A surprise in an organic experiment - Journal of Chemical Education

Crystal growth during the optical resolution of D,L-alpha-phenylethylamine by L-tartaric acid. Keywords (Audience):. Upper-Division Undergraduate. Key...
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A Surprise in an Organic Experiment Surprise outcomes in the organic laboratory sharpen students' observational skills and augment their ability to hypothesize, reason, and test. We find such a situation arises, under some conditions, in the course of the optical resolution of D,L-a-phenylethylamine(I-phenylethanamine) by L-tartaric acid, a popular organic ex~eriment.'~ Following uneventful liberation and purification (distillation) of the resolved (S)-(-)amine, students determine the specific rotation on neat samples. We have noticed that, on occasion, a crystalline material, consisting of soft white needles, deposits on the surface of samples exposed to the air. This is especially evident on loading a standard wide-mouth glass polarimeter tube. At times such crystallization has severely inhibited obtaining optical rotations. Not all student samples exhibit this behavior. In particular, long, fibrous crystals develop in direct relation to the surface area of the sample exposed. On a watch glass crystal formation can he very rapid. As an adjunct to the experiment we have had students isolate this compound. It melts at 93-95 'C and exhibits infrared absorption (CsI matrix) at 1345cm-', with multiplet peaks at 1445,1475,and 1485em-', and weak hands at 1570 and 1620 em-'; evidently the 1-phenylethanammanium carhamate is f o ~ m e d . ~ In our laboratory this experience facilitated a spontaneous and animated discussion concerning organic chemistry as an experimental science. This observation is not one expected from our manual and isnot mentioned in any of the standard texts. Studentsspeculated on what they "did" to induce crystallization to occur. Investigation of the phenomena led them to ask whether such crystals are also formed upon aerial exposure of racemic amine (they are) and to consider what evidence would he necessary to assign a structure, complete with stereochemistry. It prompted discussion of acid-base behavior, reversihility, polarity, nucleophilicity, acid gas scrubbing by amines, and, in an investigational setting, the relation between experimental procedure, technique, and outcome. We helieve this surprise occurrence in an otherwise classic experiment offers an excellent opportunity for students (and teachers) to engage in creative inquiry. 'Roberts, R. M.; Gilbert, J. C.; Rodewald, L. B.; Wingrove, A. S. Modern Experimental Organic Chemistry, 4th ed.; Saunders: New York, 1985; p 184. =Pavia. D. L.: Lamnman. G. M.: Kriz. G. S. Introduction to Owanie Laboratory Techniques-A Contemporary ~ ~ p r o a c3rd h , ed.; ~a&der;: New York, 1'988; p 496. 3 Auk, A. Techniques and Experiments for Organic Chemistry, 5th ed.; Allyn and Bacon: Boston, 1987; p 339. Ault, A. J.Chem. Educ. 1965,42,269. The 2:l stoichiometry isconfirmed by elemental analysis:C,H, N (CalcFd) C (71.30:71.50), H (7.75:7.67),N (9.78:9.72). Alan M. Rosan Drew University Madison. NJ 07940


Journal of Chemical Education