In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY-Cortland Cortland, NY 13045
Microscale Chemistry and Green Chemistry: Complementary Pedagogies Mono M. Singh,* Zvi Szafran, and Ronald M. Pike National Microscale Chemistry Center and Chemistry Department, Merrimack College, North Andover, MA 01845; *
[email protected] Microscale chemistry is a laboratory-based, environmentally safe, pollution-prevention approach accomplished by using miniature glassware and significantly reduced amounts of chemicals. Microscale chemistry can be implemented without compromising educational standards or analytical rigor, and its techniques are amenable to industrial R&D applications. Since its modest beginning at three institutions in the early 1980s (Bowdoin College, Merrimack College, and Brown University), microscale chemistry has experienced a rapid growth in the U.S. and around the world. The extent of proliferation of this technique can be judged from numerous publications in this Journal. Originally, microscale chemistry was introduced in the organic chemistry laboratory at Bowdoin College, Maine. It was later expanded to cover general, inorganic, analytical, and environmental chemistry. The National Microscale Chemistry Center was established at Merrimack College in 1992–1993 as the first center to offer formal microscale chemistry training to teachers and chemists at all levels from elementary school to university. Green chemistry (1–3) is a relatively new initiative undertaken by the U.S. Environmental Protection Agency, Washington, DC, in collaboration with the ACS and the Green Chemistry Institute, MD. The simplest definition of green chemistry is “the use of chemistry techniques and methodologies that reduce or eliminate the use or generation of feedstocks, products, by-products, solvents, reagents, etc., that are hazardous to human health or the environment” (2). While more commonly being used in industrial applications, the concepts of green chemistry can also be incorporated into educational pedagogy, which argues for the adoption of microscale laboratory methods in teaching institutions. To recognize the impact of green chemistry on the environment, several awards, such as the Kenneth G. Hancock Green Chemistry Memorial Scholarship and the Presidential Green Chemistry Challenge Awards have been instituted (see EPA’s award announcement: www.epa.gov/docs/gcc). Microscale chemistry is a laboratory-based green chemistry approach. In green chemistry, the laboratory product, rather than being the industrial 10,000 lb/h of ethyl benzene, is the understanding of the chemistry behind a particular reaction. The application of green chemistry–microscale chemistry to this laboratory product would then be a modification of the reagents, solvents, experimental methodology, and/or products to allow the gaining of this knowledge with the minimum hazard to human health or the environment. A chemist trained in this way will have a significant impact on the solution of problems related to the environment. In this paper we report on the compatibility of microscale chem1684
istry and green chemistry pedagogic programs: their benefits and impact on academia and industry. We will examine each of the fundamental aspects of green chemistry pedagogy (3b), with examples from microscale experiments performed at our center. Use of Alternative Feedstocks or Starting Materials in Academic Laboratories Reduction of laboratory hazards can be achieved by microscale techniques because quantities of chemicals used in laboratory experiments are markedly reduced. This lowers the cost of operating the laboratory, which in turn allows a wider variety of alternative reagents to be used. A dramatic example is found in inorganic chemistry, in the syntheses and properties of several carbonyl and nitrosyl compounds of rhodium and ruthenium. Carbonyl compounds are traditionally prepared by direct reaction of the metal with carbon monoxide gas. For example, trans-[Rh(CO)Cl(PPh3)] was originally prepared by adding triphenyl phosphine (PPh3) to a chloroform solution of the chloro-bridged dimer Rh2(CO)4Cl2 (4, 5), which in turn was synthesized by passing CO gas over hydrated RhCl3: RhCl3(s) + CO(g) → Rh2(CO)4Cl2(s) + other products Rh2(CO)4Cl2(s) + 4 PPh3(s) → 2 trans-[Rh(CO)Cl(PPh3)2] + 2 CO(g) This preparation is difficult to carry out in the academic laboratory, owing to the high toxicity of carbon monoxide and of the solvent, and the reaction conditions. Similarly, in preparing metal