In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
A Convenient Method for Dispensing Organometallic Reagents Thomas A. Newton Department of Chemistry, University of Southern Maine, Portland, ME 04104-9300;
[email protected] Discussion of the chemistry of Grignard reagents and organolithium compounds is standard fare in introductory organic chemistry textbooks (1). In teaching laboratories these reagents traditionally have been generated by the reaction of magnesium or lithium metal with an alkyl or aryl halide in an inert solvent (2). The resulting solution is then used in situ. One problem with this approach is that it is difficult to be certain of the concentration of the organometallic species. Titration of an aliquot of the solution is required for an accurate determination (3, 4). Commercially prepared solutions of these reagents of known concentrations are especially convenient. In teaching laboratories, however, having large numbers of students use such reagents poses logistical problems; glass syringes are easily broken, plastic syringes swell upon contact with the nonpolar solvents used to prepare these reagents, repeated insertion of needles into a reagent bottle creates leaky septa, and measuring accurately the volume of solution used is difficult, especially for beginners. This article describes a simple method for dispensing solutions of air- and moisture-sensitive materials that avoids these problems. For the past five years we have used this method in our introductory organic chemistry laboratory to dispense 1.6 M solutions of n-butyllithium. Related approaches have been described in this Journal (5). The apparatus required to use this technique is readily assembled from a three-piece buret, two syringe needles, a rubber septum, a balloon, and some Teflon tape. Prior to assembly, the buret is flame-dried. The top of the buret is then fitted with a rubber septum through which a syringe needle is inserted. Initially this needle is connected to an inert gas supply to maintain anhydrous conditions while the glass cools. Later it serves to equilibrate the pressure as reagent is removed. The top end of the buret tip is wrapped with Teflon tape before being inserted into the stopcock. The tapered end is similarly wrapped to make a leakproof connection between it and the delivery needle. The assembly is clamped to a ring stand and the organometallic reagent is cannulated under pressure into the buret. A balloon is filled with argon or nitrogen. A few twists of the neck of the balloon prevent the gas from escaping as the opening is stretched over the rubber septum. The balloon is then tied snugly with a piece of string. At this stage the apparatus is ready for use by students. Prior to adding the organometallic reagent, students place any reactants or solvents they need into a septum-sealed flask that they have swept with inert gas. They then insert the delivery needle through the septum, open the stopcock, and drain the desired volume of reagent into the flask. A vent needle ensures equalization of the pressure as the organometallic solution is added to the flask. A schematic drawing of the apparatus is shown in Figure 1. 936
Journal of Chemical Education
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balloon syringe needle rubber septum
25- or 50-mL buret
PTFE stopcock Teflon tape delivery needle
vent needle rubber septum
reaction flask
Figure 1. An apparatus for dispensing organometallic reagents.
We have used this system in our introductory organic chemistry course, which has three sections with 18 students each. Solutions of 1.6 M n-butyllithium in hexanes show no discernible change in concentration for at least a week. Students find the apparatus easy to use; they are able to measure the quantity of solution they add to their reaction mixtures to the nearest 0.1 mL. The only problems we have encountered are an occasional clogged needle or buret tip. Replacing the needle addresses the former situation; the latter may be remedied with a cleaning wire for a 10 µL syringe. Literature Cited 1. McMurry, J. Organic Chemistry, 6th ed.; Brooks Cole: Belmont, CA, 2004; pp 329–332. 2. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory with Multistep and Multiscale Syntheses, 4th ed.; Wiley: New York, 2000; pp 246–253. 3. Watson, S. C.; Eastman, J. F. J. Organomet. Chem. 1967, 9, 165. 4. (a) Ellison, R. A.; Griffin, R.; Kotsonis, F. N. J. Organomet. Chem. 1972, 36, 209. (b) Gilman, H.; Cartledge, F. K. J. Organomet. Chem. 1964, 2, 447. 5. (a) Newton, Thomas A. J. Chem. Educ. 1991, 68, A60. (b) Smith Vosejpka, L. J. J. Chem. Educ. 1993, 70, 665.
Vol. 82 No. 6 June 2005
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