In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY-Cortland Cortland, NY 13045
Wittig Reaction: The Synthesis of trans-9-(2-Phenylethenyl)anthracene Revisited
W
Christine Jaworek and Sarah Iacobucci* Department of Chemistry, Tufts University, Medford, MA 02155; *
[email protected] Several Wittig reactions that are appropriate for an undergraduate organic chemistry laboratory have been described in this Journal (1–4) and in organic chemistry laboratory manuals (5, 6 ). The synthesis of 9-(2-phenylethenyl)anthracene (1, 5) is an especially favorable reaction for the following reasons: only the trans product is generated, which readily crystallizes as thin yellow plates; the 1H NMR spectrum is ideal for illustrating how J values can be used to determine stereochemistry (7–9); and the fluorescent property of trans-9-(2-phenylethenyl)anthracene allows for an immediate analysis of the product using TLC (or the brilliant blue fluorescence can be quickly visualized by irradiating the reaction mixture with long-wave radiation). In addition, the trans-9-(2-phenylethenyl)anthracene can be used in future chemiluminescence laboratory experiments (10). In reported procedures for the synthesis of trans-9-(2phenylethenyl)anthracene, the reaction solvent was typically methylene chloride. The product was obtained by extraction from the reaction mixture and evaporation of the solvent. However, the reported workup was often found to be technically problematic for the students. For example, during the extraction steps, large emulsions were produced (as often happens when using halogenated solvents in extraction). Also, the undergraduate laboratory is often not equipped with the most effective methods for evaporating solvent, such as rotary evaporators and proper ventilation.
mixture. Carefully add 50% sodium hydroxide (50% NaOH w/w) (Fisher Scientific) (0.25 mL, using a calibrated Pasteur pipet or Beral transfer pipet) to the rapidly stirred reaction mixture. If necessary, wash any solid off the walls of the Erlenmeyer flask with DMF (ca. 5 drops). After stirring vigorously for 30 min, add 4 mL of a 1:1 mixture of 1propanol/H2O to precipitate the product. Collect the crude product by vacuum filtration. Transfer the crude product to a 10-mL Erlenmeyer flask and recrystallize using ca. 4 mL of 1-propanol. Typical student yields are 50–70% (more than enough sample to use in the above-mentioned analyses and future experiments). TLC: stationary phase, silica gel; eluent, hexane/ethyl acetate, 5:1; Rfproduct = .75, bright blue spot when irradiated at 364 nm; Rf 9-anthraldehyde = .50, yellow spot.
The Revised Experiment
1. Silversmith, E. F. J. Chem. Educ. 1986, 63, 645. 2. Pike, R. M.; Mayo, D. W.; Butcher, D. W.; Butcher, S. S.; Hinkle, R. J. J. Chem. Educ. 1986, 63, 917. 3. Lampman, G. M.; Koops, R. W.; Olden, C. C. J. Chem. Educ. 1985, 62, 267–268. 4. Warner, J. C.; Anastas, P. T.; Anselme, J.-P. J. Chem. Educ. 1985, 62, 346. 5. Williamson, K. Macroscale and Microscale Organic Experiments, 3rd ed.; Houghton Mifflin: Boston, MA, 1999; pp 464–467. 6. Mayo, D. W.; Pike, R. M.; Butcher, S. S. Microscale Organic Laboratory; Wiley: New York, 1986; pp 163–172. 7. Williamson, K. Op. cit.; pp 218–219. Coupling constants for trans alkenes typically 13–18 Hz and for cis alkenes typically 7–12 Hz. 8. Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; Wiley: New York, 1998; p 212. 9. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1992; p 620. 10. Williamson, K. Op. cit.; pp 656–661.
The following changes were incorporated in an effort to improve the workup. 1. N,N-Dimethylformamide was employed in place of methylene chloride as the reaction solvent. 2. The product was isolated by precipitating it from the reaction mixture using 1-propanol and water rather than trying to extract it. 3. The crude product was collected by vacuum filtration.
Procedure Place the following in a 10-mL Erlenmeyer flask: benzyltriphenylphosphonium chloride (200 mg, 0.514 mmol) (Aldrich Chemical Co.); 9-anthraldehyde (115 mg, 0.558 mmol) (Aldrich Chemical Co.); a small stir bar; and N,Ndimethylformamide (DMF) (0.5 mL). Vigorously stir the
Hazards Sodium hydroxide (50% NaOH w/w) is extremely damaging to the eyes and skin. Students must wear eye protection at all times. W
Supplemental Material The NMR data are available in this issue of JCE Online.
Literature Cited
JChemEd.chem.wisc.edu • Vol. 79 No. 1 January 2002 • Journal of Chemical Education
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