In the Laboratory edited by
Green Chemistry
Mary M. Kirchhoff ACS Green Chemistry Institute Washington, DC 20036
Solvent-Free Wittig Reaction: A Green Organic Chemistry Laboratory Experiment
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Sam H. Leung* and Stephen A. Angel Department of Chemistry, Washburn University, Topeka, KS 66621;
[email protected] The Wittig reaction is an important chemical reaction that is used to convert ketones or aldehydes into alkenes. Laboratory experiments based on this reaction are found in many organic chemistry laboratory textbooks (1–7) and some have been published in this Journal (8–16). In recent years, promoting green chemistry has become an important focus in the chemical industry, research laboratories, and teaching laboratories (17–19) because of environmental, health, and economic concerns. Several recently published green organic chemistry experiments (20–23) show this trend occurring in academia. Most organic chemical reactions in the laboratory take place in organic solvents. These solvents account for a great proportion of the waste material generated in a laboratory. Therefore, the development of environmentally friendly solvents or solvent-free reactions continues to be of great interest. Recently, Balema et al. reported several solvent-free Wittig reactions carried out mechanochemically by ball milling (24). Although this method is innovative and environmentally friendly, the equipment setup (ball mill, helium atmosphere, forced air-cooling) and the lengthy time (3 to 20 hours) needed for the reaction are not practical in a teaching laboratory. After some investigation, we discovered that some Wittig reactions can be carried out by grinding the reactants in a mortar with a pestle for about 20 minutes. Based on our findings, we have successfully developed a laboratory experiment involving a solvent-free Wittig reaction that can be completed in a three-hour sophomore organic chemistry laboratory class period. In addition to the traditional value of the Wittig reaction, this experiment also adds a new variety to the common reactions carried out (with solvent) in an organic chemistry teaching laboratory. In this experiment, (E )- and (Z )-1-(4-bromophenyl)2-phenylethene, 3 and 4, are synthesized by a Wittig reaction using benzyltriphenylphosphonium chloride, 1, 4-bromobenzaldehyde, 2, and potassium phosphate (tribasic) (Scheme I). These chemicals are placed in a mortar and ground with a pestle for about 20 minutes to effect the reaction. The isolation of the E isomer of the product is achieved by washing the initial crude product with water, followed by recrystallization in ethanol.
generate an ylide from the phosphonium salt, and the ylide reacts with the aldehyde to give the alkene product. In the original publication by Balema et al. (24), this solvent-free Wittig reaction was carried out in a ball mill under helium atmosphere for eight hours with potassium carbonate as the base. However, potassium carbonate did not work very well
O
P(Ph)3
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Br 2
1
K3PO4 no solvent
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Br
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+ (Ph)3P
Although this Wittig reaction is carried out without solvent, it does not change the fundamental concepts behind the Wittig reaction. A base is still required in the reaction to
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Scheme I. Synthesis of (E )- and (Z )-1-(4-bromophenyl)-2-phenylethene by a solvent-free Wittig reaction.
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In the Laboratory
in our case using a mortar and pestle. We eventually determined that potassium phosphate (tribasic) was the most effective base for our reaction. This solvent-free Wittig reaction produces both (E )- and (Z )-1-(4-bromophenyl)-2-phenylethene. The presence of these isomers can be seen on thin-layer chromatography of the crude product mixture. Students need to be reminded that even though both the E and Z isomers of the alkene product are produced (in about 70% yield), only the E isomer will be isolated by crystallization (between 20 and 26% yield) in the this laboratory experiment. If an NMR spectrometer is available for student use, the instructor may choose to extend this experiment to include NMR analyses of the crude product mixture or the purified E isomer of the product. Using these NMR spectra, students can practice determining coupling constants of the alkenyl protons from these alkene products. In comparison with other reactions that form carboncarbon double bonds, the Wittig reaction is useful in controlling the position of the double bond with high certainty. In general, however, Wittig reactions have a low atom economy owing to the reaction of a triphenylphosphonium salt and the production of triphenylphosphine oxide. The major advantage of this Wittig reaction in an organic chemistry laboratory is the absence of an organic solvent for the reaction. Hazards Benzyltriphenylphosphonium chloride, 4-bromobenzaldehyde, and potassium phosphate can cause eye and skin irritation. Ethanol is flammable. Denatured ethanol contains small quantities of organic compounds such as ethyl acetate and methyl isobutyl ketone. Students should avoid breathing the vapor. Acknowledgments The development of this laboratory experiment was supported by a Curriculum Development Grant from Washburn University. The authors would also like to thank the reviewers for their valuable comments. WSupplemental
Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
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Literature Cited 1. Cox, C. F. Experimental Organic Chemistry; Macmillan: New York, 1988; pp 301–304. 2. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Orgnanic Laboratory Techniques; Saunders: Philadelphia, PA, 1990; pp 297–302. 3. Nimitz, J. S. Experiments in Organic Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1991; pp 291–294. 4. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory, 3rd ed.; Wiley: New York, 1994; pp 297–313. 5. Mohrig, J. R.; Hammond, C. N.; Morrill, T. C.; Neckers, D. C. Experimental Organic Chemistry; Freeman: New York, 1998; pp 267–274. 6. Williamson, K. L. Macroscale and Microscale Organic Experiments, 3rd ed.; Houghton Mifflin: Boston, MA, 1999; pp 464– 470. 7. Lehman, J. W. Operational Organic Chemistry, 3rd ed.; Prentice Hall: Upper Saddle River, NJ, 1999; pp 315–322. 8. Broos, R.; Tavernier, D.; Anteunis, M. J. Chem. Educ. 1978, 55, 813. 9. Gillois, J.; Guillerm, G.; Savignac, M.; Stephan, E.; Vo-Quang, L. J. Chem. Educ. 1980, 57, 161–162. 10. Lampman, G. M.; Koops, R. W.; Olden, C. C. J. Chem. Educ. 1985, 62, 267-268. 11. Warner, J. C.; Anastas, P. T.; Anselme J.-P. J. Chem. Educ. 1985, 62, 346. 12. Silversmith, E. F. J. Chem. Educ. 1986, 63, 645. 13. Pike, R. M.; Mayo, D. W.; Butcher, S. S.; Butcher, D. J.; Hinkle, R. J. J. Chem. Educ. 1986, 63, 917–918. 14. Breuer, S. W. J. Chem. Educ. 1991, 68, A58–A60. 15. Breton, G. W. J. Chem. Educ. 1997, 74, 114–115. 16. Jaworek, C.; Iacobucci, S. J. Chem. Educ. 2002, 79, 111. 17. Singh, M. M.; Szafran, Zvi; Pike, R. M. J. Chem. Educ. 1999, 76, 1684–1686. 18. Hjeresen, D. L.; Shutt, D. L.; Boese, J. M. J. Chem. Educ. 2000, 77, 1543–1547. 19. Kirchhoff, M. M. J. Chem. Educ. 2001, 78, 1577. 20. Reed, S. M.; Hutchison, J. E. J. Chem. Educ. 2000, 77, 1627– 1629. 21. Pohl, N.; Clague, A.; Schwarz, K. J. Chem. Educ. 2002, 79, 727–728. 22. Harper, B. A.; Rainwater, J. C.; Birdwhistell, K.; Knight, D. A. J. Chem. Educ. 2002, 79, 729–731. 23. Palleros, D. R. J. Chem. Educ. 2004, 81, 1345–1347. 24. Balema, V. P.; Wiench, J. W.; Pruski, M.; Pecharsky, V. K. J. Am. Chem. Soc. 2002, 124, 6244–6245.
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