In the Laboratory
Raman Spectroscopy in the Organic Chemistry Laboratory: The Formation of N-Carboxy-2-imidazolidone Rosa E. Rivera-Hainaj Department of Biology and Chemistry, Purdue University North Central, Westville, IN 46391;
[email protected] The use of Raman spectroscopy as an analytical tool has expanded mainly because of enhancements in computer technology that have allowed for instrumentation to become readily available (1). Raman spectroscopy, similar to Fourier transform infrared spectroscopy (FTIR), shows vibrations related to specific functional groups present in molecules, making the Raman technique complementary to and as informative as FTIR (2–4). The educational community has taken advantage of this progress by introducing the study of Raman spectroscopy in the undergraduate chemistry curriculum (1, 5–9). A two-week experiment for the introductory organic chemistry course is presented in which students combine laboratory techniques (e.g., recrystallization, melting point determination, and Raman spectroscopy) to study organic molecules. This experiment is scheduled during weeks two and three of the first semester of the organic chemistry course, allowing students to study functional groups in the laboratory as they are introduced in the lecture. Furthermore, this experiment introduces spectroscopy early in the organic chemistry course giving additional time for students to understand and apply spectroscopic techniques to chemical analysis.
round clear glass vials for Raman analysis. Spectra were collected via 30 s acquisition time and analyzed with Grams 32 software (Thermo Scientific, Waltham, MA) and Excel (Microsoft Inc., Seattle, WA). Sample Preparation Students recrystallized N-methoxycarbonyl-2-imidazolidone, 2, from water for further analysis (5). Samples for melting point determination were ground into a fine powder and placed in melting point capillary tubes. Samples for Raman spectroscopy were prepared by dissolving each compound in distilled deionized water to a 0.5 M concentration and filtered with a 0.45 μm filter prior to spectra collection. To study the decarboxylation reaction, 0.150 mL of 2 M KOH was added to the aqueous solution of 2 in the Raman sample vial. Data Acquisition Students determined the melting point of 2 before and after recrystallization. The melting point of 2-imidazolidone, 1, was also determined. As a preamble to the spectroscopy portion of this experiment, students were introduced to the principles of vibrational spectroscopy with emphasis in Raman and FTIR.1,2 The vibrational signatures of the different functional groups were discussed as well. Raman spectra of 1 and 2 were collected and used to identify the vibrations corresponding to the different functional groups present in the molecules. Lastly, students collected spectra during the conversion of 2 to N-carboxy-2imidazolidone, 3, and the spontaneous decarboxylation of 3 to 1 (Scheme I).
Experimental Details Chemicals and Equipment N-Methoxycarbonyl-2-imidazolidone, 2, was synthesized by the instructor and provided to students (Scheme I and the online materials; refs 10–12). 2-Imidazolidone, 1, chloroform, methylchloroformate, and potassium hydroxide were purchased from Sigma–Aldrich, Inc. (St. Louis, MO). Melting points were determined using a Mel-Temp apparatus (Barnstead–Thermolyne, Boston, MA). Raman spectra were collected with a Delta Nu Advantage NIR spectrometer (Delta Nu, Inc., Laramie, WY) in the region 400 to 2000 cm–1 with 5 cm–1 spectral resolution. The samples were placed in 1.0 mL
Hazards KOH is a corrosive and toxic substance. N-methoxycarbonyl-2-imidazolidone and 2-imidazolidone are irritating to eyes, skin, and respiratory. Safety glasses and proper ventilation
O
O HN Scheme I. Reaction scheme for the production of N-methoxycarbonyl-2-imidazolidone, 2, (top) and the spontaneous decarboxylation to 2-imidazolidone, 1, in the presence of base (bottom).
