In the Laboratory
Synthesis and Characterization of a Gasoline Oxygenate, Ethyl tert-Butyl Ether
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Craig J. Donahue,* Teresa D’Amico, and Jennifer A. Exline Department of Natural Sciences, University of Michigan–Dearborn, Dearborn, MI 48128;
[email protected] Background Few compounds have been in the headlines more this past decade than the controversial gasoline oxygenate methyl tert-butyl ether or MTBE (1–3). In the 1970s MTBE was first used in low concentrations in gasoline (1–3 vol %) as an octane booster and a replacement for tetraethyllead. This occurred because of the passage of the Clean Air Act of 1970, which mandated drastic reductions in auto emissions. In response to this legislation, automobile manufacturers developed the catalytic converter. Since this catalyst system was poisoned by lead from tetraethyllead, unleaded gasoline had to be used in new vehicles with catalytic converters (4, 5). The role of MTBE changed from octane booster to gasoline oxygenate with the passage of the Clean Air Act Amendments (CAAA) of 1990. Out of this legislation came a federal mandate that certain noncompliance areas in the USA had to use winter oxyfuel and other areas reformulated gasoline (RFG). Oxyfuel is intended to reduce CO emissions and RFG is intended to reduce emissions of photochemical ozone precursors (3). The refiners turned primarily to MTBE to supply the oxygen required in these new fuels because of its many desirable properties, including its low cost compared to other potential oxygenates, its compatibility with the hydrocarbons found in gasoline, its nontoxicity, and its high octane value. Winter oxyfuel, which must be 2.7 wt % oxygen, contains 15% MTBE (if that is the oxygenate used—ethanol is another option) (3). From the time MTBE was first added to gasoline to the present, MTBE production has skyrocketed. In 1999, global demand for MTBE surpassed 7 billion gallons, and the USA consumed 61% of the MTBE produced (6 ). MTBE’s troubles began when it started to be detected in drinking water in California and elsewhere. Leaky underground storage tanks and pipelines were determined to be the primary sources of MTBE contamination. Drinking water contaminated with very low levels (ppb level) of MTBE tastes and smells bad (3). In 1996 a U.S. Geological Survey study reported that MTBE was frequently found in the urban groundwater supplies sampled (7). To compound the problem, MTBE is quite water soluble, does not readily adsorb to soil particles, travels faster and farther than other gasoline constituents, and is slow to biodegrade (3). In response to these problems/findings, a growing number of states have passed legislation or issued executive orders to ban use of MTBE in gasoline. In June 2001, the EPA turned down California’s request to have the oxygenate requirement of the CAAA of 1990 waived (8). The final chapter of the MTBE story has yet to be written. The federal government has yet to ban MTBE or modify or repeal the CAAA of 1990. Part of the current debate is whether to ban just MTBE and related dialkyl ethers or to eliminate the oxygenate requirement altogether. If MTBE is specifically banned but the oxygenate requirement is retained, ethanol’s fortunes are expected to rise (8). 724
This article describes the synthesis and characterization of the compound ethyl tert-butyl ether, or ETBE, which has also been used as a gasoline oxygenate but to a much lesser extent than MTBE. We elected to prepare ETBE over MTBE because it is less volatile; MTBE’s boiling point is 55–56 °C, whereas ETBE’s is 72–73 °C. The procedure described here could also be used to make MTBE. Target Audience On our campus this lab has been performed by secondsemester general chemistry students; however, it should also be of interest to instructors who teach an introductory organic chemistry laboratory course. The lab was developed to support a new general chemistry sequence for engineering students. To make this two-semester sequence more relevant to the audience the overarching theme of “Chemistry and the Automobile” is folded into every topic covered. The lab was designed to achieve multiple goals. It illustrates several techniques used to synthesize and purify organic compounds, including manipulation of standard-taper glassware, indirect heating with the aid of a thermowell, simple distillation, liquid–liquid extraction, and use of a drying agent. In addition, the students are exposed to two modern instrumental techniques, gas chromatography (GC) and infrared (IR) spectroscopy, which they use to establish the purity and identity of their product. To understand why the ETBE can be separated from the reactants when their boiling points are nearly identical, the students also need to have an understanding of azeotropes. Finally, since both ETBE and MTBE have been used as gasoline oxygenates, this lab serves to showcase the theme of Chemistry and the Automobile. The students are introduced to organic chemistry and the concept of functional groups in lecture, before they complete this lab. They also enter this lab with a basic understanding of IR spectroscopy. They have already completed a lab that utilizes IR spectroscopy to identify organic functional groups. This is their first exposure to gas chromatography. Methods for Synthesis of Ethers The synthesis of a number of ethers has been reported in this Journal, but not of ETBE or MTBE. Reports in this Journal include the synthesis of methyl ethyl ether (9) and benzyl phenyl ethers (10) by the Williamson synthesis and of p-cresyl propyl ether (11) and benzyl butyl ether (12) with the aid of a phase-transfer catalyst. The preparation of dibutyl ether by the acid-catalyzed bimolecular dehydration of 1butanol has been described twice in this Journal (13, 14). Two reports describe the detection and quantification of MTBE in gasoline by GC (15) and GC–MS (16 ). The synthesis of ethers by the acid-catalyzed bimolecular dehydration of an alcohol is usually restricted to unhindered
Journal of Chemical Education • Vol. 79 No. 6 June 2002 • JChemEd.chem.wisc.edu
In the Laboratory
primary alcohols. Secondary and tertiary alcohols subjected to the same conditions are prone to undergo unimolecular dehydration to yield alkenes. This method does not work for unsymmetrical ethers either, because complex mixtures of three ethers generally result. The exception is the reaction of a tertiary alcohol with a primary or secondary alcohol in the presence of dilute acid. The rapid formation of the tertiary carbocation under these mild conditions followed by attack by the other alcohol allows the preparation of the unsymmetrical ether in high yield (17 ). This is the approach we used to prepare ETBE. It was first described by Norris and Rigby (18).
