In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY–Cortland Cortland, NY 13045
Notes on Converting to Microscale Antonio Herrera and John Almy* Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense, E-28040, Madrid, Spain At the Universidad Complutense de Madrid, sets of small glassware (1) were used instead of commercial microscale kits.1 The only exception is the commercial distillation equipment, which is rotated among the students. Below we describe the equipment and report four microscale experiments that emphasize microscale techniques. Projects involving long periods for reflux or distillation are scheduled with short natural products isolations to allow multitasking (2).
Figure 1. Continuous extraction apparatus.
Equipment Test tubes are used for most reactions involving refluxing (3, 4). Heat is carefully controlled (sand baths heated by hot-plates) so as to maintain the drip ring in the lower half of the tube. Syringe pipets (5), made from 1-mL tuberculin syringes fastened to Pasteur pipets with 1-cm lengths of Tygon tubing, are used to measure and transfer liquids. Recrystallizations are carried out in 5-mL beakers; solids are removed with a microspatula and filtered by suction filtration (6). These beakers are kept from being toppled by fitting them snugly into flat spirals wound from 10-cm pieces of wire 0.7–1.0 mm in diameter. Graduated pipets (10 and 25 mL) are used for elution chromatography. A small plug of cotton is inserted to support the stationary phase, which is transferred to the column as a slurry using a pipet. After the stationary phase has settled the supernatant solvent can be pipetted away rather than run through the column. There is little need for stopcocks; if flow must be suspended for short times, the top of the column is sealed with a septum cap. Techniques Extractions are carried out in test tubes to allow the dispersion of emulsions by centrifugation. Short experiments in which benzil is extracted with methylene chloride and with ether provide training for the separation of layers inside syringe pipets. When not being shaken the tube rests in a beaker of ice-water to prevent losses during the transfer of volatile solvents. A clamped filter/drying pipet (7) then directs the extracted solution into a weighed flask for concentration. Boiling points are taken using a modification of Siwoloboff’s method (8). A single 10× 75-mm test tube is clamped with one quarter immersed in a small silicon oil bath. Into this tube are placed an inverted capillary tube (about 5 mm long) and 5 drops of the liquid to be measured (9). The thermometer is clamped so that its tip is inside the tube, 1 mm above the bottom. Spinning-band and Hickman distillation sets are shared. In one experiment a sample of “hexanes” solvent is *Corresponding author. Permanent address: Department of Chemistry, California State University, Stanislaus, Turlock, CA 95382.
fractionated by the spinning-band column; the fractions are analyzed by gas chromatography and a GC-MS analysis of the initial mixture is provided for the identification of peaks. Since some of the components are C6 alkenes, the order of distillation of the components does not follow the order of elution by gas chromatography. Modifications of Published Procedures
Synthesis of 4,6,8-Trimethylazulene The procedure of Garst et al. (10, 11) is scaled down by a factor of ten except for the cracking of dicyclopentadiene, which is carried out at conventional scale for the class. Extraction of the product mixture (with 2 mL of hexane) is greatly simplified in the scale-down by carrying it out in a 15 ×95-mm test tube and breaking the resulting emulsion by centrifugation. The initial separation between the layers is not visible, but when the upper 2-mL layer is transferred to a small test tube and mixed with 2 drops of water, the layers are clearly separated. The water is returned to the centrifuge tube and the hexane layer is combined with later extracts and washed with saturated aqueous NaCl. Students follow the progress of the extraction by noting the progressively lighter color of successive hexane extracts. The dried and concentrated product is then chromatographed using a 10-mL graduated pipet (see above), and rechromatographed if necessary. Yields after crystallization are 5–10 mg. Isolation of Cholesterol by Continuous Extraction The procedure was adapted from Williamson’s method (12) except that the cholesterol is separated from the gallstones (autoclaved and ground) by continuous extraction at the boiling temperature of the solvent. The apparatus is fashioned from a 50-mL beaker and a paper cone made from a filter disk (9-cm diameter), which rests on the flare of the beaker. The cone is not fluted, but a small notch is cut to allow vapors to pass around it. It rests on the flare of the beaker and is held in place inside the bead of the beaker’s rim. A watch glass containing 2–3 g of ice rests on top of the assembly to act as the condenser and hold the paper cone in place. The water in the watch glass is replaced with ice from time to time. The extraction is carried out in the hood because some solvent escapes and needs to be replaced periodically. The heat source for the beaker should be set
JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
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In the Laboratory close to the solvent’s boiling point to reduce excess escape of vapors. The extraction is complete in 30 minutes, and the recrystallization is done in the same beaker. Recrystallized yields from 100 mg of ground gallstones average 70 mg, m.p. 144–145 °C. This apparatus is also appropriate for hot filtrations.
