Mechanisms in Class, Mechanisms in Lab

I have always used a mechanistic approach when teaching organic chemistry. Every class I have taught, I started the first day saying, “Do you want to try to memorize hundreds, if not thousands, of individual reactions, or do you want to learn to understand how about ten reactions take place, so you can apply them to hundreds, if not thousands, of situations?” Inevitably, students always respond with the latter. Unfortunately, the way traditionally-organized texts are written encourages students to neglect their own preference and attempt to memorize reactions rather than understand them. Last year, I switched to Karty’s text because it laid out the concepts of organic chemistry the way I was already attempting to force another textbook to lay them out. The only challenge was organizing my lab to parallel this teaching approach.

My institution uses a two-semester lab and I try to mirror the lab experience with the lecture portion of the course. Many traditional lab courses use a “synthetic approach” to learning organic chemistry. The students mix A and B to create C which they then isolate and run a couple of chemical, physical, and spectrometric tests on to verify the identity of their product. I found that my students were so focused on “Did I get the right thing?” that they were missing the most important part of the lab: why A and B produce C. And labs typically lack experiments that emphasize understanding mechanisms—primarily because most mechanistically heavy experiments are more complex and time consuming than a three-hour lab period can allow. When you teach a mechanistically organized course you spend a lot of time (rightfully) on understanding the nature of molecules, electrons, and bonds; you don’t jump straight into reactions. So how do you have a lab experience that promotes teaching a mechanistically organized course? There are a couple of things that have worked for me.

I still spend several weeks on seemingly simple experiments such as distillation (multiple types, multiple scales), recrystallization, and liquid-liquid extraction to teach the students basic techniques they will use later on. But now I use these labs to reinforce the concepts of intermolecular forces, which are emphasized in Chapter 2 (Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties). I specifically tailor the experiments and the writing assignments associated with these labs to ensure that connections are made between the lab and lecture. The way physical properties of compounds and solvent-solute interactions are emphasized early really helps later on when we start thinking about how reaction conditions affect reaction mechanisms. Each of these experiments is coupled with an instrumental technique that is based on similar concepts—for example, using gas chromatography to reinforce the principles of distillation.

I also like to get to spectroscopy early. Therefore, I find myself jumping from Chapter 5 (Isomerism 2) to Chapters 15 (Structure Determination 1) and 16 (Structure Determination 2). I find that when students understand spectroscopy prior to learning reaction mechanisms, they better understand why certain reactions follow certain mechanisms. And because we have just covered molecular orbital theory in Chapter 3 (Orbital Interactions 1), why molecules behave the way they do, spectroscopically, is fresh in their minds.

The advantage of teaching spectroscopy early has benefits that I have seen throughout the rest of the course. It allows us to mirror all lab experiments with the chapters we are covering in greater detail, so that when we get to reaction conditions that promote either SN1 or SN2, students can carry out an experiment using a series of electrophiles under SN1 and SN2 conditions, and note the difference in reaction rates. This helps to reinforce the importance of a unimolecular rate determining step versus a bimolecular rate determining step. But because they have already learned to acquire and interpret spectra, they can also isolate and characterize the products. They gain a much greater appreciation for carbocation rearrangement when they run the NMR of their SN1 product and find that their secondary alkyl halide has now become a tertiary alcohol! For many of my students, the concept of mechanistic difference or how a chemical reaction occurs becomes “real” to them when they see it firsthand.

For me, switching to a mechanistically organized textbook made sense. Under this new text, I wanted my lab series to accomplish two things: give students a systematic approach to learning how to conduct lab work and give them experiences which reinforce an atmosphere of learning to understand why things happen rather than memorizing. In many ways this meant creating labs at the beginning of the course which focus not only on technique but exemplify the concepts of mechanistic based learning.

-Nathan Duncan, Maryville College

Click here to learn more about Prof. Duncan

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