edited by
ELLEN and JAY BARDOLE
Computer Simulation in Organic Chemistry -Thin Layer Chromatography Douglas Bond Riverside City College 4800 Magnolia Auenue Riverside, California 92506 There are a number of laboratory techniques in organic chemistrv which are taueht to students a t the introductorv level. ~ & t e r yof these teihiques involves two separate levels; (a) nsvchomotor-in which the students learn to handle the equipment and perform the operations; and (h) cognitive-in which they learn to make the decisions which will optimize the outcome of the experiment. Although most lab manuals do a good job in discussing the cognitive aspects of thin-layer chromatography, the experiments provided do not reinforce this learning. Typically, the student isolates some mixture from a commercial or natural source (leaf pigments are favorites), spots the slide, and develops it in a solvent system that is provided in the experiment directions. When they are finished, the students have some practical experience, hut if they were turned loose, would they he able to come up with a solvent system on their own? When I was a brand new instructor I decided that I would produce only first-class organic chemists. Therefore, I desecond to termined to eive the students a learnine- exnerience . none. The assignment consisted uf a two-component and a three-comuonent mixture for umhich thev had to find a suirnble solvent system that would develop the-mixture into identifiable spots. Then the components had to be identified using solutions of the known compounds for comparison. The experience was totally frustrating for the students. The best students were able to finish the experiment in the three lab periods allotted, the rest muddled into the time for the following experiment. I omitted the three component mixture, cut the time to two lab periods. It was still insufficient. The problems with my design were easy to spot. Students were havingdifficulty making the series of decisions that goes with doing tlc in the lab. The same questions were occuring over and over again: ?
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the cognitive decisions and immediately follow with the results of that decision, the learning could he speeded up. From the nucleus of this idea, I have written the computer program, TLC. The program mimics the laboratory experience, in that the students receive an unknown and-att&npt to find a suitable solvent for development. The studenrn make all the decisions that thev would make in the laboracorv. u,ith the added advantage that they receive either positive or negative reinforcement immediately after the decision has been made. If a wrong decision has been made, the student knows immediately and is told what helshe should have done. Proeram execution looks like this: The comDuter asks for the &dent number and when it is located on the file, assigns one of 23 possible unknowns to the student. They are given directions and a list of solvents, and asked to choose an initial A.
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When this is done, the system responds with the display, shown in the figure, which contains the followine information 1) The tlc slide exactly as it would look if the experiment had been dona in the lah -.-~. ~~.~.. --. 2) The list of solventa 3) The solvent code for three previous solvents, the name and code for the solvent used to develop the slide shown to the left, and 4) A question: Is your Ri acceptable?
The student must now make hisher decision: Yes or No. Based on that, the program follows one of iwo branches de. nendina on whether he,she is riaht or wrone. If the student Is right; helshe is given a new unknown if tlhe RI was satisfactory or asked to choose a new solvent if it is not. 1f the student is wrong, helshe will get some kind of a prescriptive message. If the Rt value was acce~tahle,the computer points this out and passes the student on to the next unknown. If it wasn't, there are several messages that the student will encounter depending on the number of errors that have been made.
Which solvent should I use first?
Are these Rf values OK?
The R, on my last slide is too low (high), what solvent do I try now? What solvent is less polar than chloroform,etc? As I ran around the lab from one student to the other, I began to see why the authors of many lab manuals provide canned directions. My resolve began to weaken. There was no problem with the psychomotor activities. Students became adept a t spotting, developing, and visualizing the slide in a very short time. It was the cognitive decisions that were causing difficulties. If a computer simulation program could he written that would allow students to make Volume 55,Number 5,May 1978
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There are eieht (8) . . different solvents which can he used. and any of these eight may he comhined with one of the others to form a solvent pair. Two solvents may he comhined in either the ratio 1:2 o r 2 1 making a total of % comhinations. Singly or in combination, there are 64 different solvents which mav he attempted. The 23 unknowns are real substances. Although the name of the unknown is never given to the student, it is stored in the computer file with the Rf data for that compound. We ran each of these 23 compounds under all 64 of the solvent conditions. Considering the unknowns which were examined and rejected and running three substances to a slide, as well as errors, it was necessary to run over 600 slides to accumulate the data base upon which the program operates. Students must complete a t least four of these unknowns, the last two usine four solvents or less and without accessine the help f e a t u r e . ~ h epurpose of limiting the numher of soc vents on the last two is to force them t o use some thought - in the selection of the solvent. When a student finishes the unknowns, helshe is moved onto mixtures. At the present, they only do two component mixtures, hut since some of them are difficult, the challenge is still very real. There are 191different mixtures, each made un of two of the original 23 unknowns. There are ohviouslv many more possibilities, hut these represent those that cah be separated by one or more solvent systems. The requirement for completing mixtures is simply to finish two. Thereareno restrictions as to minimum numher of solvents, or use of the help feature. When these two are done, the system will tell himlher that helshe is done, and then ask if helshe wants to do more just for the practice. About 45% do additional mixtures. The h e l section ~ is desiened for flounderine students. We point out before the computer assignment thgt this section is available and to use it if thev are stuck. For the student who is bewildered a t the beginning, who can't even make that first choice of solvent, the computer will make an initial suggestion. Farther down the line, help is provided in the form of prescrintive messaaes or more comnliCated suaeestions. These suggestions are-hased on the p&arity of the student's last choice, the resulting Rr value and the remaining untried solvents. When the studeit is working with an unknbwn mixture, there are a numher of branches that can he taken each of which depends on the outcome of each of the components of the mixture. The time required for completion of the program varies from 17 to 67 with an average of 37 minutes. There is no requirement for students to finish the program in one sitting, since they can come hack and pick up where they left off at anvtime. The records of student nerfomance are k e ~on t the diik files and include the follo
