Making Backward Mechanism Arrows An Endangered Species

I’ve just finished grading the first exam for my Organic II course and I’ve experienced something I never have before. Of the more than 50 students that took my Organic I course last semester, none of them drew a single mechanism arrow in the wrong direction [1]. Yes, some of these students did get parts of mechanism questions wrong, but it wasn’t because they drew arrows incorrectly. The students that did draw arrows incorrectly were students that took Organic I at a different institution or previously used a different textbook. How do I account for these results? I credit Joel Karty’s approach, and his book, because of the excellent job it does stressing the fundamentals in the beginning (first semester) to lay the groundwork for what I just experienced in Organic II (second semester).

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Making a Commitment (But Not to Traditions)

I did not realize my commitment to traditions—in my personal life and in the classroom—until recently. In my personal life, I discovered that I was married to a person who did not know that: Christmas trees are decorated while listening to Christmas music and not with a basketball game on in the background; salads are eaten after the main meal; there is no TV in the morning; and soda is a restaurant-only drink. In the classroom, my closely-held tradition was to introduce free radicals first. Traditions are fine, important things, which is why we observe them. But at what point do we rethink traditions? Are we traditionalists open to new, improved ideas?

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Lone Pairs and Aromaticity

One of the things my students find most challenging about aromaticity is whether to include lone pairs as part of a cyclic π system. If a lone pair is included, then the number of π electrons increases by two, and a student’s prediction about whether a species is aromatic will also change. What I think makes this challenging is that the rules appear to change depending on the nature of the ring and the nature of the atom that has the lone pair. This issue is apparent when we compare pyridine, pyrrole, and furan:

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Pyridine and pyrrole both contain a nitrogen atom with a lone pair of electrons, but, whereas the lone pair in pyridine is not included in the π system, the one in pyrrole is. In furan, one of the lone pairs on oxygen is included but not the other.

Students can certainly memorize these specific results for pyridine, pyrrole, and furan, but the true problem manifests itself when students are asked to make predictions about the aromaticity of unfamiliar molecules that contain atoms with lone pairs. For many years my students struggled with this, but with an additional tool I now teach, they have become quite good at making these kinds of predictions.

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It’s Elementary: First Steps to Active Learning

For most of my teaching career I have organized my course by mechanism class rather than by functional group. Even so, year after year I observed my students struggling with reaction mechanisms. Neither elaborating mechanisms on the board in class nor assigning challenging mechanism problems in practice or homework sets seemed to improve my students’ ability to demonstrate mechanistic competencies on exams, especially when it came to mechanisms that were not direct analogs of those discussed in class or in the text. A simple, foundational pedagogical change in Joel Karty’s forthcoming textbook, however, helped me to break through this barrier and reach my students.

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