistry, Beloit College, Beloit, WI 53511
We have developed a novel way to introduce NMR spectroscopy into the general chemistry curriculum as part of a week-long aspirin project in our one-semester introductory course entitled “The Structure and Properties of Materials”. In a guided-inquiry-based experiment, students use researchquality instruments and work in collaboration with other students to develop a correlation chart for NMR chemical shifts. Using this chart, they are able to interpret the NMR spectra of their aspirin product. The Experiment Aspirin is synthesized by reacting salicylic acid and acetic anhydride in the presence of a phosphoric acid catalyst (eq 1).1 Salicylic acid (2.00 g), 5.0 mL of acetic anhydride, and 5 drops of 85% phosphoric acid in a 125-mL Erlenmeyer flask are maintained at 75 °C in a water bath for 15 min. Two milliliters of water is added to decompose any unreacted acetic anhydride, producing some hot acetic acid vapor; then the flask is removed from the water bath and 20 mL of water is added to the reaction mixture. After cooling for a few minutes in air, during which time crystals of aspirin should begin to form, the flask is put in an ice-water bath to hasten crystallization. The product is isolated by vacuum filtration, washed with three 5-mL portions of ice-cold water, and then dried at 70 °C. Purity, as percent acetylsalicylic acid content, is determined by titration with NaOH to the phenolphthalein endpoint. Purity of the aspirin product is also determined qualitatively using IR and NMR spectroscopy. O C OH OH
O C CH3 +
O
H3PO4
O C OH O O C CH3
(1)
+ CH3
O C OH
pounds chosen to include some of the structural features of aspirin: benzoic acid, acetic acid, propionic acid, phenol, ethanol, salicylic acid, phenyl acetate, benzene, cyclohexane, acetone, 3-pentanone, ethyl acetate, benzaldehyde, propionaldehyde, and acetophenone. IR and NMR spectra are posted in a common study room so that students have access to all class data. Each student obtains both an NMR and an IR spectrum of the aspirin product and either an NMR or IR spectrum of one of the reference compounds to post for class use. Spectra are run by the student under the supervision of the instructor or a teaching assistant. Students weigh out 20 mg of their aspirin product into an NMR tube, fill the tube to the specified depth with CHCl3-d, and place the sample in the spectrometer. The instrument has been previously shimmed by the instructor, so usually no additional shimming is necessary. The spectrometer collects the data; students adjust amplitude and phase of the peaks and plot the spectrum. With modern FTIR and FT-NMR instruments, individual spectra can be obtained in five minutes or less. In our experience, 24 students in a laboratory section have obtained all of the needed spectra in less than 4 hours of lab time with only modest waits to use an instrument (which also provides time to discuss the instrument and spectral interpretation while waiting for spectra to be printed). Since students run the IR, NMR, or titration portions of the experiment in any order, efficient use of the instruments is maximized, and students spend less time waiting in line.
a
C CH3 O
salicylic acid
acetic anhydride
aspirin
acetic acid
WARNING: Acetic anhydride and phosphoric acid are corrosive. Avoid skin contact and wear eye protection. Students obtain and compare the IR and NMR spectra of the aspirin product they synthesized to a series of reference spectra obtained by the class as a whole. IR spectra were obtained using a Mattson Galaxy Series 3000 FTIR spectrometer. Samples were run in mineral oil (or neat, for some reference compounds), and scans were from 4000 to 500 cm᎑1. A Varian Gemini 200 MHz NMR spectrometer was used for 1H spectra. Samples were run in CHCl3-d as solvent and scans were from 0 to 12 ppm. From earlier experiments, our students have some experience identifying major C–H, C–O, and O–H stretching and bending frequencies in the IR spectrum. Using the tables of IR vibrational frequencies from previous experiments, they are able to identify the major peaks in the IR spectrum of their aspirin product. NMR spectroscopy, however, is a new concept. To facilitate interpretation of the aspirin spectrum, we start the NMR portion of the experiment by having the students collect 1H NMR spectra of a series of reference com470
b
Figure 1. Student-run 200 MHz 1H NMR spectra in CHCl3-d for (top, 1a) acetone and (bottom, 1b) acetic acid. See questions 1–-5 in the text.
