Is Organizing by Mechanism Necessarily “Higher Level”?

Because organizing by mechanism really helped me turn around my own organic chemistry course, I was eager to share what I had learned. Many instructors I’ve talked to over the years have been very receptive; some have even adopted this organization themselves. But I’ve encountered a good amount of apprehension as well. Some instructors worry that organizing by mechanism is too “high level,” implying that such an organization requires a level of student that they simply don’t have.

Probing a little, I came to understand that instructor apprehension is linked to a few previously published textbooks that are organized according to mechanism, the first of which was written by Cram and Hammond (1959). Instructors who taught from these earlier books seemed to agree that an organization by mechanism was logical, but after witnessing a lot of students struggling, they hastily concluded that the organization itself was the problem. I had my doubts about this conclusion because of the great success I was having in my own courses. From what I had experienced, an organization by mechanism actually made learning organic chemistry a lot more palatable and rewarding for students in the long run. So, I wondered, what could account for the success of my own course but the failure of these earlier textbooks?

To solve this riddle I acquired a copy of Cram and Hammond and a couple other organization-by-mechanism textbooks and I looked for differences in what I was doing in the classroom and the way that things were handled in the books. Indeed, these earlier textbooks appear to have had incredibly high expectations of students, especially with regard to “chemical intuition.” With very little (and often no) discussion in the book, it seemed to me that students were expected to understand and routinely apply a variety of critical concepts necessary to work comfortably with mechanisms. Such concepts include: Rationalizing how the curved arrow notation in each elementary step of a mechanism reflects the flow of electrons from an electron-rich site to an electron-poor site; employing concepts of charge stability to judge whether a reaction is energetically favored; and how to deal with changes to a mechanism when the conditions are acidic versus basic. Personally, I think that all of these are challenging concepts for students to apply to mechanisms, and represent a relatively steep part of the learning curve. They are not concepts that should be glossed over—they ought to be dealt with head on.

Learning from the missteps of these earlier textbooks, I make sure to teach “chemical intuition” before holding students accountable for predicting the outcomes of reactions. I have an extended discussion in the context of acid–base reactions to help students develop a deep understanding of charge stability. I follow that with a segment on the ten most common elementary steps in organic mechanisms. For each elementary step, my students and I examine how the curved arrow notation is consistent with the flow of electrons from an electron-rich site to an electron-poor site, and we also examine how charge stability contributes to the driving force. And we spend time on rules-of-thumb surrounding the acidic–basic conditions of reactions. In essence, what I do is provide tools that are critical for understanding mechanisms. And I do this before holding students accountable for predicting products or devising syntheses.

I believe that spending time on the mechanism tools early helps out tremendously later. The tools provide students a means by which they can build and hone their chemical intuition with each new mechanism. The tools enable students to climb the steep part of the learning curve with relative comfort, so that they are rewarded with a flatter part of the curve later on. Therefore, with regard to the question of whether an organization by mechanism is necessarily high level, I think the answer is “No.” It does not require high-level students. Rather, if the right tools are provided early on, an organization by mechanism can help students in any organic course perform at a very high level.

— Joel Karty

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