Chemical Education Today
Letters Design of Experiments
The author replies:
The undergraduate lab simulation on combinatorial chemistry describing how students run an esterification under unique conditions (1) gives me hope that someday I’ll convince educators to simulate the way industry seeks optimal synthesis conditions. There the procedure is to construct a response surface in which yield, for example, is the dependent parameter to be optimized and all the significant reaction variables are independent variables. Any number of computer programs can handle such a multidimensional response surface, but the principle can be demonstrated by using just two independent variables, for example reaction time and temperature. What I propose is that each student run the chosen reaction at a unique combination of time and temperature and then each student’s yield be plugged into a multiple regression program, including interaction terms, of course. Not only will preferred operating conditions be revealed, but also the concept of interaction and the fallacy of onevariable-at-a-time experimental design will become obvious. If one wants to embellish the experience, one can recover starting material and teach the difference between yield and conversion. One can use the data thus developed to grade the students by examining how far from the value predicted by the response surface each student’s yield falls. Variations on this theme could replace time and temperature with such continuous variables as reactant ratio, concentration, rate of addition, and agitator speed. One can also use discrete variables such as order of addition, solvent type, and workup method. Furthermore, one can use statistical design in studying purification methods where purity, by melting point for example, is used as the criterion of goodness. Instead of suggesting specific experiments (I’ve been out of university too long to try that) I’d like to challenge teachers to try their hands at doing so and to share their experiences in these pages.
Dr. Luberoff ’s letter serves as a challenge to those of us in academia to continue to make the laboratory experiments relevant to our students. Personally, I will try to keep this in mind; however, I see several major challenges to the implementation of Luberoff’s proposal. The first is finding a reaction whose rate is appropriate at accessible temperatures to give reasonable variations in yield during a single lab period. The esterification reaction we used is not appropriate in this regard. The second is controlling the variables with sufficient precision and accuracy. For example, small changes in temperature can lead to large changes in rate; I would not want this to be overly challenging from a technical perspective to the students. Additionally, it is a challenge to design and implement an analytical scheme that would allow determination of yield, conversion, and purity by several hundred students. Finally, there is an emotional–educational problem. It is potentially frustrating for students who will have poor yields because of where they happen to fall on the response curve described by Luberoff. We have designed our esterification with several additional surprise esters that also have easily recognizable odors. With these caveats, the goals described by Luberoff are well worth the effort that would be required to implement this experimental design. The organic chemistry laboratory would be a good place in the curriculum to introduce it. I look forward to seeing someone rise to this challenge put before us. David M. Birney Department of Chemistry and Biochemistry Texas Tech University Lubbock, TX 79409-1061
[email protected] Literature Cited 1. Birney, D.; Starnes, S. J. Chem. Educ. 1999, 76, 1560. Benjamin J. Luberoff 261 Fearrington Post Pittsboro, NC 27312-8556
[email protected] JChemEd.chem.wisc.edu • Vol. 77 No. 12 December 2000 • Journal of Chemical Education
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