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?
In the traditional, functional group approach, alkyl halides (RX) are the first functional group introduced in most organic textbooks, and one of the first reactions and mechanisms students learn is free radical halogenation. This is the way I learned and, in turn, the way I’ve taught for nearly a decade. Free radical halogenation is not selective and leads to mixtures of products. In the example below, free radical chlorination leads to 4 different monochlorinated products, and students can calculate the relative percentages of each product.
In order to understand the relative percentages of each monochlorinated product, students need to understand the mechanism that leads to the production formation: initiation, propagation, and termination. Homolytic bond cleavage involves the “fish-hook” (single-head) arrow notation. To me, this was not a complicated mechanism; three simple steps.
The university where I currently teach uses a mechanistically organized textbook, and the radical chapter appears at the very end. The first few times I taught organic chemistry organized by mechanism, I followed the organization of the textbook and taught radicals at the very end of Organic II. However, last fall I decided to follow tradition and teach the free-radical halogenation mechanism in the first unit of Organic I. I felt that the first unit of instruction needed to be more than a general chemistry review sprinkled with some organic topics.
Later in the fall I found myself repeating, “Forget about homolytic bond cleavage. Stop using the fish-hook arrow. Arrows move in one direction when showing electron movement for two electrons.” I kept seeing fish-hook arrows and homolytic bond cleavage attempts on each of the subsequent unit exams and the final. It was the first mechanism they learned, and they could not get it out of their minds. I was sorry I had taught it at all.
Below are some examples of mechanism attempts I saw on my final exam. While I did not ask any questions about radical mechanisms on the final, I saw attempts at radical mechanisms over and over again. Students did not understand when to use the single versus the double arrow, nor the proper flow of electrons, and therefore kept going back to what they learned first. I sadly realized that, even though I was teaching from a mechanistically organized text, my students did not really understand mechanisms by the end of Organic I. I had confused them by my commitment to tradition.
| Using Two-head Arrow Improperly | |
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| Using Single-head Arrow Improperly | |
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Table: Incorrect Mechanism Attempts On My Final Exam
Unfortunately, I learned the hard way that the comfort of tradition isn’t a good reason not to change. Radical mechanisms are complicated and show electron movement unlike any of the other mechanisms in sophomore organic. In Joel Karty’s textbook, students are slowly and properly introduced to mechanisms by learning ten elementary steps (Chapters 6 and 7). Students are given a simple yet solid foundation upon which to build increasingly complicated multi-step mechanisms, and radical mechanisms are not covered until Chapter 25. Even though tradition says to teach free radical halogenation first (in a survey of ten other organic textbooks, all introduced radical mechanisms within the first eight chapters), I’ve decided that it’s not the best route for student understanding of mechanisms. I’m also considering TV in the morning…
— Marie Melzer, Old Dominion University
Marie Melzer teaches a mechanistically organized course at Old Dominion University. She plans to use Joel Karty’s book in the fall (2014). Click here to learn more about Prof. Melzer.
