The Puzzle-Oriented Organic Laboratory Miles Pickering Princeton University. Princeton, NJ 08544 Some vears aeo. I nuhlished an opinion essav on the aDparent degenerafion iforganic lab into mere Eookbooking ( I ) . The article advocated that organic lab experiments he ;sed to solve a problem in org& chemistry, not to do synthesis only. My article seemed t o inflame organic chemists from Maine to Vladivostok, and basically the responses could be summarized as, "if you're sosmart, d o better!" This article is therefore a report t o the community of teaching organic chemists on an attempt t o create a new style of organic laboratory experience involving thought as well as recipes. I should point out that I am not the first to have this dream. In the heady days of the sixties, one Wilmer Fife published an article (2) on the "nrohlem oriented lab". but his was a small organic lab, in a good small college, and the article hinted a t deep problems. Since no sequels appeared. one assumes that his-efforts were short-lived. ~ o w a d a y s , because of new small scale technology and widespread availability of instruments, Fife's idea might now be feasible, in a way that i t was not in his day. In particular, the speed of small-scale work opens whole new vistas for lab design. The challenge is not the majors lab, hut the large introductory service course with a large clientele of sophomores of marginal skill, m w t of which will not major in chemistry. Twicallv also such courses are TA-run. and TA aualitv . " and d&ee of commitment varies greatly. There is also a ~ h i l o s o ~ h i cauestion al here for laboratorv , w d d like our teaching labs to mi& education. ~ d e a l l iwe the process of scientific discovery. Thus, the object of lab is to create a situation where the student can get the answer to aquestion as the result of a set of laboratory operations. In a measurement lab, the answer to the question put to nature is simply the number being sought. But organic chemistry is typicalofmorequalitativeditlciplines,and there is realdifficulty in extending this model of laboratory education from the auantitative to theoualitative. The resultsol thiscurriculum project give real hope that students can use the lab in an intellectuallv" sienificant wav. - . even if thev are not eneaeed in pure measurement.
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The Laboratory Environment The main oreanic lab a t Princeton registers between 150 and 200 sophokores, meets for 3 h a week, and the TAI student ratio is fairly good, about one TA for every 15 students on a given afternoon. The lab can handle up to 60 students on an afternoon, has only seven hoods, and a modest assortment of instrumentation (a Perkin-Elmer R24 NMR and several 1310 IR's). Thus, in many ways it is avery typical beginning organic lab course. The curriculum to be described was created by the author, designed to parallel the lecture as much as possible. All experiments were semiprojects, lasting as long as three weeks, both to add depth and to keep the burden of report writing and grading under control. Every single unknown, everv derivative, and everv reaction was personally tested hv the author to minimize the number of unpleasani surprises. Theimmenseamount of time that is required for this kindof exhaustive testing and documentation is almost certainly the major reason why such an idea has not been previously attempted. However, i t became rapidly clear that if X 232
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Table 1. Organic Currlculum Fall Term 1. l ~ ~ l a t i o ofnan unknown acid from e headache plll. Acidlbase chemistry. Recrystalliratlon from hot water, malting point. 2. - Canniram reaction on unknown aldehvde. Extraction. Recrvstalllzatlan 01 acia. melting point to characterize. Alcono ident I sd by IR 3 Unknown a canol idernlfieo oy convers8on lo arena. ootn producl and reactant identifled oy IR. Dlstil at on. 4. Hydrobarationof 1-methyl-1-cyclohexene. norbomene, lndene to determine steric course of reaction (4). 5. Carbene addirion to 1.6cydooctadlene. Reflux. Adapted from William~~
son (9). Spring Term 1. Aromatic substitution. Position of substlMion determined In acetyiation of 2-chlorotoluene, methoxy substitution of 3,441chloronitrobenm
(3). 2. Did-Alder reaction. Exo or endo determined far m l e i c anhydride addition for furan. 1.3-cyclohexadisne (5). 3. Cycimddltian of 19dlpoles. Addition of 1.3dipole from benzaldoxime to stilbene, indene, styrene 10 determine stereospecificity and regiospeCificity (6). 4. Aldol c a n d e n ~ t i o nof unknown ketone and unknown aldehyde. Aldol product ~haracterlzedby NMR. Derivative made of aldehyde and ketone, IR of aldehyde for Identification.Extended t o m Hathaway (IO).