O
+
NH
Cl
O
CH3
CH3Cl
Δ
2
O
O 2
CH3
O
O O
N HN
HN
methylchloroformate
1
O
N
CH3
O
N
KOH
O spontaneous
HN
HN
NH
O 3
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 11 November 2009 • Journal of Chemical Education
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In the Laboratory Table 1. Melting Point Determinations for 2-Imidazolidone and N-Methoxycarbonyl-2-imidazolidone mp/°C mp/°C Error (expt) (lit) (%)
Compound 2-Imidazolidone, 1
127–128
129
1.16
N-Methoxycarbonyl-2-imidazolidone, 2 172–175 (before recrystallization)
180
3.61
N-Methoxycarbonyl-2-imidazolidone, 2 178–179 (after recrystallization; 85% recovery)
180
0.83
7 6
Detector Count / 104
827
2-imidazolidone, 2 N-methoxycarbonyl2-imidazolidone, 1
5 930
4
716
3 980 1000
2
1103
1 0 1800
1600
1400
1200
1000
800
600
400
∙1
Wavenumber / cm
Figure 1. Raman spectra of the compounds 2-imidazolidone, 1 and N-methoxycarbonyl-2-imidazolidone, 2.
Detector Count / 104
6
time after KOH addition: 5 min 30 min
5
↓827
4 ↑1665 3
↓715
↑1018 ↓1445
2
↑1048 ↑1068
↑930
1 0 1800
1600
1400
1200
1000
800
600
400
∙1
Wavenumber / cm
Figure 2. Time course of the production of N-carboxy-2-imidazolidone, 3, and its spontaneous conversion to 2-imidazolidone, 1. Up-arrows indicate the increase in intensity of the indicated peaks as the reaction progresses. Down-arrows indicate the decrease in intensity of the indicated peaks as the reaction progresses.
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should be used at all times. Laser safety should be followed at all times, as students may be exposed to radiation light from the laser in the Raman spectrometer. Results and Discussion Students studied the compounds 2-imidazolidone, 1, and N-methoxycarbonyl-2-imidazolidone, 2. The melting point of 1 was compared to that of 2. Determination of the melting point before and after recrystallization of 2 allowed students to assess the compound’s purity (Table 1). The melting point of 1 was 127–128 °C. The published value is 129 °C (4), giving a 1.16% error in the determination. The published value for the melting point of 2 is 180 °C (10, 11). The melting point range for 2 was 172–175 °C before recrystallization (giving a 3.61% error) and 178–179 °C afterwards (giving a 0.83% error). Raman spectroscopy was used to study the vibrations corresponding to the functional groups on molecules 1 and 2 (Figure 1). Even though both compounds show several vibrations in the Raman spectra, students focused on the study of the vibrations at 827 cm–1 and 930 cm–1. The vibration at 827 cm–1 corresponds to the carbamate moiety, N−CO2, which is absent from 1. Conversely, the vibration at 930 cm–1 corresponds to the symmetric ring breathing mode of 1, which is not present in the spectrum of 2 (13). Raman spectroscopy was also used to study the conversion of 2 to N-carboxy-2-imidazolidone, 3, and the spontaneous release of CO2 to form 1 (Scheme I). Compound 3 is a molecule of interest as it is used as a substrate analog in the study of biotin decarboxylation reactions (13). Students collected a Raman spectrum every 5 minutes for 30 minutes (Figure 2). This period of time allowed for the hydrolysis of the ester of 2 to form 3 and for the decarboxylation to occur. Students focused on the vibrations indicative that the decarboxylation process is underway and those indicative of the formation of 1. The increase in intensity of the peak at 930 cm–1 during the collection time of the experiment shows that decarboxylation is taking place as the COO− is being released from 3 and 1 is being generated. The vibration at 827 cm–1 corresponds to the COO scissor mode in 2. The peak at 1665 cm–1, which is small 5 minutes after KOH addition, corresponds to the COO− moiety. The low intensity of this peak initially indicates that the carboxy species has not formed completely at the time the spectrum was collected. However, this vibration is present after 10 minutes of KOH addition, signifying the formation of the N-carboxy species (data not shown). The 1665 cm–1 peak is still present after 30 minutes of the KOH addition owing to the fact that the carboxy species is stable at high pH and takes longer than 30 minutes to completely decarboxylate. A spectra collected after 60 minutes of KOH addition showed that the 1665 cm–1 vibration had decreased in intensity considerably, indicating that the decarboxylation process has progressed (data not shown). The spectra collected over the 30 minute period also show several other vibrations that correspond to the formation of byproducts from the carbamate hydrolysis and decarboxylation. The peak corresponding to the formation of methanol by the carbamate hydrolysis is shown at 1018 cm–1 (due to the C−O stretch of methanol). Additionally, the vibration corresponding to HCO3− formation can be observed at 1048 cm–1.
