One of my most common refrains in my organic chemistry classes is that students should strive to understand and apply the foundational principles, as opposed to trying to memorize each example reaction that they’ve encountered. I strongly believe that a mechanistic organization discourages the memorization behaviors that students are almost forced to adopt in a functional group-organized text, which is a large part of why I love the Karty textbook.
When I first flipped through this textbook four years ago, I was drawn by the student-friendly writing, visual design, and green chemistry examples. But what really sold me was Chapter 7: An Overview of the Most Common Elementary Steps. In addition to the objectives defined in the textbook, my primary goal for this chapter is to teach students to see patterns of reactivity. Once students recognize the patterns, they can take a logical (rather than a memorization) approach to learning new reactions.
To experienced chemists, the patterns jump out immediately because we’ve spent years studying and performing a huge library of reactions. But to novice chemists like our students, this chapter can be a real information dump. I knew that teaching this chapter in a lecture-based format could be boring and overwhelming. I feared that it might send my students the message, “You need to know ALL OF THIS!” How could I teach elementary steps in a way that emphasizes a logical approach to organic chemistry? Could this be an opportunity to teach students how to think like an expert chemist who understands the general principles of reactivity and applies them to new reactions? And how could I make this lesson more active, instead of passive, for students?
I begin my 2.5-hour class period with the familiar proton transfer reaction between ammonia and hydrochloric acid. In addition to the arrow pushing and acids/bases that students learned in the previous chapter, I add new terminology: nucleophile, electrophile, and leaving group. Next, we review the structural features that are commonly found in these species. For example, I emphasize that leaving groups are typically negative or neutral once departed and that they have the ability to stabilize negative charge (like stable conjugate bases.) Nucleophiles, on the other hand, could appear in one of four ways: a negatively charged species with a lone pair, a neutral species with a lone pair, a highly polarized sigma bond, or a non-polar pi bond. My goal in reviewing these features is not to offer an all-inclusive list, but to provide students with a general sense of how the criteria for nucleophile, electrophile, or leaving group might be met so that they can apply these principles to a myriad of contexts.
The remainder of the class is dedicated to group work where students study the nine remaining elementary steps using two handouts. The first handout has a page for each elementary step with its name, some key notes about the unique features of each reaction type, and an example mechanism. Students label any nucleophiles, electrophiles, acids, bases, and/or leaving groups. They also identify bonds broken and bonds formed and label the partial charges (d+ and d-) on the atoms involved in bond breaking and forming. Lastly, students identify another sample mechanism from the second handout that fits each elementary step category.
By the end of the class, students have discussed each elementary step at length; practiced recognizing nucleophiles, electrophiles, and leaving groups; and have compiled an organized, complete set of notes. These notes emphasize the similarities between all elementary steps (like electron flow from electron-rich to electron-poor atoms), but also underline the unique aspects of each reaction. Furthermore, this activity reinforces line structure thinking, such as recognizing full octets, reading implicit and explicit lone pairs, and identifying non-zero formal charges.
I believe that this approach to Chapter 7 emphasizes that organic chemistry is a logical science with predictable patterns of reactivity, as evidenced by the confidence and speed with which students finish the handouts toward the end of class. For any instructors wondering how to teach Chapter 7, I encourage you to try out this approach (or modify it to suit your students’ needs) and see how it goes! I would love to hear about how you might adapt it for shorter class periods.
-Grace Ferris, Lesley University
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