nitrosyls, nitric oxide gas is used. Apart from being toxic, nitric oxide is implicated as one factor in atmospheric ozone depletion. Further, although metal carbonyls and nitrosyls are important classes of inorganic compounds and serve as excellent tools for pedagogy, the reagent toxicity combined with the high cost of platinum salts rules out their use at the traditional scale in the laboratory. Using microscale chemistry, one can avoid these problems. The rhodium carbonyl product, trans-[Rh(CO)Cl(PPh3)2], can be prepared using an alternative solvent, dimethylformamide (DMF, (CH3)2NCH=O), which serves both as the source of CO (rhodium abstracts the CO from DMF) and as the solvent (6, 7). Use of DMF thus eliminates the use of two toxic reagents, while at the same time increasing the atomic utilization (for the concept of atom utilization or atom economy concept, see refs 8a,b). The reaction is readily carried out at reflux temperature of DMF (~155 °C) and atmospheric pressure, using a 10-mL round-bottom flask equipped with a microcondenser. On adding solid PPh3 to the reaction
Journal of Chemical Education • Vol. 76 No. 12 December 1999 • JChemEd.chem.wisc.edu
In the Laboratory
mixture, one obtains trans-[Rh(CO)Cl(PPh3)2]. DMF
RhCl3?xH2O → trans-[Rh(CO)Cl(PPh3)2] PPh 3
Whereas selection of the rhodium metal system would be prohibitively expensive at the standard laboratory scale (1 g of RhCl3?xH2O costs ~$175), the reaction can be easily carried out at the microscale level using only 10 mg of rhodium chloride hydrate, resulting in a cost per student of $1.75. The product can then undergo oxidative addition with halogens (X = Cl, for example) forming mer-[Rh(CO)Cl3(PPh3)2], or SO2 adduct formation to give green Rh(CO)ClSO2(PPh3)2. The metal can then be recovered and recycled. Similarly, it is possible to prepare the metal nitrosyl compounds [RuCl3(NO)(PPh3)2] and [Ru(NO)2(PPh3)2] in ethanol using Diazald (CH3C6H4SO2N(CH3)NO) as the source of nitrosyl ligand (7). In organic chemistry, dichromate anion (Cr2O72{) in concentrated sulfuric acid is often used to oxidize alcohols. This is problematic in academic laboratories because K2Cr2O7 is carcinogenic. In the microscale laboratory (9), the oxidation of cyclohexanol can be accomplished using household bleach as the oxidizing agent. Another example involves the use of Cu2+ as a catalyst in the conversion of benzoin to benzil. Thus, microscale chemistry allows the green chemistry concept of feedstock substitution to be used, resulting in milder reaction conditions and reduced environmental exposure. This, coupled with the “maximum atom utilization theory or atom economy concept” (8) serves the basis for using green chemistry in laboratory pedagogy. Nature of Reagents or Nature of Transformations The laboratory pedagogy involving microscale chemistry allows the use of altered reagents, new reaction pathways, and alternative solvents, yielding less hazardous products and by-products. The ubiquitous “transformations of copper” experiment illustrates the effect of changing the nature of the reagent on a chemical reaction. Here, copper metal (less than 100 mg) is dissolved in dilute nitric acid in a 10-mL Erlenmeyer flask covered with an inverted beaker (10–12). The use of dilute nitric acid requires mild warming for the reaction to proceed, giving the student accurate control over the rate of reaction and almost eliminating the production of NO2 fumes. The gas dissolves in the more aqueous medium, instead of escaping into the atmosphere. The copper(II) is then transformed into several other products, from which the metal is recovered in quantitative yield. In the process, the amount of wastewater can be reduced by replacing standard filtration (using an aspirator) by use of either a Pasteur filter pipet or centrifugation. At larger scales, the reaction takes much longer and generates a copious amount of NO2 gas. Another example involving use of an alternative solvent is the preparation of zinc iodide (11, 12). The desired product here is the determination of the stoichiometry of the compound, rather than the compound itself. Thus, unlike most iodides, which are prepared in organic solvents so that they can be crystallized, the ZnI2 is prepared in water by the reaction of 100 mg of iodine with 200–250 mg of zinc. Subsequently, the stoichiometry is determined by weighing the leftover zinc metal to determine the amount of zinc that actually reacted. The solution is then recycled to recover the iodine.
