In the Laboratory
The Isolation of Rubber from Milkweed Leaves
W
An Introductory Organic Chemistry Lab Lisa Volaric*† and John Hagen Department of Chemistry, Chatham College, Pittsburgh, PA 15232; *
[email protected] Students are motivated by the connection between chemistry and the everyday world. To tap into this interest, many organic chemistry laboratory courses include natural-product isolation experiments, such as the isolation of caffeine from tea (1) and the isolation of oil of clove (2). With the exception of DNA, RNA, and protein extraction experiments (3–5), nearly all undergraduate natural product isolation experiments involve only small molecules (6–9). In an effort to provide a natural product isolation experiment that complements the polymer synthesis experiment already in our organic chemistry laboratory curriculum, we developed an experiment that involves isolating rubber from milkweed. To teach the basic techniques needed for organic syntheses, most organic chemistry laboratory courses begin with a series of experiments that frequently consume the first month of the course. One of our goals was to design a single laboratory experiment that teaches many basic techniques while simultaneously introducing the students to polymers and natural-product isolations. Our procedure for isolating rubber from milkweed exposes the students to the proper use of organic laboratory equipment and the techniques of refluxing, vacuum filtering, extracting, centrifuging, and IR fingerprinting. Validation of the procedure was completed by conducting both microscale and macroscale versions of this experiment in college-level organic chemistry laboratories. Background This procedure involves the extraction of rubber from milkweed plants, which was of significant commercial interest several times in recent history. Access to rubber plantations was restricted during the First World War, causing Allied companies to search for a domestic source of rubber. Thomas Edison, at the request of Henry Ford, identified plants native to the United States that could be practical sources of rubber. One such plant was milkweed (10), Asclepias syriaca, which contains approximately 3% rubber. Ford also found that rubber could be extracted from the domestic goldenrod. Other Allied nations were also conducting research in this area and identified the guayule plant in Mexico and the Russian dandelion in Uzbekistan (11) as possible sources of rubber. Allied access to rubber plantations in the East Indies was again lost at the beginning of World War II, encouraging further research into northern rubber sources and development of synthetic rubber. The development of synthetic rubber reduced the need for a domestic source of natural rubber until the oil shortages of the 1970s led to further research in the milkweed rubber extraction process (12–15). † Current address: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599.
Materials and Methods Milkweed plants were obtained in August from a field near Pittsburgh, PA. The leaves were removed from the fibrous stems because the stems are difficult to grind, making them unsuitable for our experiment. The leaves were placed in a plastic bag and stored frozen prior to conducting the experiments. Leaves used in the microscale experiments had been frozen for three months, whereas leaves used in the macroscale experiments remained frozen for five months. The acetone and cyclohexane used in the experiment were reagent grade from Fisher Scientific. The microscale and macroscale procedures are analogous, but for clarity, the microscale procedure is described first and then the differences between the microscale and macroscale experiments are highlighted. As the microscale experiments were conducted first and the time required to complete them was unknown, we boiled the milkweed leaves in water for one hour before the beginning of the experiment. To determine the dry mass of the milkweed leaves, the instructor gave the students the mass of 10 leaves prior to boiling. The students then calculated an average mass for one leaf and used this as the mass of their sample. Each student obtained one leaf and ground it into a paste with a mortar and pestle. The paste was then transferred to a 10-mL roundbottom flask with 5 mL of acetone and refluxed for 30 min. The acetone reflux extracts glycerides, isoprenoids, terpenoids, and sterols. A microscale filtration apparatus consisting of a Hirsch funnel, a thermometer adapter, and a 10-mL roundbottom flask was used to vacuum-filter the sample. The sample was then washed with acetone until the filtrate changed from green to colorless as chlorophyll and other colored compounds were removed. The solid was allowed to dry, then placed in a 10-mL round-bottom flask with 5 mL of cyclohexane and refluxed for another 30 min to extract the rubber from the solid. The viscous mixture containing the polymer was centrifuged to separate the solid from the supernatant. The supernatant was removed and dried by passing the liquid through a Pasteur pipet packed with anhydrous sodium sulfate. A thin-film IR spectrum of the polymer was obtained and compared to the IR spectrum of synthetic rubber, cis-polyisoprene. After the IR spectrum was obtained, the solvent was evaporated under a stream of air to collect the rubber. The students tested the elastomeric properties of the rubber extract by stretching it with a microspatula. The mass of the rubber was measured and the percent content of rubber in the milkweed leaf was calculated. The macroscale procedure is like the microscale procedure with the following exceptions. Because the microscale experiment took only 2.5 hours to complete, the students boiled their own leaves for this experiment. Each student weighed 10 leaves and boiled them for one hour. The leaves were then
JChemEd.chem.wisc.edu • Vol. 79 No. 1 January 2002 • Journal of Chemical Education
91
In the Laboratory
Number of Students
7 6
Microscale Macroscale
5 4 3 2 1 0 0.0-0.9
1.0-1.9
2.0-2.9
3.0-3.9
4.0-4.9
5.0-5.9
6.0-6.9
7.0-7.9
% Rubber Figure 1. Percent recovery of rubber from milkweed leaves for microscale and macroscale methods. The average recovery was 2.4 ± 1.8% for the microscale method. The macroscale method yielded an average recovery of 1.8 ± 0.7%.