worked t o give Y, methyl X might not give methyl Y, or only if there were subtle chanees made in the procedure to account for things like solubility differences. ' The final curriculum is shown in Table 1.This is only one of many possible sequences of experiments, and many others would do as well. I t is the style, not the sequence, of experiments that matters. How This Currlculum Is Dmerent The Univeml PuPurr8 Approach There are no experiments in the course that are purely preparative or exist solely to demonstrate a technique. Instead all experiments in this course are puzzles. The puzzles can be of two sorts. The fist sort of puzzle is where the starting material is known, hut the reaction can plausihlv yield A, B, or C, or a mixture The student runs thk reaction, works out the product, infers something about the mechanism. This seems to m e t o he real organiE chemistry, in distinction to the cookbook prep strategy. Most of the mechanistic puzzles used have been invented by this group (3-6) and either have appeared or will shortly appear in this Journal. The other sort of problem is akin to the unknowns beloved of advanced organic qualitative analysis labs but rarely used in mainstream labs. a t least not until the verv end. A known reaction takes an uiknown to a characterizable product from which one can work backwards, The procedures used have only slight latitude for students. We have found that even the best students will anxiously query the TA if there is the slightest ambiguity left in the procedure. Such techniques as recrystallization, which requires several judgements, are particulary difficult for this kind of student. So the procedures are complete, and the few branch points clearly delineated by the language.
The use of canned procedures may seem a regression to the cookbook, hut in the end even established scientists do not work out every procedure from scratch every time. It seems t o us that the job of interpreting the data is hard enough without adding the devising of procedures to this task. Efficient Use of Student Lab Time
One resource that is rarely used well is the student's lab time, and much improvement is pmsible. One of the unused resources is the week-long periods between the lab sessions. Reflux time is a particularly inefficient use of time. One or two reflux experiments are scheduled every year to illustrate the technique, but when you have seen one reflux, you have seen them all. A wide variety of organic reactions will run to completion a t room temperature if the reagents are mixed in a test tube and allowed to stand for a week in a student desk. In this wav several reactions can run at the same time. making co&parisons easy. Often the products are purer, and a great deal of work and time is saved. T o run a reaction under reflux is to use up several hours, if set-up time, warming time, cooling time, and cleaning up time are added to the actual reflux period itself. ~ e f l u x w a s t e swater and energy and is incredibly boring for students. Another slow step that can he done in student lockers is slow evaporation steps. As much as 30 mL of methylene chloride will evaDorate in a week without anv nroblem. This eliminates anotf;er source of laboratory boiLdom. For the fairly small scale reactions now typical of organic lab, desk drawer evaporation is practical for a t least some solutions. Naturally this procedure should not he used with particularly toxic, flammable, or malodorous solvents, and there has to be an adequate airflow through - the laboratory to avoid accumulation of vapors. Efficient use of lab time means that more reactions can be run, more puzzles posed, and the intellectual intensityof the lab vastly increased. Purity, Not Yield
The near universal emnhasis on vield seems to me to be misplaced. While imporiant for career chemists, in the hands of beeinners it deeenerates into the habit of leavine the boiling :hips in the product, turning in wet or impure material, or outright falsification of data. We encourage product quality in two ways. First, to get a good answer to puzzles requires some skill a t recrystallization. Second, we have undertaken five rigorous "product quality checks". Students turn in labeled vials a t the stockroom, where thestaff checks them off on a list. The samplesare then sorted into four categories on the basis of appearance by the instructor. and a few selected meltine ooints are taken at random to see'that students are in fact tt&ing in the material they claim. For 150 students, this process takes perhaps an hour of instructor time, as the actual recording of grades is delegated to an assistant. The central collection point eliminates the missing sample and facilitates waste disposal. The results of the quality checks improved strikingly as the year went on, and t h e students &ned more skili, as shown in Table 2. The emphasis on purity made students much more conscious of what their products looked like and led to several discussions about crystals and crystalline shapes. The yield also improved, since students came to realize that good yield meant having enough material to turn in good samples. As skill goes up, so will yield, and the emphasis on i t is misplaced for beginners. Grading To Emphasize the important Things
The grading strategies used in this course are taken from those previously teated in freshman chemistry, with little
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There is one lab exam, to which students may bring their notebook and reports as references. The exam provides a
Table 2. Success 01 Recwstalllzatlon % of Studenb Getting Full Credit Fall Term Week 8 (Exp. 4) Week 10(Exp. 5) Spring Term Week 3 (Exp. 1) Week 6 (Exp. 2) Week 9 IExo. 31
uniform grading standard across sections and, moreover, has significant educational value since it forces the student to ask why the workups work, why particular techniques are used, and in general to pull the material together. Puttine the eradine emnhasis on the lab exam has another - - ~ advantage. I t Frees t i e ~k to root for the student, t o be an elder sibline fieure instead of a stern parent. Oreanic lab is much less gainful if the TA is an a l l i a n d resouke person, not an overseer. The reports are graded by TA's, hut a strong emphasis is placed on empirical thinking; that is, does the conclusion match the data? This is a difficult point for TA's, because it is so different from the get-it-right-or-else strategies used when they were trained. There is a continuing need to educate TA's because often they insist that all data must match mechanistic paradigms, and cannot cope with refutation by real data from nature. In many ways this is one of the most difficult conrrtraints on this style of laboratory education. Students are encouraged to resuhmit their reports with corrections (of reasonine. not of fact). a svstem modelled on the process by which papers are accegtedfor scientific journals. Since no credit is given for "right" answers, there is little incentive t o try to defeat the problem and find out the answer from other than experimental sources. At the end, an instructor-TA conference is held, and it becomes clear who bas made an honest attempt and who has not. A lump of credit is then assigned on an all-or-none basis. This system has been previously validated for freshman chemistry and seems to work well in organic as well.
