Spectroscopy: Seeing (and Using) the Big Picture

Like many other instructors, I do the majority of spectroscopy instruction in my laboratory. It seems natural to integrate spectroscopy problems into lab exercises, and to use the molecules we make as the platform for understanding how to analyze them. Most organic texts I have seen introduce spectroscopy towards the end of the first semester (often after alkynes and before dienes). These chapters are somewhat self-contained, with an overview of theory and simple functional groups as examples. These chapters lend themselves to lab fairly easily, except for one major drawback: the simple functional group examples are….simple. Following the functional-group approach, each subsequent chapter highlights a functional group (ketone, carboxylic acid, etc.) with an introduction of physical and spectroscopic properties. While this was fine for reinforcing spectroscopy in lecture from lab, it was less than useful for laboratory. I want my laboratory to work alongside my lecture, and I don’t want to hold off on a reaction just because we haven’t officially learned how to analyze that type of product yet!

In his preface, Joel explains that his chapters on spectroscopy (Chapters 15 and 16) are self-contained and can be taught earlier, depending on instructor wishes. I really appreciated this approach even before I read the chapters, since it treats spectroscopy as I feel it ought to be treated. Spectroscopy methods are the tools by which we can “see” molecules, powerful tools in their own right, and not simply a topic to introduce briefly and then tag-on as an afterthought to a discussion of different functional groups. These chapters cover all the functional groups, making them rich with real examples of complex compounds that the students are actually able to analyze.

I started my first lab with a one hour lecture on IR (15.4-15.6) and 13C NMR (16.1-16.2, 16.13) spectroscopy that I made clear was closely aligned with the text. Some of this lecture was a modest review, since our general chemistry curriculum uses an instrument-heavy laboratory approach. Enough gets forgotten over the summer, though, that a review is very worthwhile! We then worked through problems in the text, of which there were many that were useful at multiple levels of difficulty. The simple problems (such as “determine how many signals should appear in the 13C NMR spectrum”) serve two useful purposes: they not only helped reinforce the spectroscopy that the students had just learned, but they also set the stage nicely for chirality later on in the semester. Many texts take a similar approach, but I enjoyed Joel’s examples more than any I had used before, since there was such a variety of functional groups in the examples—no more endless alkyl halides, alcohols, and alkenes! I also appreciated the more complex problems that let students refine their problem solving skills in assigning values to spectra of a diverse mix of real-life molecules.

I introduced proton NMR (16.3-16.12) about three weeks into the first semester, when we were getting ready to analyze products from the acid/basic/neutral extractions. It was great to have text and problems that tied in with what we were doing in an early lab. Mass spectrometry and UV-Vis came later in the first semester, so that I could address carbocation stability and conjugation at the same time we were analyzing our lycopene through UV-Vis and GC-MS.

Treating spectroscopy as a stand-alone unit did not minimize its use in my course, it maximized it, giving me the freedom to use it as the valuable tool that it is to determine structure and address the fundamental questions of the course: What do these molecules look like and do?

-Michelle Boucher, Utica College

Click here to learn more about Prof. Boucher

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