Nature of Reaction Conditions Microscale chemistry allows reactions to be chosen that require less extreme reaction conditions—shorter reaction times, ambient pressure instead of high pressure, and reduced temperature requirements (often achieved with microwave heating). These are supported by extensive use of catalysis. Shorter overall reaction times are a result of several factors at the microscale level. First, the glassware is connected by o-ring cap-seals, resulting in reduced set-up times (9, 13). The reaction time itself is reduced, since there is a larger surface-to-bulk ratio for the reagents at the microscale level, enhancing mass transfer. The smaller amounts of product speed up the workup and recovery steps. Overall, microscale experiments can be performed in approximately 50% of the time of their traditional-scale counterparts, as was shown under controlled conditions for the Grignard synthesis of triphenylmethanol (13). In the multistep synthesis of hexaphenylbenzene (9), benzil is condensed with 1,3-diphenylacetone to form tetraphenylcyclopentadienone. This reaction can be carried out using microwave activation, requiring no other heating and resulting in a shorter reaction time. In step one of the overall synthesis, the cyanide catalyst can be replaced with the vitamin thiamine hydrochloride (14 ). Nature of the Final Product or the Target Molecule Since the educational product is the knowledge of the chemistry behind the particular reaction, as opposed to any particular compound, alternative safer products or target molecules can be synthesized using microscale techniques. Two such examples were discussed above. In the preparation of ZnI2, the goal was learning to determine stoichiometry, not isolation of the actual product. In the synthesis of [Rh(CO)Cl(PPh3)2], since the educational product was the synthesis of any metal carbonyl, the choice of rhodium metal was made because it allows the most benign reaction conditions. Wilkinson’s catalyst, [RhCl(PPh3)3], can be prepared in quantitative yield from RhCl3 under mild conditions (7). The catalyst is well known in reacting with aldehydes or amides, from which it abstracts a carbonyl group, resulting in the corresponding alkane or amine: RhCl3?xH2O → [RhCl(PPh3)3] (in ethanolic PPh3 solution) [RhCl(PPh3)3] + CH3(CH2)5CHO → trans-[Rh(CO)Cl(PPh3)2] + C6H14 + PPh3 The overall result is the synthesis of a metal carbonyl under mild conditions without use of CO or chloroform. Pollution Prevention The advantages of using microscale chemistry as a pollution-prevention methodology that promotes source reduction of chemical wastes are well known. The principle of the 3 R’s (reduce, reclaim, recycle) is an integral part of microscale chemistry. An interesting example illustrating these ideas is seen in the preparation of lead(II) iodide. This is a crystalline compound, which can be prepared using microscale methods from the lead ores galena (PbS) or cerussite (PbCO3) (11). The sequence of reactions (see below) in this synthesis replicates the extraction of lead from its ores using oxidation–
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reduction, precipitation, and thermal decomposition reactions. PbCO3(s) + 2HCl(aq) → PbCl2(s) + CO2(g) + H2O(,) PbCl2(aq) + 4HNO3(aq) → Pb(NO3)2(aq) + Cl2(g) + 2NO2(g) + 2H2O(,) Pb(NO3)2(aq) + 2KI → PbI2(s) + 2KNO3(aq) This experiment graphically illustrates the 3 R’s principle, as the reaction is carried out on a 50-mg scale, and after the synthesis, basic lead carbonate is recovered by treating the iodide with 6 M nitric acid followed by sodium bicarbonate. This also generates solid iodine, which can be skimmed from the surface of the solution. The construction and use of a microscale buret for microscale titration (15) and a microscale pycnometer for density determination (16 ) provide other good examples of pollution prevention. By using 2-mL microburets instead of 50-mL burets, microscale titrations can be carried out using only 4% of the traditional amounts of chemicals. The precision of the microburet is the same as of the buret, and the smaller delivered volume allows multiple runs to be performed in the same amount of time. This results in greater experimental accuracy (17 ). Moreover, microscale burets are made from graduated pipets, which are relatively inexpensive. Another example of pollution-prevention pedagogy is found in applications of microscale electrochemistry in the laboratory (18). Starting in the Fall 1998 semester, we have introduced recycling of cold water for cooling microcondensers used in reflux and distillation techniques in inorganic and organic laboratories. This is easily achieved by using inexpensive (~$20) water circulation pumps (the type used for making decorative table-top water fountains) and it reduces production of waste water in the laboratory by hundreds of gallons per day (19). Conclusion The benefits of implementing a microscale–green chemistry laboratory program include reduced reaction times, improved safety, and major cost savings (20, 21). A dramatic example of this cost effectiveness was reported in a recent paper (21) presented at the 5th North American Chemical Congress (Cancun, Mexico). In that meeting, the Autonomous University of San Luis Potosi, Mexico, reported a return of $599,000 M. on an initial outlay of $195,000 M. within one semester. The savings were realized mainly in three areas: reduced cost for laboratory supplies (including chemicals), reduced disposal cost, and economy of time. Green chemistry and microscale chemistry are complementary pedagogies, allowing the ideas of source reduction, material substitution, and exposure minimization to be brought effectively into the academic laboratory. Acknowledgment We acknowledge the support received from the Massachusetts Toxics Use Reduction Institute (TURI, MA).