Journal of Chemical Education • Vol. 86 No. 11 November 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
Questions for Students Students were given the following questions as a guide for data analysis and writing a formal laboratory report:
1. Based on the experimental and literature data, which compound, N-methoxycarbonyl-2-imidazolidone, 2, or 2-imidazolidone, 1, has a higher melting point? Is this what you expected based on your knowledge of physical properties of organic molecules? Explain.
2. Compare the Raman spectra of N-methoxycarbonyl-2imidazolidone, 2, and 2-imidazolidone, 1. What are the differences and similarities in the spectra?
3. Compare the Raman spectra during the decarboxylation experiment. Which signature vibrations appear and disappear during this time period? To which functional groups can these vibrations be assigned to?
4. What changes in the Raman spectra exemplify the decarboxylation process?
Conclusion The experiment presented herein was assigned to students in weeks two and three of the introductory organic chemistry laboratory. The combination of benchwork and spectroscopic methods allowed students to experience a multiple technique approach to the study of organic molecules. There was no waiting period to use the melting point apparatus, as half the class worked with them and half the class worked in the Raman collection and switched midway in the laboratory period. This laboratory experiment permits students to learn about physical properties of organic molecules, their functional groups, and spectroscopy early in the introductory organic chemistry course. As a result, students increase the number of learned laboratory techniques they can then use in future experiments in the organic chemistry laboratory and other chemistry courses. Acknowledgments
with ATR capability. ATR is needed if the reaction is set up in aqueous KOH, as the water signal will interfere in the IR spectrum obtained in a regular FTIR instrument. However, if an ATR attachment is not available for the FTIR instrument, the reaction can be set up in D2O and use KOD as base to avoid interference from the water signal. Raman and FTIR spectroscopies are complementary techniques, so students will be able to observe the changes explained in this article. 2. Students were provided with instructions (in the form of handout and demonstration by the instructor) concerning the operation of the Delta Nu Advantage NIR Raman spectrometer. A pre-laboratory lecture was given to students about vibrational spectroscopy.
Literature Cited 1. Abhyankar, S. B.; Subramanian, S. Spectrochimica Acta A 1995, 51, 2199–2204. 2. Lambert, J. B.; Shurvell, H. F.; Lightner, D. A.; Cooks, R. G. Organic Structural Spectroscopy; Prentice-Hall: Upper Saddle River, NJ, 1998; Chapters 8, 9. 3. Horiba Jobin Yvon Web site: Raman Spectroscopy Tutorial. http://www.horiba.com/scientific/products/raman-spectroscopy/ raman-resource/ (accessed Jul 2009). 4. Sigma-Aldrich Catalog. http://www.sigma-aldrich.com/ (accessed Jul 2009). 5. Carey, P. R. J. Biol. Chem. 1999, 274, 26625–26628. 6. De Graff, B. A.; Hennip, M.; Jones, J. M.; Salter, C.; Schaertel, S. A. Chemical Educator 2002, 7, 15–18. 7. Comstock, M. G.; Gray, J. A. J. Chem. Educ. 1999, 76, 1272–1275. 8. Aponick, A.; Marchozzi, E.; Johnston, C.; Wigal, C. T. J. Chem. Educ. 1998, 75, 465–466. 9. McClain, B. L.; Clark, S. M.; Gabriel, R. L.; Ben-Amotz, D. J. Chem. Educ. 2000, 77, 654–660. 10. Clarkson, J.; Carey, P. R . J. Phys. Chem. A 1999, 103, 2851–2856. 11. Schaeffer, H. J.; Bhargava, P. S. J. Pharm. Sci. 1964, 53, 137–143. 12. Carey, P. R. Annu. Rev. Phys. Chem. 2006, 57, 527–554. 13. Caplow, M. J. Am. Chem. Soc. 1965, 87, 5774–5785.
The author gratefully acknowledges support from the National Science Foundation (MCB-0650797) and the Department of Biology and Chemistry at Purdue University North Central.
Supporting JCE Online Material
Notes
Supplement
1. In case that instructors do not have access to a Raman instrument, this experiment can be completed using an FTIR instrument
Student handouts
Instructor notes
http://www.jce.divched.org/Journal/Issues/2009/Nov/abs1319.html Abstract and keywords Full text (PDF) with links to cited URLs and JCE articles
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 11 November 2009 • Journal of Chemical Education
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