In the Laboratory
Derivatization of Fullerenes: An Organic Chemistry Laboratory
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Charles T. Cox, Jr.* and Melanie M. Cooper Department of Chemistry, Clemson University, Clemson, SC 29634; *
[email protected] This article focuses on the adaptation of two previously published derivatization techniques: the Bingel (carbene insertion) reaction and the Prato (1,3-dipolar addition) reaction (1, 2). Both reactions involve addition to the fullerene nucleus and, in both cases, the products are distinct enough to allow separation by thin-layer and column chromatography. In addition, the experimental procedures can be performed in a large-enrollment organic laboratory course. The experiments are most appropriate for a second-semester organic laboratory where students have been introduced to carbonyl chemistry and cycloaddition reactions. This article serves to further increase the body of fullerene experiments available for students, which currently contains only two articles from 1996 (3, 4); one of which is aimed more at introductory organic or general chemistry students (4). Fullerenes, C60, have interesting spectral properties particularly with regard to NMR. C60 has no peaks in 1H NMR; therefore, this characterization method depends only upon the structure of the substituent(s). With respect to 13C NMR, a single peak around 145 ppm is observed for C60 (5); however, following derivatization, up to 60 peaks can be observed because of the loss of the molecule’s symmetry. While the NMR properties are interesting and can be readily interpreted, NMR spectra can be very difficult to obtain, particularly 13C NMR because of the solubility of the fullerene. Thousands of scans are needed for a 13C NMR, which can take as much as eight hours or more, but informative 1H NMR spectra can be obtained with as few as 64 scans using a 500 MHz NMR or 96 scans with a 300 MHz NMR. It is plausible that students can obtain a 1H NMR for their derivatized product. The C60 behaves as an ordinary alkene or aromatic group with respect to IR spectroscopy and, again, the most informative peaks for the product depend on the identity of the substituent(s). The UV–vis spectrum for C60 has absorption bands at 213, 230, 257 (main), 329, and 406 nm (the values of these peaks will vary depending on solvent) (5). With regard to use in characterizing derivatives, UV–vis spectra are used to confirm the occurrence of the Bingel or Prato addition by the presence of an absorption band at approximately 430 nm (6). The absorption band is generally weaker than the other bands because it represents a singlet–singlet forbidden transition (1). Overall, because of the highly absorptive nature of C60 and its derivatives, this experiment can be used as an excellent introduction to UV–vis spectroscopy. Most traditional syntheses do not yield products with multiple absorptions, and this synthesis would provide an excellent lead in to a discussion of UV–vis absorptions. Experimental The Bingel experiment is designed to be completed in a single laboratory session of approximately three hours, and the Prato experiment is designed to be completed in two three-hour laboratory sessions. These experiments require students to purify their products using column chromatograwww.JCE.DivCHED.org
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phy; the products are easily identified because they are brightly colored. When conducting the Bingel experiment, students will spend most of their time completing the column. However, the Prato derivatization requires a 90 minute reflux period followed by product analysis using TLC. The following week the products should be purified using column chromatography.
The Bingel Experiment This experiment is based upon a procedure first described by Camps and Hirsch (2), but it has been modified for use in an organic laboratory. The synthesis of the Bingel adduct involves mixing and stirring four materials: C60, diethyl malonate, carbon tetrabromide, and 1,8-diazabicyclo[5.4.0]undec7-ene (DBU) in that particular order. Upon mixing, students must carefully monitor the progress of the reaction using TLC, otherwise, multiple adducts may form, which cannot be easily separated using column chromatography. The Bingel reaction is also unique in that it can be easily monitored using the colors of the reaction mixture. Upon initial mixing, a purple solution is formed and, as the reaction progresses, the mixture will distinctly change from purple to a reddish-brown color indicating the formation of the Bingel adduct, which has a reddish-brown color. Purification can be readily achieved with column chromatography. The unreacted C60 will elute unretained in the silica column with toluene, and then the product will elute. This elution can be monitored using color: the C60 band will be purple and the derivatized fullerene band will be red. For this experiment, we determined that adding toluene to elute C60 and using dichloromethane to elute the product yielded the best separation. A period of less than three hours was needed to complete the experiment, but this did not include product characterization using IR and NMR, which occurred a week later. The spectral data and chromatographic data for the Bingel product are listed in Table 1. The nature of C60 lends itself nicely to product characterization using 1H NMR because only peaks corresponding to the substituent will be observed. Other malonates can be used to form Bingel adducts using the same mole ratios; synthesis of appropriate malonate derivatives is described in reference 7. Using o-dichlorobenzene as the solvent instead of toluene tends to make the reaction proceed more quickly and substituting a volume of 8 mL of o-dichlorobenzene for 25 mL of toluene works well. HowTable 1. Characterization Data for the Bingel Product Method
Result
1
δ 1.38 (6H, t, -CH3); δ 4.32 (4H, q, -CH2)
IR (Nujol)
3000, 1780, 1600, 1225, 775 cm᎑1
TLC (SiO2:CH2Cl2)
Rf = 0.7
H NMR (500 MHz, CDCl3)
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In the Laboratory
ever, this reaction is much more difficult to control, and it is more likely to yield a larger mixture of multiple adducts than using toluene. Considering the nature of the reaction with odichlorbenzene, it is recommended that beginning or intermediate undergraduate students use toluene as the solvent. It is possible to generate the α-bromomalonate in advance for use during the synthesis in lieu of doing the reaction in situ. However, this preparation can be difficult and isolation of the required product quite complex (2); therefore, it is recommended that the reaction be done in situ. This not only reduces time, but also can reduce the cost of materials.
