Michael M. King New York University New York, New York 10003
A Practical Laboratory Examination for Organic Chemistry
Traditionally, introductory undergraduate lahoratory courses conclude each semester with a written final examination. Such an examination does not, however, lend itself in all instances to a realistic appraisal of the course content, particularly when the focus of that material is on lahoratory skills and techniques. Ideally, an examination should serve to ascertain the degree of learning that has taken place during the period covered by that examination. In the case of the laboratory course this learning is often the mastery of certain lahoratory and manipulative skills and the development of the necessary judgment required to know when and how to apply these skills and techniques. Unfortunately, simple written exams cannot serve adequately to evaluate this kind of learning. Without commenting on the merits of or objections to tests in general or final examinations in particular, i t was felt that sharing our experiences at New York University with a practical laboratory final examination for the first term of organic chemistry might prove useful to individuals who are presently dissatisfied with their own present lahoratory examinations. This description is also prompted by the recent appearance in this Journal of several reports2.3 of a related nature dealing with practical laboratory examinations in the areas of general and analytical chemistry. By way of introduction to the examination itself, it should he pointed out that the lahoratory program which has been developed for the first semester of our course focuses its attention on the teaching of common lahoratory techniques rather than on synthetic reactions. The semester's work concentrates on the introduction and illustration of the basic techniques used to purify and characterize organic suhstances as well as a number of different methods u s e d t o separate complex mixtures. Of course, wherever possihle the experiments are also chosen to illustrate the material discussed in the lecture hall. Efforts are also made to select substrates for these experiments which are familiar to the students (caffeine, plant pigments, etc.), although we recognize that the student need not know the structure of henzene to carry out its purification by distillation. A copy of the directions for the examination as relayed to the students in their syllabus follows. Examination Directions
structions you make for yourself. You are encouraged to he totally familiar with the appropriate chapters of your lahoratory manual. Grading: Your grade on the examination will he based on your lahoratory technique, the quality of product, and the purity of the sample (based on melting point). Discussion
Samples are chosen for this examination on the basis of the ease with which they can be recovered in high yield and in high purity following extraction and crystallization. Oftentimes the substrates are student preparations, retained from the previous year. It is also desirable that the suhstances have clearly defined solubility characteristics such that the search for a good solvent for crystallization can he rapid and provide an unmistakable choice. The list of solvents usually consists of methanol, ligroin (or petroleum ether), and chloroform. (By the nature of the examination the latter solvent would certainly be a poor choice and the differences in the former two are sufficiently great as to make the choice of solvent quite evident.) Benzoic acid and p-toluidine or p-anisidine have usually served quite well as the acidic and basic impurities, but the choice of impurity is really not critical. Liquid acids or bases might even he more interesting contaminants. We have found that a group of about seven different suhstrates can serve our six 30 student sections adequately. The examination fits nicely into a 3-hr lahoratory period, although the students do find it helpful (and time saving),to have cleaned their glassware during a previous period. Of course the nature of the examination is such that numerous adaptations, variations, and embellishments are ~ossibleto fit into other circumstances and time requirements. It is possible, for example, to further check thk purity of the product by thin layer chromatography, to isolate the acid or base, or to further characterize the product by some spectroscopic or chromatographic technique. It is important, however, not to rely on any single device or instrument which could possibly result in a delay or waiting period for the student. The evaluation of the student's performance on the examination must he a com~ositemade, consisting of both objective and suhjective ebaluati&s. It is relatively easy to quantify such factors as the purity of the prod. or made . uct; its appearance, the percentage of recovery, etcT Appropriate penalties can also he allotted for spilled samples or failure to complete the examination in the period provided. The suhjective evaluation is clearly more difficult. It is based on the opinions of the two graduate assistants who are normally responsible for the lahoratory section. They walk around the lab during the course of the exam assessing the student's technique and skill, while keeping in mind such questions as: was the work planned out properly (good preparation); were quantitative transfer techniques applied; was the proper procedure used to look for a good crystallization solvent; was the correct flask or funnel used a t the appropriate stage; were the various ~
Nature of the exam: The examination will consist of the purification of an unknown solid by extraction and crystallization. You will be given 1.5 g of a substance which is contaminated with 0.5 g of either an acidic or basic substance which you will have to remove by an appropriate extraction process. (You will he told whether your impurity is acidic or basic). All of the suhstances will he soluble in methylene chloride. After removal of the contaminate, you must further purify your product by an appropriate crystallization. You will he supplied with a list of possihle solvents, only one of which will he appropriate for your compound. You must decide which would he suitable for the recrystallization. Isolate the product, take its melting point, and turn it in to your lahoratory instructor. Work a t your own rate and turn in whatever you isolate. Texts: No texts will be allowed in the laboratory. You may, however, bring your lahoratory notebook, which will contain your previous observations concerning extractions and crystallizations plus any additional comments or in-
'Present address: Department of Chemistry, The George Washington University, Washington. D.C. 20006. ZHayes, D., J. CHEM. EDUC., 49,710 (1972). SHarris, W. E., and Kratochvil, B., J. CHEM. EDUC., 48, 543 (1971).
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pieces of glassware and equipment handled properly; etc. Later the two assistants . get toaether to discuss their oh. servations, examine the products, and assign some numerical and/or letter grade for each student's performance. Conclusion
We have found this practical laboratory examination to he a valuable teaching tool for our students. As there is no experiment exactly like this one in the semester's work, the exam requires them to examine each of the appropriately related experiments and then adopt a special procedure of their own for the exam. They are thus faced
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with the peed to evaluate the semester's work in terms of the kinds of techniques which they have learned and then to apply these techniques to a different situation. A "cookhook" approach to this experiment will simply not work. The student must develop his own procedure, decide for himself on the quantities of solvents and solutions, determine how many extractions of which solution will he necessary, choose the correct solvent to use for crystallization, and decide how much, etc. In short, the exam focuses on much of the semester's work and provides for an assessment of how well the student was ahle to learn that material. This after all is what a test should he used for and this is in fact what we have been ahle to do.