Journal of Chemical Education • Vol. 75 No. 4 April 1998 • JChemEd.chem.wisc.edu
In the Laboratory
Students are then asked to analyze the data and develop a correlation chart for chemical shifts in the NMR spectrum (note that we are only interested in chemical shifts here and do not introduce spin–spin splitting, which is largely obscured by the chosen wide scan range anyway). The process, which is done in small groups of students working together, is guided by the following series of questions. 1. Based on its chemical structure, how many different kinds of hydrogen atoms are present in acetone? 2. How many peaks are there in the NMR spectrum of acetone (Fig. 1a)? 3. Based on its chemical structure, how many different kinds of hydrogen atoms are present in acetic acid? 4. How many peaks are there in the NMR spectrum of acetic acid (Fig. 1b)? 5. Based on your answers for questions 1 & 2, match the peaks in the acetic acid spectrum with the appropriate portion of the acetic acid chemical structure by drawing the structure of acetic acid and labeling each group of hydrogen atoms with the ppm of the corresponding peak. 6. Look at the chemical structures of benzoic acid and phenol. What structural feature do they have in common? 7. Look at the NMR spectra of benzoic acid and phenol (Fig. 2). What range of peaks do the two spectra have in common? Give the range in ppm. 8. Based on your answers to questions 6 & 7, give the ppm value for the carboxylic acid peak in benzoic acid. 9. Based on your answers to questions 6 & 7, give the ppm value for the alcohol peak in phenol.
This same type of reasoning is applied to the remainder of the reference compounds. In doing so, students can identify the peaks in the NMR spectrum that correspond to some
Figure 3. Student-run 200 MHz 1H NMR spectrum in CHCl3- d for aspirin. The broad carboxylic acid peak at 9.5 ppm is not always observed
of the common functional groups, with particular emphasis on those present in aspirin. Students are then asked to assign the peaks in the NMR spectrum of their aspirin product and to identify any impurities. They are further asked to relate percent purity by titration with spectral results and percentage yield. Discussion We have found this discovery-based approach to NMR spectroscopy to be very successful. Students are able to assign unambiguously the peaks in their aspirin spectra (Fig. 3) based on the compiled class data. Furthermore, common impurities such as water, salicylic acid, or acetic acid can be readily identified. Liquids (water) that do not mix with CHCl3-d and adhere to the sides of the tube may lead to a broad peak in the NMR spectrum between 4 and 5 ppm. This may account for an overall yield calculated to be greater than 100% and a calculated purity of less than 100%. The presence of unreacted salicylic acid may be indicated by several (usually small) peaks in the aromatic region of the NMR spectrum that are not due to aspirin, and can often explain why the titration calculations led to a purity of greater than 100%. The aromatic peaks of salicylic acid fit between the aromatic peaks of acetylsalicylic acid in a 200-MHz spectrum. Acetic acid CH3 also shows up nicely in some samples. Relating spectral results with percent purity by titration and percentage yield is an integral component of this experiment. This focus encourages students to analyze their data by applying concepts such as polarity, limiting reactant and acid– base titration, which were learned in previous experiments. This project is particularly appealing in that it involves the synthesis and detailed analysis of a familiar material, aspirin, using some of the powerful tools that practicing chemists routinely use. Factors such as electronegativity and electron shielding that affect NMR chemical shift are fairly well understood at the introductory level. Combined with IR spectroscopy (which we have found does not lend itself as well to this type of inquiry analysis) and acid–base titration, the classic aspirin synthesis experiment becomes a larger project that is wellsuited to the guided-inquiry-based introduction of NMR spectroscopy. Note
Figure 2. Student-run 200 MHz 1H NMR spectra in CHCl3- d for (top) benzoic acid and (bottom) phenol. See questions 6–9 in the text.
1. Experimental procedures for the synthesis of aspirin are available in most standard organic laboratory manuals—for instance Campbell, B. N., Jr. & Ali, M. M. Organic Chemistry Experiments, Microscale and Semi-Microscale; Brooks/Cole: Pacific Grove, CA, 1994.
JChemEd.chem.wisc.edu • Vol. 75 No. 4 April 1998 • Journal of Chemical Education
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