In my previous post, I described how happy I’ve been with my students’ ability to process the relatively complex interplay between kinetics and thermodynamics to understand the outcome of a competing set of chemical reactions. The specific example I gave involved the competition between transesterification and the Claisen condensation reaction, and that got me thinking about how far my students have come since they first started learning organic reactions last semester. In Chapter 7 of my book, students are given an overview of the ten most common elementary steps in organic reaction mechanisms—the same elementary steps that make up transesterification and the Claisen condensation reaction. Nevertheless, I don’t think students should be expected to deal with these kinds of complexities immediately after learning the elementary steps—they are simply not ready. Instead, students need time to digest what they learned in Chapter 7, and they also need significant scaffolding of additional material before being held accountable for these higher level decision making processes—material such as: how to incorporate proton transfer steps reasonably in a mechanism, relative nucleophile strengths, reversibility, and the ideas surrounding kinetic versus thermodynamic control.
What, then, should students be expected to do upon completing Chapter 7? I think it boils down to the following:
1. Use curved arrow notation to depict bond breaking and bond formation. Given the reactants and products of an elementary step, students should be able to add the appropriate curved arrows.
2. Follow curved arrow notation. Given the reactants and the curved arrow notation, students should be able to draw the products of the elementary step.
3. Name the various elementary steps. Given the reactants and either the curved arrow notation or the products, students should be able to provide the elementary step’s name.
4. Justify the directionality behind electron flow in an elementary step. Students should be able to identify the relatively electron-rich and electron-poor sites in the reactants, and should consistently see that curved arrows are drawn from the former to the latter.
5. Understand the contributions to the driving force of an elementary step. Students should know that charge stability and bond energy are two major factors that contribute to the driving force of an elementary step, and should be able to predict which side of the reaction each factor favors.
6. Recognize which pairs of elementary steps are the reverse of each other. Students should see, for example, that nucleophilic addition and electrophilic addition steps have corresponding elimination steps. This helps simplify what students need to know, especially in terms of an elementary step’s driving force.
Users, like Steve Pruett, have described chapter 7 as a “game changer.” Having these six skills under the student’s belt, the stage is then set for scaffolding-in the chapters to come. Students will continue to learn and will deepen their understanding without becoming overwhelmed.
-Joel Karty
