I have been teaching organic chemistry with a mechanistic approach for a long time. Unfortunately, the book that I used prior to Karty tried but ultimately dropped this approach midway through the second semester of topics. In Joel’s text, mechanisms and thinking through a different lens pervade, which has prompted a number of my students to declare, “While other classes say they are going to push you up Bloom’s taxonomy, in this class we actually do it!” (Yes, my students actually reference Bloom’s taxonomy in casual conversation!)
A mechanistically organized textbook de-emphasizes rote memorization of reagents in favor of emphasizing the core causes and bottlenecks of polar mechanisms. This sort of analysis directs students towards a deeper understanding of why. Students are enabled to move from simply remembering a new set of reagents for each set of functional group transformations towards applying that information to different functional groups or even creating new mechanisms. The material is reinforcing students’ base knowledge while challenging them to evaluate that information in new settings.
While teaching the SN1/SN2/E1/E2 section, I gave my students two linked and sequential activities which illustrate that a mechanistic approach can improve a student’s higher order cognitive skills. This section of organic I is particularly difficult for students because they have to choose which pathway(s) to follow as well as memorize new reagents. It is truly the first time that they have to make a number of decisions to simply “predict the product.”
In the first activity, I pair reactions that might appear to be similar but have diverging paths. I ask the students to write the mechanism and to compare and contrast the two reactions in light of product distribution. By having students evaluate what factor is important to explain diverging pathways, this activity is intended to enable students to choose the correct reagent or to predict the correct product. At this point, students are aware of the competing factors but aren’t sure when to use which reason. Using the following example, I want them to see the difference between a nucleophilic and non-nucleophilic acid (i.e. why doesn’t hydrogen sulfate add as a nucleophile) as well as to explain how we can see elimination with sulfuric acid but not with HBr.
The students usually make a number of errors in this first assignment, both in their mechanisms and their explanations. I collect our student errors and combine them into a follow-up assignment where I ask them to determine what’s wrong with the mechanistic step(s) as drawn. There is value in assessing mistakes before just looking at the correct answer; they learn how to avoid the common traps and how to edit/correct themselves.
For the example used above, I ask:
- Why doesn’t the bromide act as the nucleophile and do SN2 to kickout hydroxide?
- Why doesn’t the bromide (or hydrogen sulfate) do either E1 or E2 on the protonated alcohol (or carbocation) to form an alkene?
- Why doesn’t the hydrogen sulfate add to the carbocation?
To their credit, there are students who see all of this complexity immediately, but for the student who is still trying to memorize reagents and products, seeing the incorrect mechanisms written out triggers their ability to take a wider view. It didn’t take long for the students to complete this activity and to get reasonable answers. Ultimately, a lot of the students that I will teach in organic chemistry will never use the reagents again, but they will be faced with complex decisions. I want my medical professionals, for example, to be able to evaluate numerous bits of (maybe conflicting) data and not jump to conclusions. Teaching mechanistically helps to instill this type of reasoning, and this is valuable to them even beyond the classroom.
-Tara Kishbaugh, Eastern Mennonite University