The Prato Experiment This procedure is based on one described by Maggini and Scorrano (1). The synthesis of the Prato adduct involves refluxing a mixture of paraformaldehyde, sarcosine, and C60 for no less than 90 minutes. This reaction is much more controlled than the Bingel experiment and multiple adducts do not form with high frequency. Also the success of the reaction does hinge upon students’ ability to closely monitor the reaction with TLC; therefore, this experiment is likely a better lead in to fullerene chemistry than the Bingel experiment. Purification can achieved using column chromatography. We found that the best separation involved first eluting with toluene to remove the unreacted C60 and then with a mixture of 3:1 methylene dichloride:toluene to recover purified product. The progress of the column can be monitored using the colored bands: the C60 band is purple and the product band is reddish-brown. Other amino acids may be used in lieu of sarcosine using the same mole ratios; however, it is recommended that the literature be consulted first. The spectra and chromatographic data for the Prato reaction are listed in Table 2. Experimental Summary The yields are typically 20–30% for the Bingel and Prato fullerene adducts with 20–30 mg of product obtained. Approximately 25–35% of the C60 starting material is recovered (10–20 mg) during column chromatography and can be reused for later experiments to minimize the experiment costs. With the exception of C60, the other materials are fairly inexpensive. Hazards Toluene and dichloromethane are flammable. DBU is a strong base and therefore is caustic. Students should take extra care when using DBU. Gloves and a lab coat are recommended. A syringe is needed to add DBU slowly to the solution. This poses a potential hazard, and students should be monitored carefully. The syringes should be disposed of in an appropriate sharps container. Instructors should ensure that each syringe is placed in the container. Dichloromethane is volatile and known to cause liver and kidney problems from long term exposure Table 2. Characterization Data for the Prato Product Method
Result
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δ 2.98 (3H, s, -CH3); δ 4.38 (4H, s, -CH2)
IR (Nujol)
3432, 1632, 1055, 758, 662 cm᎑1
TLC (SiO2:Toluene)
Rf = 0.8
H NMR (500 MHz, CDCl3)
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therefore students should wear gloves and a lab coat. Also working in a fume hood, if available, is recommended. Discussion These experiments were evaluated positively by the students, most of whom agreed that the reactions should be incorporated into the curriculum. These experiments have been used for three terms with 76 students total. The Bingel reaction has an 80% success rate, and the Prato reaction has a 96% success rate. The Bingel experiment was thought to be more difficult than the Prato experiment because students were required to conduct the experiment in one laboratory session. Because of the success and the positive student responses, these experiments will be continued in the lab curriculum. It was observed that students’ developed a more in-depth understanding of the importance of TLC as a laboratory method particularly from the Bingel experiment and because of the ease of the column purification, students were introduced to this technique as well. Column chromatography is known to be a tedious and difficult technique, but students were successful because these experiments yield products that are colored thereby allowing students to easily monitor the column. However, it must emphasized that students were required to periodically analyze the column fractions with TLC. This served as a means of further emphasizing the importance of TLC while at the same time emphasizing how columns are run for colorless compounds. Students were told that fullerenes are at the forefront of chemistry research and are being studied at many institutions. This evoked greater interest among them; they commented that they enjoyed doing an experiment that was not only different but covered a relatively new topic in chemistry. The IR and 1H NMR spectra were useful for the students to verify the identity of their products. The spectra were simple to interpret and only two peaks appeared in the 1H NMR spectrum. While a 13C NMR and UV–vis spectra were not obtained for the Prato and Bingel adducts, they can be readily obtained; however, it must be emphasized that multiple scans are necessary for the 13C NMR, otherwise poor resolution should be expected. In the future, we plan on obtaining one 13C NMR spectrum for the Prato and the Bingel products and distributing it to the class as it is not practical for every student to run a 13C NMR spectrum. We did not obtain a UV–vis spectrum because that is part of the analytical curriculum and is not introduced in organic chemistry. Supplemental Material Instructions for the students and notes for the instructor are available in this issue of JCE Online. W
Literature Cited 1. 2. 3. 4.
Maggini, M.; Scorrano, G. J. Am. Chem. Soc. 1993, 115, 9798. Camps, X.; Hirsch, A. J. Chem. Soc. Perkin Trans. I 1997, 1595. Almy, J.; Herrera, A.; Martín, N. J. Chem. Educ. 1996, 73, A105. Hildebrand, A.; Hilgers, U.; Blume, R.; Wiechoczek, D.; J. Chem. Educ. 1996, 73, 1066. 5. Taylor, R. Fullerene Notes on Fullerene Chemistry: A Handbook for Chemists; Imperial College Press: London, 1999; p 137. 6. Hirsch, A.; Grösser, T.; Siebe, A.; Soi, A. Chem. Ber. 1993, 126, 1061. 7. Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Isaacs, L.; Anderson, H. L.; Faust, R.; Diederich, F. Helv. Chim. Acta 1995, 78, 1334.
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