Synthesis of Benzoic Acid via a Grignard Reaction Williamson’s procedure (13) is followed, except that 16 × 175- or 16 ×198-mm test tubes are used to form the phenylmagnesium bromide and for the reaction of the dry ice. The tubes are clamped side by side during the transfer (by syringe pipet) of the Grignard reagent. The second tube is used for the extractions. Yields are about 120 mg after crystallization, m.p. 118–119 °C. Synthesis of m-Tetraphenylporphyrin The procedure of Pasto, et. al. (14) is scaled down by a factor of 5. A 16 × 198-mm test tube is used for the reaction. The cooled reaction product (fine crystals) and subsequent methanol rinsings are transferred to suction filtration with a pipet. Acknowledgments We warmly thank Josefa Rodríguez Yunta, Fernando Gómez Contreras, and Nazario Martín León for their support and assistance during the conversion to microscale. J.A. gratefully acknowledges financial support from DGICYT (grant PB95-0911) and the Universidad Complutense through its program of assistance to foreign guest researchers on sabbatical leave (“Estancias de Científicos en Régimen de Año Sabático en la Universidad Complutense”).
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Note 1. Microscale equipment currently sells in Spain for about double the prices quoted in the Ace Catalog 1600 (1996).
Literature Cited 1. Bishop, R. D. J. Chem. Educ. 1994, 71, A252–A253; Silberman, R. G. J. Chem. Educ. 1994, 71, A140–A141; Crouch, R. D.; Nelson, T. D.; Kitner, C. M. J. Chem. Educ. 1993, 70, A203–A204; Breuer, S. W. Educ. Chem. 1991, 28, 75–77. 2. Alty, L. T. J. Chem. Educ. 1993, 70, 663. 3. We have had success with this use of standard test tubes even though they do not have the ideal geometry: Williamson, K. L. Instructor’s Guide for Macroscale and Microscale Organic Experiments, 2nd ed.; Heath: Lexington, MA, 1994; p vii. 4. Flash, P.; Phiri, S.; Mukhergee, G. J. Chem. Educ. 1994, 71, A5–A6. 5. Henley, T; Munchausen, L. J. Chem. Educ. 1993, 70, A311; see also Swanson, G. C. J. Chem. Educ. 1994, 71, A202 for the measurement of small quantities. 6. Claret, P. A. Small Scale Organic Preparations; Pitman: London, 1961; p 15. The nail head should not be perfectly round; otherwise the flow will be impeded. 7. Vestling, M. M. J. Chem. Educ. 1990, 67, 274. 8. Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman: Essex, England, 1989; p 242. 9. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory, 3rd ed.; Wiley: New York, 1993; p 45. Describes the ultramicro technique, which requires more sample preparation time but considerably less unknown. 10. Garst, M. E.; Hochlowski, J.; Douglass, J. G.; Sasse, S. J. Chem. Educ. 1983, 60, 510. 11. An alternative method for preparing cyclopentadiene is equally effective: Williamson, K. L. Macroscale and Microscale Organic Experiments, 2nd ed.; Heath: Lexington, MA, 1994; p 308. 12. Ibid., p 153. 13. Ibid., pp 369, 372. 14. Pasto, D.; Johnson, C.; Miller, M. Experiments and Techniques in Organic Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1992; p 473.
Journal of Chemical Education • Vol. 75 No. 1 January 1998 • JChemEd.chem.wisc.edu
