Schiff Base Puzzle Project David Todd Pomona College Claremont, CA 91711 Most first-year organic chemistry laboratory courses start off witha sequence of technique and one-step preparation experiments, the details of which are laid out quite exolicitlv; There then follows. rather abruotlv. a se&ent devotedUtoqualitative organic analysis. fhe"eons:quent demand for a shifting of mental gears can be quite distressing to the student. In order to ease the transition between these segments of the course, it appeared sensible to put between &em an ex~erimentin which an unknown is to be identified. and in wkch a modest amount of strategy is needed. ~ h e e x ~ e r i ment devised for this purpose has now been field-tested on 87 students, and has been found to achieve its purpose (Note 1J. Each student picks an unknown substituted aniline (A), (one of five), and a substituted benzaldehyde (B), (also one of five,, produces the corresponding ~ c h i fbase f from them, and compares its melting point to those ofthe 25 possible Schiff bases. their structures and meltine ooints beine given. The reaction is carried out on a l-mmol scale (Note 2). The reaction is
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A second Schiffbase is then made, the choice of A and B being made so as to maximize the chance of f d y identifymg the fust Schiffbase. Usually this strategy suffices to solve the puzzle, but sometimes a third Schiffbase must be made to clinch the matter. It was found necessary to recrystallize each Schiff base to ensure that bv use of the usual k5 T rule the melting point can be relied upon.
the m.p. is found a decision is made whether to vary the A or the B component, and the second Schiff base is prepared.The problem can then usually be solved; in case an ambiguity remains, the preparation of a third Schiff base settles the matter (Note 3). It should be noted, as the matrix data shows, in some cases quite different melting points have been reported by different investieators. The reasons for these differences can be the subjct of useful class discussion, the syn-anti isomerism possible in the class being a point of departure. Should any reader care to have a copy of the handout our students received, I shall be glad to send it. Notes 1.There have been several papers appearingearlierinthis journal describing experiments in which students are given a wide choice of reagents to use in a general reaction. T w o of these are: "Knoevenagel Condensation to a-Phenylcimamonitrile",S. S. Kulp, J. Chem. Ednc. 1988,65,742; "An Aldol Condensation Experiment Using A Number of Aldehydes and Ketones", B. A. Hathaway, J. Chem. Educ. 1987, 64, 367. But in these experiments the student starts with known campounds -there is no ~uzzleumblem to be solved. 2. It should be pointed out to th= students that all of the materials used-in particular the aromatic amine-are potentially hazardous, and appropriatecare should be used in handling them. 3. Note that some of the ten reagents are solids at mom temperature, and a fairly obvious strategy that could be used is to isolate a reaeent and find its meltine mint. Of the 87 students who went through thisexpenment, not one thought ofthis pnis~hility!If the instructor cares to drop a hint in this regard. it should be accumpanled hy a warning that beeausenfsupcr-cooling,failure ofan oil to crystallize at O' C is not reliable evidence that the compound melts below O' C
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Schiff Base Structure Amines
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Experimental Nine 1M solutions in absolute ethanol, corresponding to the amines and aldehydes shown in the matrix below-note that the m-nitroaniline solution is 0.5 M because of its limited solubility--are made by the iustmctor, and each provided with a 6in. Pasteur pipet marked to deliver 1 mL of solution. These are labelled Al, A2...and B1, B2... for the aldehydes and bases, respectively. A dropping bottle of 1 M HCl is provided.) The student puts 1mL of an A and 1 mL of a B, and one drop of 1M HCl, in a 4-in. test tube. The tube is heated in a beaker of hot water on the steam bath for 5 min, then the solution is scratched as it is cooled in ice water. I t is sometimes necessary to add some water to the hot alcohol solu82-83;86 tion before a solid product can be obtained' The precipitate and aNumbers separated by a dash indicate the meiting range; numbers separated by a semicolon rec~stallized When represent values found by two different researchers.
584
Journal of Chemical Education
