When teaching mechanisms, I try to impress upon my students that the concepts tied to mechanisms are not confined to the chapters that they appear in within the Karty text, but rather, that they are a continuation of connected topics across the discipline as a whole.
Today’s class focused on Sections 13.1-13.3, which elaborate on the art of synthesis. I spent the entire 50-minute class period showing my students how to write a reaction, catalog a reaction, and think backwards from a product (retrosynthesis).
What I like about Chapter 13, in particular, is that it takes the components of the previous chapters and encourages students to look at organic chemistry as a unified story. Oftentimes, I view a reactionary step as a singular plot element, whereas I view a mechanism as the complete description of the story from start to finish. This is because the mechanism shows every step of the process: from the starting material, to the intermediates, and to the end goal of an isolated product.
To illustrate my analogy above, because I’m fond of the Lavoisiers for their contributions to chemistry, let’s say that Antoine and Marie-Anne get married and live happily ever after. This statement represents a singular plot element (i.e., a reactionary step), meaning that it doesn’t allude to the entire story of their lives (i.e., the mechanism), such as how they met, what exactly they accomplished together as chemists, and how Marie-Anne was pivotal to establishing the concept of nomenclature, to name just a few examples. (I should note, however, that Marie-Anne’s contribution to the scientific community was, at the time, glossed over.)
Returning now to my earlier point about how to arrange and write “proper” reactions, we all understand the figure below: the reagents are on the left; the products are on the right; and any variables are written by the reaction arrow. Students, though, tend to skip over this “art of writing reactions,” but I believe this is such an important skill to learn. So to help boost my students’ interest, I explain the rationale behind why the components are placed in specific spots, while noting that reagents, like variables, can be placed over the reaction arrow. Because a reaction needs to be planned out to get the maximum yield and a pure, single product, I explain to my students that this is ultimately why there is only one variable shown to be the product.
While I was teaching my students how to write reactions, I was led to wonder whether there was anything that I was teaching differently to support the mechanism organization of organic chemistry. In an attempt to answer my own question, we worked through an exercise in class together. For example, in Your Turn 13.6, the problem asks for the description of a function group (FG) modification or a change in the carbon skeleton (carbon). In addition, I asked my students to provide the appropriate reagents and to support their answer with a rationale. Not only did this exercise serve as a “self-check,” or diagnostic tool, for students to confirm their understanding of the material, but it also enabled students to work through the objectives of Chapter 13 while simultaneously reviewing the material that they had learned in previous chapters.
As shown above, each step in Your Turn 13.6 is discussed according to a function group (FG) or a change in the carbon skeleton. To help my students break down all these different components and make this problem easier for them to understand, I separated out the steps in the reaction and asked the following questions:
- What changes do you see when comparing the starting material and the product?
- What pieces are kept consistent or unchanged?
- Do you see an addition of carbons? How many? Where on the starting material do you see this?
In the reaction that applies to Step 3, which, in this case, made the acetylene (alkyne), I designated the reactionary step as a function group (FG) transformation and a carbon skeleton extension. However, I made it clear to my students that this reaction could also be perceived as a change to the parent molecule, because only carbon atoms were added.
After, I tied in the following questions:
- When did we learn these reactions?
- Do we remember the reagents or the named reactions?
- Does the placement matter?
Later, when arranging the reaction of the alkyne as it was transformed into the aldehyde, I asked my students:
- What causes the carbonyl to be on the end? What key reagent is needed?
One of my students suggested oxymercuration. While commending the student for participating, because I want to maintain as interactive a classroom as possible, I explained that this would actually give the ketone as the alternate product, which would be formed with Markovnikov’s rule in mind. To nudge my students towards the right direction, I went back to my two original questions above: What causes the carbonyl to be on the end, and what key reagent is needed? This sparked an “a-ha” moment in one of my students, who called out “peroxide.” And she was exactly right—the aldehyde is achieved via hydroboration oxidation, and the peroxide is the second set of reagents.
Even though mechanisms might seem dry and stagnant at first glance, I’ve found that using various analogies and practice problems not only helps add a sense of versatility to my classroom, but it also helps my students more effectively reinforce and apply the material they learned from previous chapters. By approaching mechanisms in a holistic manner, then, I’m more easily able to show my students “the big picture” in organic chemistry: each reagent has a working part and a key place in the reaction scheme. This macroscale approach also makes mechanisms more fun for me to teach and more fun for my students to learn—what I consider to be a double win!
-Kerri Taylor, Columbus State University
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