placed in a food processor and ground into a paste. The refluxing steps for the macroscale method were the same as for the microscale method except that a larger (100-mL) round-bottom flask was used and the amount of solvent was increased to 50 mL. After the cyclohexane layer was dried and the IR spectrum was obtained, a rotary evaporator was used to evaporate the solvent and collect the rubber. Hazards Acetone and cyclohexane are inflammable. Use a heating mantle or other spark-free heating device when refluxing. In addition, milkweed sap is an irritant. Contact with eyes must be avoided when harvesting the leaves. Results and Discussion All of the students who performed the experiment were able to isolate rubber from the milkweed leaves. Figure 1 illustrates the distribution for the percent recovery of rubber from the milkweed leaves. Students isolated rubber with a recovery of 2.4 ± 1.8% and 1.8 ± 0.7% for the microscale and macroscale procedures, respectively. These values are comparable to the published value of 1–2% (12). There is a larger spread in recovery for the microscale method than for the macroscale method. This is a result of using average leaf mass to calculate percent recovery in the microscale procedure. Even though similar-sized leaves were selected, some leaves were larger than others. If a student had a particularly large leaf, the percent recovery calculated would be larger than the class average. This would account for the outlying point in the distribution of the class data. Two tests, one physical and one chemical, were conducted to identify the material extracted from the milkweed leaves. The physical test was a qualitative measure of the elasticity of the polymer. All students noted that their samples displayed elastic qualities, taken as a positive indication that the isolated material is polymeric. The chemical technique used to identify the extract was IR spectroscopy. A thin-film spectrum of the extract was obtained and compared to the thin-film spectra 92
Figure 2. IR spectra of (A) synthetic rubber (Perkin Elmer 1600 Series FTIR; 4 scans, 4 cm᎑1); (B) student-isolated rubber from milkweed using the microscale method (Perkin Elmer 1600 Series FTIR, 4 scans, 4 cm᎑1); and (C) student-isolated rubber from milkweed using the macroscale method (Mattson FTIR, 8 scans, 2 cm᎑1).
of commercially produced synthetic cis-polyisoprene. Sample spectra of the synthetic rubber and of the rubber isolated using both microscale and macroscale methods are shown in Figure 2. The spectra of student-isolated rubber have characteristic IR peaks consistent with those of synthetic rubber. These primary peaks are identified as a C–H stretching band at 3000 cm᎑1, a C–H bend in the 960–980 cm᎑1 range, and the C=C stretch at 1660–1675 cm᎑1. The physical and chemical tests support the conclusion that the extracted material is natural rubber. Summary and Conclusions The students using both microscale and macroscale methods were able to isolate and extract rubber from milkweed leaves with recoveries consistent with literature values. Students used elasticity and IR analysis to determine that the product they isolated was natural rubber. Overall, the student feedback was very positive. Students who conducted the macroscale procedure generally felt that the experiment was too long at >4 hours; however, none of the microscale students felt this way because their procedure was much shorter, 2.5 hours. To avoid this problem, we suggest boiling the leaves for the students before they start the experiment. This will save at least 1 hour of lab time and
Journal of Chemical Education • Vol. 79 No. 1 January 2002 • JChemEd.chem.wisc.edu
In the Laboratory
make it possible for students to complete the experiment during a typical laboratory period. All students who provided written feedback said they enjoyed the experiment and found it to be a good review of laboratory techniques. Acknowledgments We would like to thank Chatham College and Seton Hill College for providing the resources to run this experiment and allowing us to add the experiment to the laboratory schedule. We would also like to thank all of the students who conducted the experiments and provided critical feedback that helped us to refine the experiments. WSupplemental
Material
A student handout, lab prep sheet, and CAS registry numbers of chemicals used are available in this issue of JCE Online. Literature Cited 1. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Organic Laboratory Techniques, 2nd ed.; Saunders: New York, 1995; pp 87–92.
2. Lehman, J. W. Operational Organic Chemistry: A ProblemSolving Approach to the Laboratory Course, 3rd ed.; Prentice Hall: New York, 1998; p 81. 3. Buccigross, J. M.; Hayes, J. W. II; Metz, C. S. J. Chem. Educ. 1996, 73, 273–275. 4. Hagerman, A. E. J. Chem. Educ. 1999, 76, 1426–1427. 5. Deal, S. T.; Hurst, M. O. J. Chem. Educ. 1997, 74, 241–242. 6. Anderson, A. M.; Mitchell, M. S.; Mohan, R. S. J. Chem. Educ. 2000, 77, 359–360. 7. Onami, T.; Kanazawa, H. J. Chem. Educ. 1996, 73, 556–557. 8. Adam, D. J.; Mainwaring, J.; Quigley, M. N. J. Chem. Educ. 1996, 73, 1171. 9. Hampp, A. J. Chem. Educ. 1996, 73, 1172. 10. Johnson, M. Edison, a Biography; McGraw Hill: New York, 1959; pp 470–474. 11. Seymour, R. B.; Carraher, C. E. Polymer Chemistry; Dekker: New York, 1988; pp 199–201. 12. Buchanan, R. A. Extraction of Rubber or Rubberlike Substances from Fibrous Plant Materials; U.S. Patent 4,136,131, 1978. 13. Hager, T.; MacArthur, A.; McIntyre, D.; Seeger, R. Rubber Chem. Technol. 1979, 52, 693–709. 14. Neilsen, P. E.; Nishumura, H.; Otvos, J. W.; Calvin, M. Science 1977, 198, 942–944. 15. Swanson, C. L.; Buchanan, R. A.; Otey, F. H. J. Appl. Polym. Sci. 1979, 23, 743–748.
JChemEd.chem.wisc.edu • Vol. 79 No. 1 January 2002 • Journal of Chemical Education
93
