Macro versus Microlab: A Controlled Study of Time Efficiency Miles Pickering and Joseph E. LaPrade Princeton University. Princeton. NJ 08544 One of the most interesting recent innovations in laboratory teaching is the use of micro or semimicro synthetic procedures. Various dramatic claims for speed and efficiency have been made for this innovation ( I ) . However, few studies have been done that have provided concrete data that either supported or refuted these claims. Also there has been almost no use of micro-style experiments in large TArun labs. This paper reports a controlled educational experiment in which a Grignard reaction was run by two groups of students a t two different scales. The auestion. then. that this DaDer addresses is: how much does thk scale bf a student lagoratory synthesis affect the time taken to carrv i t out? And. secondlv. can a micro or semi micro experimentbe carried o"t in a TA-IU lab?
Elapsed Tlme lor Completlon ol Certain Steps Control group "Flames out" (assembly) First student adds benzophenone W i a n students adds benmphenone Median time to crude product (neubaiization with HCi, emact. dry e h r , evaporate ewer) Median time to finished product
Experimental group
47 min 110
23 min 52
137
87
222 330
142 204
tions.
Experimental
Two groups of about 25 sophomores in the second semester of Princeton's organic chemistry lab were used as subjects for this experiment. Both groups made triphenylmethan01 bv treatine nhenvlmaenesium bromide with benzoohef k o w e d a standard textbook (2). none. he con&l We oicked the Grienard reaction as a test case because of its virtually universal presence in organic lab curricula. The two groups were drawn randomly from one single afternoon, so were roughly matched for ability. A new ~rocedurefor the Grienard reaction was provided for the edperimental group. ath her than go t o the-expense and possible unforeseen problemsof true microlab work, the students worked a t about one-tenth scale compared to the standard textbook procedure. The reaction was run with about 350-400 mg of Mg in about 5-10 mL of ether in a 25mm-diameter test tube. The only significant procedural difference between the two groups was that the micro group used sonication to start the reaction ( 3 4 ) , since the introduction of impurities a t this scale is a serious Droblem. All student mixt;res started with 5-10 min of smication in ultrasound cleaning baths. Two baths were sufficient t o serve 30 students, and no problems (headaches, etc.) were reported hv the students. The test tube was corked with a C ~ Ctubeduring I~ the starting process, then equipped with a 10-mm-diameter test tube used as a cold finger. The written handout strongly encouraged the efficient use of time, by, for example, suggesting that the calculation and weighing of the benzophenone be done while the reaction was in progress. But other than this, no attempt was made to interfere with the natural Dace of the lab. I t proved difficult to keep track of the time when each student finished each step. Hence the data reported in the table are times when the class passed certain landmarks. The landmarks monitored include, for example, the time a t which all flames are extinguished and ether is first issued to the students. The elapsed time to this point (''flames out") is a measure of the time for assembly of apparatus, weighing out magnesium, and flaming dry. The next landmark was the quenching of the Grignard by addition of benzophenone. The time from "flame out" to this is a measure of sonication and reaction time. The time to crude product from quench time is a rough measure of the time efficiency of the extraction and solvent evaporation steps. Thus we have data not only on overall time hut on each block of laboratory opera-
Dlscuslon
There is a striking saving of time by use of a semimicro procedure. This time is saved a t all steps, but the saving is most marked in the assemblv o~erationsand during the factor of two faster.~he workup. The assembly time ;as reaction time itself was about 70% of that for the control group. The workup time was also much shortened. The experimental group was weighing pure product when the control group was just beginning to strip off the solvent ether from the extraction steps. I t might be objected that the sonication speeded the reaction abnormally, but time was saved in all parts of the procedure, not just during the reaction. We feel that simplicity of assembly is a major contributor to this time saving. The workup was much faster (108 versus 62 min), since there was much less solvent t o evaporate, and the warming and cooling required for recrystallization was muchfaster. The overall conclusion is that what used to take 3 h can now be done in two. The time saved can be used for extra experiments, characterization of products and other purposes. The amount of experimental work in the semester canbe significantly increased. The reader should avoid generalizing too far from these results. First, the Grignard reaction may not he typical. The need for scrupulous dryness adds to the assembly time atypicallv. Some ooerations. such as column chromatoeranhv. . .. may well be very much faster at the micro or semimicro scale and Home unaffected by scale. Second, it is possible that at least some of the time economy could be arhieved by careful streamlinine. of standard ~rocedurea.Third, our results may have been iistorted in some unknown wiy by the ~ a i thorne effect, the fact that change initself isoften beneficial. We have not examined in rigorous detail the trade-off between yield and speed. However, the experimental group produced enough material to characterize in all cases. Student calculated yield data suffer from so many uncertainties (wet . nroduct. miscalculation. etc.) that we are warvof takine i t seriously f& either experiken&l or control groups. I t may also be that a true micro lab would be significantly faster than the semimicro procedure tested here. We feel that any saving in time may well be offset by the difficulty of manipulating tiny amounts of material. At our scale one has attained 90% of the ultimate savings in air pollution, solvent cost, and toxicity, and there is no need to buy special glass-
a
.
-
Volume 63 Number 6 June 1986
535
ware. The optimum scale may well turn out to be somewhere between micro and macro. The choice of scale also means automatically that some techniques will be emphasized more than others, hut this raises philosophical problems beyond the scope of this paper. Certainly the last word on the question of optimal scale for student synthetic work has not been written. Acknowledgement The teaching assistants of the experimental sections, Joe
536
Journal of Chemical Education
Pant and Peter Yeh, made a number of useful suggestions, and also helped gather data. Literature Cited (11 Rawls, R.Ckm. E w . News 198462. (6). 20. (2) Pavia, D. L.; Lampman. G. M.: K&, G. 8. "Intzoduetion ta O~ganieLaboratmy Teehniguaa,'. 2nd ed.;Seuadeni: Philadelphia, 1962. (3) Lah,T.D.;Beny,D.J.Chem. Edue. 1985,62,85. (41 LU*.. J. L.;oamiano, J. C. JACS 19m.102.7926. (51 Sprich. J. D.:Lerandoa, G.S. Inorg. Chim. Acto 1983,76(4),L24l. (61 L-~,T.D.J.chsmE ~ U CL985.62.720. .