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Literature Cited 1. Benign by Design: Alternative Synthetic Design for Pollution Prevention; Anastas, P. T.; Farris, C. A., Eds.; ACS Symposium Series 577; American Chemical Society: Washington, DC, 1994. 2. Green Chemistry: Designing Chemistry for the Environment; Anastas, P. T.; Williamson, T. C., Eds.; ACS Symposium Series 626; American Chemical Society: Washington, DC, 1996. Designing Safer Chemicals: Green Chemistry for Pollution Prevention; DeVito, S. C.; Garrett, R. L., Eds.; ACS Symposium Series 640; American Chemical Society: Washington, DC, 1996. Green Chemistry: Frontiers in Benign Chemical Syntheses and Processes; Anastas, P. T.; Williamson, T. C., Eds.; Oxford University Press: Oxford, UK, 1998. 3. (a) Anastas, P. T.; Warner, J. Green Chemistry: Theory and Practice; Oxford University Press: Oxford, UK, 1998. (b) Collins, T. J. J. Chem. Educ. 1995, 72, 965. 4. Hieber, W.; Legally, H. Anorg. Allgem. Chem. 1943, 251, 96. 5. McCleverty, J. A.; Wilikinson, G. Inorg. Synth. 1966, 8, 211. 6. Singh, M. M; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1990, 67, A180. 7. Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience; Wiley: New York, 1991. 8. (a) Trost, B. M. Science 1991, 2, 1471. (b) Sheldon, R. A. Chemtech 1994, 24, 38. 9. Mayo, D.; Pike, R. M.; Trumper, P. K. Microscale Organic Chemistry, 3rd ed.; Wiley: New York, 1994. 10. Szafran, Z.; Pike, R. M.; Foster, J. C. Microscale General Chemistry Laboratory with Selected Macroscale Experiments; Wiley: New York, 1993. 11. Singh, M. M.; Pike, R. M.; Szafran, Z. Micro and Macroscale Experiments for General and Advanced General Chemistry; Wiley: New York, 1995. 12. Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Chemistry for High Schools, Vols. 1 and 2; Kendall/Hunt, Dubuque, Iowa, 1996– 1998. 13. Pickering, M.; LaPrade, J. E. J. Chem. Educ. 1986, 63, 535. 14. Williamson, K. L. Microscale Organic Experiments; D. C. Heath: Lexington, MA, 1987. 15. Singh, M. M.; Szafran, Z; Pike, R. M. J. Chem. Educ. 1991, 68, A125. Singh, M. M.; McGowan, C.; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1998, 75, 371. 16. Singh, M. M; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1993, 70, A39. 17. Singh, M. M.; McGowan, C.; Szafran, Z.; Pike, R. M. J. Chem. Educ., in press. 18. Singh, M. M; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1993, 70, A36. Ibanez, J.; Singh, M. M.; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1997, 74, 1449. 19. First demonstrated to one of us (MMS) by Victor Obendrauf of University of Graz, Austria. A paper describing the method is under preparation. 20. Szafran, Z.; Singh, M. M; Pike, R. M. J. Chem. Educ. 1989, 66, A263. 21. Urizar, G.; Villar, C. Microscale in the Faculty of Chemistry of the University of San Luis Potosi; presented at the 5th Chemical Congress of North America, Cancun, Mexico, Nov. 11–15, 1997; Abstract 230; and private communication.
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