One of the most important experiments students conduct in the second semester of organic chemistry laboratory, CHM 2211L, is the structural analysis of an unknown compound. This assignment represents the final project in which students are able to put together many if not all the techniques they have learned in two consecutive semesters of organic lab.
Accordingly, students are given an unknown compound, a liquid or solid, whose structure they are to determine using a combination of spectroscopic and chemical tests. In terms of student performance, this experiment is critical because it accounts for approximately 15% of the final grade. Because of the importance and difficulty of the experiment, students are allowed three laboratory sessions, or about 10.5 hours, to complete their analysis and propose a structure for the unknown.
In the current practice , simple solubility tests are conducted to determine whether the unknown is an acid or a base, a polar or a non-polar molecule. Furthermore, students are provided with a pre-recorded infrared spectrum of their unknown. From the IR spectrum, the main functional group of the unknown can be established which determines the reactivity of the molecule. Finally, a series of standard chemical reactions is undertaken to prepare solid derivatives from which the exact chemical structure can be established.
It should be emphasized that the infrared spectrum is the single most important variable that determines student success in this experiment. If the actual functional group is not diagnosed correctly, students can waste valuable time preparing derivatives that correspond to a different molecule than that present in their unknown!
Experimental design and implementation. The fundamental factor in creating experiments for the undergraduate organic lab concerns the varying skills of the students. Therefore, each experiment must be carefully tested to yield reproducible results. Furthermore, when hands-on implementation using costly instrumentation is involved , care is necessary to create a method that is straightforward and involves a minimum number of steps. By contrast to the current practice to hand out a pre-recorded spectrum, students learn to use a small, freshly distilled, sample of their unknown to obtain the corresponding IR spectrum. Moreover, after digitizing the recorded spectra they use Microsoft Excel to process the data.
Before recording the FTIR spectra, all unknown compounds must be purified by simple distillation. The latter is a very time-consuming, tedious, but necessary step. For example, depending on the unknown's boiling point, each distillation may take from 30 to 45 minutes. In some cases, more than one distillation is necessary to completely purify the compound such that a useful IR spectrum is obtained!
There are about 50 different organic compounds from which a student unknown may be chosen. Hence, the first step of the project was to create a library of FTIR spectra using highly purified, distilled samples against which student spectra may be compared. These reference spectra are stored permanently in the FTIR's computer and are accessible to instructors to assess the initial purity of the student’s sample before they undertake chemical analysis of the unknown.
Assessment and Evaluation. In order to validate the analytical method, the IR library was compared against published spectra in the chemical literature. The spectrum of each compound was found to be the same within experimental error to that reported in the literature. Therefore all of the standard reference compounds were successfully purified and the compiled IR data are free of impurities or contaminants that can lead to incorrect functional group assignments when students record spectra using their own samples.