This unique chapter is the game changer for how students perceive organic reactions. Whenever I discuss Joel’s textbook with colleagues, this chapter is the first aspect of the book that I mention.
Chapter 7, “An Overview of the Most Common Elementary Steps,” briefly surveys ten steps:
- Proton transfer
- SN2
- Bond formation (coordination)
- Bond breaking (heterolysis)
- E2
- Addition of a nucleophile to a polar pi bond
- Elimination to form a polar pi bond
- Addition of an electrophile to a nonpolar pi bond
- Elimination of an electrophile to form a nonpolar pi bond
- Carbocation rearrangements
This survey covers the elementary steps that students will see in a reaction mechanism until they study pericyclic reactions and reactions involving free radicals.
One benefit, which I mentioned in an earlier post (“Liberation in the Laboratory”), is that this presentation allows students to work with almost any mechanism and identify the steps that occur. This gives the students and instructor a common vocabulary that they can use throughout the course.
In addition, this presentation starts the process by which students assess steps in terms of thermodynamics and kinetics. Specifically, we examine each step as being reactant-favored or product-favored in terms of charge stability and bond strength. In the chapters that follow, the text discuss reversibility of reactions, the role of LeChatelier’s principle, and rate laws in the context of substitution and elimination reactions.
In the short term, this introduction of elementary steps allows students to answer questions like:
Consider the following elementary steps:
Provide a description of each step:
(a)
(b)
(c)
(d) Which step, (a), (b), or (c), is most likely to be product-favored? Justify your answer.
In the long term, students apply these ideas regularly in classroom discussions. Students go from being passive receivers of presentations of “This Is What Occurs” to active participants in discussions that begin with questions such as: “How do we describe this step?”; “Do we expect this step to occur?”; “Why does this step appear unlikely to occur?”; “What could occur?”; “Is this step reversible or irreversible?”; “What is the driving force for this step?”; “How is the reaction similar to previous reactions? How is it different? What additional step occurs?”
The downside of including this chapter is unavoidable: it means that there is an extra chapter in the first semester curriculum. For me, this extra chapter requires two days of class discussion.
The upside is the quality of understanding that students bring to class. The first time I taught the course using Joel’s text, there was a point in Organic II where I recognized that students’ perceptions of reactions were clearer and deeper than those of students in previous years’ courses. Our discussions were richer than any I had had in prior years. This led to a professional assessment, in arguably unprofessional language, that Chapter 7 rocks.
— Steve Pruett