Synthesis and Purification of ETBE The preparation and purification of ETBE is completed in one 4-hour period. Students perform the synthesis of ETBE in groups of four. Each group is assigned their own hood. The following setup utilizing 19/22 standard-taper glassware is preassembled for the students. A Claisen head is attached to a 250-mL round-bottom flask, and a stillhead is attached to the side arm of the Claisen head. A water-cooled condenser equipped with a vacuum adapter and receiving flask is attached to the stillhead. Attached to the top of both the Claisen head and the stillhead are thermometer/thermometer adapter assemblies. The thermometer attached to the Claisen head is used to monitor the reaction mixture. Seventy-five milliliters of 15% sulfuric acid followed by 30 mL of 95% ethanol is added to the round-bottom flask. This mixture is heated to 70 °C with the aid of a heating mantle. The thermometer/thermometer adapter assembly is then removed from atop the Claisen head and replaced with a dropping funnel. To the funnel is added 23.3 mL of tertbutyl alcohol. Caution: The tert-butyl alcohol will freeze if its temperature is allowed to fall below 25 °C. Approximately onethird of the tert-butyl alcohol is added to the reaction mixture immediately and the remainder is added in two batches 10 minutes apart. Shortly after the first addition of tert-butyl alcohol, the low-boiling azeotrope (bp 64 °C) of ETBE and water starts to distill over. The students are instructed to collect the azeotrope for 45–60 minutes, maintaining the temperature at the stillhead at or near this temperature and no higher than 70 °C. A small sample (ca. 1 mL) of the crude product is set aside for analysis. The remainder is subjected to purification by liquid–liquid extraction. The crude ETBE is purified by liquid–liquid extraction with distilled water (10 mL of H2O, 6–8 times) using a separatory funnel, and then dried over anhydrous MgSO4. The dried ETBE is separated from the drying agent by pipetting it away from the MgSO4 into a clean dry vial (rather than removing the drying agent by gravity filtration, which greatly reduces the yield of product). A small sample (ca. 1 mL) of the purified product is set aside for analysis. Analysis of the Crude ETBE The gas chromatographs of the crude and purified ETBE are obtained outside the lab period by student assistants. During the lab period the students view a short PowerPoint presentation on GC and are given a demo of the GC that is used to analyze their samples. The gas chromatographic separations are performed on a Varian 3350 gas chromatograph equipped with a thermal conductivity detector. A 6-ft × 1 ⁄ 8 -in. stainless steel column packed with Chromosorb 101 (80/100)
Figure 1. Gas chromatograph of crude ETBE.
is used. The operating conditions are as follows. The initial column temperature of 110 °C with no hold time is ramped at a rate of 10 °C/min to a final temperature of 250 °C followed by a hold time of 10 min. The helium carrier gas flow rate is 30 mL/min and the sample size is 0.5 µL of the fraction (undiluted). The IR spectra are recorded from a neat liquid between NaCl plates. Chromatograms of the crude ETBE invariably reveal the presence of four species—water, ethanol, tert-butyl alcohol, and ETBE. The retention times for these species under the operating conditions specified are water, 0.9 min; ethanol, 2.0 min; tert-butyl alcohol, 3.5 min; and ETBE, 5.2 min. The percentage of ETBE in the crude product (the distillate) varies considerably from one student sample to the next. Based on 25 student samples, the fraction of ETBE in the crude distillate ranged from 10% to 65%. Figure 1 is a typical chromatogram of the crude ETBE distillate.