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Scale Choice
The scale should he chosen to make the lab safe and as easy and as inexoensive as oossible. We feel that runnine reakions in t e s t tubes in the 300-mg-to-1-g range to be approximatelv right; and onlv in one case. the distillation. do &exceed this scale. True microscale requires the teaching of a lot of technique to students, many of whom will never use i t again. Worse yet, i t focuses both student and professor on questions of technique, instead of the molecular questions that are the real business of chemistry. The advantages of microscale, speed and economy, are almost as large a t t h e 0.5-g scale as a t the 0.1-g scale, and for most reasonable materials toxicity does not become a problem a t this scale. Yet the level of frustration and the number of restarted reactions seem reduced. TA Training and Organization
We work on TA preparation in several ways. First, TA's are expected to keep a teaching notebook, to summarize the procedures in these notebooks, and at TA meetings are picked by lot to explain the experiment, working from their notes. This is a simple system to keep them current without a lot of wasted time (8).Special techniques required by the test-tube scale are explained in depth once a t the beginning of the fall term. as is the course ohilosonhv. Our TA's norkally run two 3-h labs perweek. T o ease the suoervisow burden, we have selected certain TA's to have oniy one section (on the first day of the lab cycle) and to Volume 68 Number 3 March 1991
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serve as "officer of the day" for another afternoon later in the week. This means that there is always an experienced TA for the student to appeal to and an extra person to get help, solve problems, and otherwise be the lab instructor's proxy. Since we have a significantly more difficult curriculum, it seems only appropriate to offer additional help. Concluslon The puzzle idea worked quite well here a t Princeton. The appeal shifts toward the more able students, the potential majors, but i t is not beyond the ordinary mainstream student. Whether this idea isgenerally applicable is not clear a t the moment, and danger lies in extrapolation to other colleges and other student populations. It is, however, enormouslv difficult and time consuming to set up and test such a c u r r h u m . At this stage, doing it this way is a tour de force, rather than a routine practicality. Eventually enough nneale emeriments will so it will ~ ~ - accumulate ~ - in the ~ literature ~ ~ become as easy and straightforward to teach in this way as in anv other. But not tomorrow or the dav after! - -'?he desirability of the strategy is convincing, however, because the sort of intellectual inauirv in this lab is the kind that real organic chemists engage ii if they are about the technician level. Our avvroach diminishes boredom and, as Fife writes, "gives meaning to all the operations that the student does in the lab." I t is the absence of this "meaning" ~~
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that makes the students feel that the traditional lab is cookbook and pointless. The organic teaching lab cannot forever ignore the mechanisticrevilution that has swept organic chemistry. Sooner or later we must once again make our teaching labs exercises in discovery, practice in arguing from evidence, and training in reconciliation of ambiguous data. Such skills are not only essential for scientists and . nhvsicians. " . thev" are the verv stuff of intellectual maturity itself. Acknowledgment I am indebted for innumerable helvful discussions with graduate students Alex Muehldorf, jim Geiger, and Dan Melamed. Collaborators Joe LaPrade, David Todd, and Henry Gingrich have contributed important ideas to the program. I am also grateful to Maitland Jones for his moral support of this project. Llteraiure Clted 1. Pickering, M. J. Cham.Educ. 1988,65.143. 2. Fife. W. J. Chsm.Edur. 1968.45, 416. 3. Todd. D.;Pickering. M.J. Chrm.Educ. 1988.65, 11W. I . Pickering, M.J. Chem.Edur., 1990.67.436. 5. Pickerins. M.J. Chem. Educ. 1990.67.524, 6. Gingrieh, H. L.: PLkering, M. J. Chcm.Educ., in press. 7. Mnnts.D. L.;Pickoring. M.J. Chrm.Educ. 1981,58,43 8. Pickerin& M. J. Call. Sci. Tchng. 1988. 17.385. 9. Williamson. K. L. Marroaeale and M i c r a s c o l ~Orgonir Experimenb: Heath: Lexington.MA. 1989. 10. Hathaway. 8.A. J. Chem.Educ. 1988,65,367
