Efficient Distillation in the Microscale Laboratory Ronald G. Lawler and Kathlyn A. Parker Brown University. Providence, RI 02912 The virtues attributed recently to the use of microscale techniques in the introductory organic chemistry laboratory' have led us to explore this approach in our own undergraduate course2. Rather than carrying out scaled-down versions of traditional large-scale experiments3, however, we have sought simple protocols designed specifically to handle 10-100 mg of liquids and solids. Consequently we have abandoned the use of glassware with ground glass or other connectors and rely instead on the use of simple, essentially disposable apparatus constructed in large part by the students themselves. Separation and purification by liquid-liquid extraction or recrvstallization using small centrifuge tubes and Pasteur pipe&, techniques traditionally taught in upper-level qualitative organic analysis courses, were easily mastered by our second-semester freshmen. Simple distillation of a small quantity of liquid reaction product, on the other hand, is a techniaue without strong- orecedent in the undereraduate . curriculum. Our solution to this problem involves the use of a one-~iecedistillation tube insoired bv the Kueelrohr a. m.a ratus which has become a fixture of modern organic research laboratories4.Our version, however, is smaller than commercially available tubes and is constructed from a disposable Pasteur ~ t student as needed. In the course of a . .i ~by.e each semester each student runstructs six to eight of these tuhes. \Ye have yet to enwunter anyone who, with a little practice, cannot construct a serviceable tube in a few minutes and distill a few drops of liquid using the heat from a gas-air microburner.
The Pasteur Plpet Mlcrwtlll
The figure illustrates the steps required to construct a microstill and to distill and collect a small sample (-100 rL) of relatively nonvolatile liquid (see below). The tip of a disposable pipets is sealed at one end and necked down near the other (a) and small, hemispherical bulbs (walls not too thin) are blown in the side (b) and end (c) . . of the tube. The samole is laced in the end bulb using a fine-pointed disposable pipet id) and distilled (e) by heating in a low microburner flame. The tube is then sealed a t the constr~ctiun(il, the end bulb is broken off after sn~ring(g), and the distillate is shaken or centriiuaed - into the tip (h) for storage or further use. Aside from the usual hazards associated with broken or hot glass and flames, we have experienced no safety prohlems using this apparatus in introductory organic chemistry. The glass surface between the two bulbs is adequate to condense the vapors of liquids with boiling points as low as the-C-I alkenes (bp -90 OC) without additional c ~ o l i n g . ~ However, ether or other volatile, flammable solvents must be removed from the sample before it is distilled. Wc are indelned to Susan Hurenberg, our teaching assistant. and to the 12 Hrown llniversitv underaraduates in our original section for their cheerful p&ticipahon in this proiect. Michael Ssuillacote has also tested most of our procedures with the help of an additional small group of undergraduates. Financial support for our effort to develop microscale experiments was provided by Brown University.
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Mayo. D. W.; Butcher. S.S.; Pike. R. M.; Fwte. C. M.; Hotham. J. R.; Page, D. S. J. Chem. Educ. 1985, 62, 147. The first three semesters of the chemistry curriculum at Brown at the time of this project consisted of one semester of introductory chemistry followed by two semesters of organic chemistry. The laboratory techniques described here were introduced in the first semester of the organic chemistry course. The majority of the participants were second-semester freshmen. Many of our experiments were adapted from Pavia, D. L.; Lampman, G. M.; Kriz, G. S.. Jr. "Introduction to Organic Laboratory Techniques". 2nd ed.; Saunders: Philadelphia. 1976. A conceptually similar microdistillation tube was used in the early days of organic chemistry. One such apparatus is described in Pregl. F. "Die Quantitative Organische Mikroanalyse"; Springer: Berlin. 1917; p 179. Also of interest is a simple apparatus for fractional distillation: Emich, F. "Microchemicai Laboratory Manual"; Wiley: New York, 1932; pp 36-38. This tube is similar in design to a K-gelrohr tuDe but was used in a venlcal pos~t8on We nave used both Pyrex (Fsher Sclentlf c. No 13-678-20D)and son qlass (VWR Sc!ent!ficfnc No 14672-3801plpels Most students find the laner to be easier to work in a gas-air microburner flame. Stills made of soft glass have a tendency to shatter, however, if a bead of glass is left on the tip after sealing in step (f). We are grateful to a referee for pointing out the utility of this apparatus for removing traces of stationary-phase contamination from high-boiling samples collected using preparative gas chromatography. There are, no doubt. other worthy applications that will come to light as the method is adopted more widely.
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Steps in
1012
constructing the Pasteur pipet microstill.
Journal of Chemical Education