Analysis of the Purified ETBE Chromatograms of the purified ETBE show ETBE to be present at an average level of 92%, with a range of 88% to 97%. Absent from the IR spectrum of the purified ETBE is the broad –OH stretch observed in crude ETBE samples. The fingerprint region of the IR spectrum of the purified ETBE matches that of the Aldrich IR spectrum of ETBE (within ± 0.5 cm᎑1) (19). The yields of ETBE ranged from 7% to 27%. Two-thirds of the student groups obtained yields greater than 15%. There are several reasons why yields are not higher: (i) collection of the low-boiling azeotrope is often curtailed before it is complete to permit this experiment to be completed in one lab period, (ii) owing to its high volatility some of the recovered ETBE is lost during the purification, and (iii) the students are novices at performing the purification procedures. Nevertheless, more than enough ETBE is collected to perform the characterization. When the same procedure is performed by more advanced undergraduate students the yield of ETBE ranges from 40% to 50%. We have found that to obtain ETBE at a purity level of 98% and higher at least a dozen extractions are needed. The purified ETBE can also be examined by NMR spectroscopy, although this procedure is not included in this lab experiment. The 1H NMR spectrum of the purified ETBE in CDCl3 contains a quartet at 3.41 ppm due to the methylene
JChemEd.chem.wisc.edu • Vol. 79 No. 6 June 2002 • Journal of Chemical Education
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In the Laboratory
hydrogens of the ethyl group, a singlet at 1.20 ppm due to the methyl hydrogens of the tert-butyl group, and a triplet at 1.17 ppm due to the methyl hydrogens of the ethyl group. There is partial overlap of the latter two peaks. The 13C NMR spectrum of the purified ETBE in CDCl3 has four peaks, at 72.47, 56.70, 27.53, and 16.24 ppm. The 13C NMR chemical shift values of the purified ETBE match those of the Aldrich 13 C NMR spectrum (within ± 0.1 ppm) (20). Hazards Ethyl alcohol, tert-butyl alcohol (2-methyl-2-propanol), and ethyl tert-butyl ether are flammable liquids and irritants. Dilute sulfuric acid is toxic and corrosive. The entire synthetic and purification procedure should be performed in a hood and students should wear safety goggles and gloves when handling these chemicals. Ethers may form reactive peroxides and become an explosion hazard if stored too long. Conclusion Students and lab instructors have responded very favorably to this experiment. They like its multifaceted nature. The general chemistry students are excited by the sophisticated glassware and complex setup they get to use and are intrigued by the gas chromatographic technique. By examining both the crude and purified ETBE samples by GC, they can see the impressive change in the purity of their sample—the outcome of liquid–liquid extraction and the subsequent drying process. They can also see a difference in the IR spectra of the crude and the purified ETBE. Finally, students gain a better appreciation of the controversy surrounding MTBE’s use in gasoline because, besides learning about it in lecture they have now prepared and analyzed its cousin, ETBE, in the laboratory. Acknowledgments We gratefully acknowledge the financial support of this work from The Camille and Henry Dreyfus Foundation, Special Grant Program in Chemical Sciences, and an NSF
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Course, Curriculum, and Laboratory Improvement grant, DUE-0088729. We thank Bette Kreuz for assistance in developing the GC method for this lab. W
Supplemental Material
Detailed instructions for students, with worksheets and diagrams of the experimental setups, are available in this issue of JCE Online. Literature Cited 1. U.S. Environmental Protection Agency. MTBE Fact Sheet #1: Overview; U.S. Government Printing Office: Washington, DC, 1998; EPA 510-F-97-014. 2. Hanson, D. Chem. Eng. News 1999, 77 (Oct 18), 49. 3. Franklin, P. M.; Koshland, C. P.; Lucas, D.; Sawyer, R. F. Environ. Sci. Technol. 2000, 34, 3857. 4. Taylor, K. C. Chemtech 1990, 20, 551. 5. Heck, R.; Farrauto, R. Automotive Eng. 1996, 104, 93. 6. Thayer, A. Chem. Eng. News 2000, 78 (Apr 17), 25. 7. Squillace, P. J.; Zogorski, J. S.; Wilber, W. G.; Price, C. V. Environ. Sci. Technol. 1996, 30, 1721. 8. Watkins, K. Chem. Eng. News 2001, 79 (Jul 23), 21. 9. Casanova, J. Jr. J. Chem Educ. 1963, 40, 41. 10. Ditts, K.; Durand, M. J. Chem Educ. 1990, 67, 74. 11. Rowe, J. E. J. Chem Educ. 1980, 57, 162. 12. Hill, J. W.; Corredor, J. J. Chem Educ. 1980, 57, 822. 13. Yohe, G. R.; Yohe, M. T. J. Chem Educ. 1932, 9, 1268. 14. Smith, W. B. J. Chem Educ. 1962, 39, 212. 15. Brazdil, L. J. Chem Educ. 1996, 73, 1056. 16. Quach, D. T.; Ciszkowski, N. A.; Finlayson-Pitts, B. J. J. Chem. Educ. 1998, 75, 1595. 17. Vollhardt, K. P. C. Organic Chemistry; Freeman: New York, 1987; Chapter 9. 18. Norris, J. F.; Rigby, G. W. J. Am. Chem. Soc. 1932, 54, 2088. 19. Pouchert, C. J. The Aldrich Library of FT–IR Spectra; Aldrich Chemical Co.: St. Louis, MO, 1985. 20. Pouchert, C. J.; Behnke, J. The Aldrich Library of 13C and 1H FT–NMR Spectra; Aldrich Chemical Co.: St. Louis, MO, 1992.
Journal of Chemical Education • Vol. 79 No. 6 June 2002 • JChemEd.chem.wisc.edu
