Taking Students Beyond the Limitations of a Functional Group Organization

I’ve written earlier about the advantages of the chapter on elementary steps and how students’ benefits from a mechanistic organization surfaced in the second semester course. In this post, I’d like to offer an example of how this approach allows students to solve problems that I once considered too “advanced” for typical organic students.

A functional group textbook focuses on A plus B produces C. For example, students learn the following reactions:

post12_imgA

Certainly, instructors and the textbook will show students the mechanisms and emphasize the differences and similarities between the reactions. Even so, we have limited the students to recognizing only a certain number of reactions.

With a mechanistically organized approach, one that emphasizes the elementary steps involved, the scheme that students generate is different. Although the following representation is not presented in Joel’s book, it summarizes the way students organize the information after using his text:

post12_imgB

Let me restate that this is not how the concept is presented to students; rather, they learn it from repeatedly considering questions such as:

  • “How would you describe this nucleophile? Strong or weak?”
  • “How could it react with the substrate?”
  • “What will happen?”
  • “Is there a good leaving group present?”
  • “What do you expect to happen next?”
  • “Do we expect a species with a charge to be the final product?”

In fact, Joel’s books has “Solved Problem” and “Your Turn” sections that lead students through these steps.

Note that because the reactions are presented in the context of a particular type of elementary step (e.g., Chapter 17: Nucleophilic Addition to Polar Pi Bonds 1: Addition of Strong Nucleophiles and Chapter 20: Nucleophilic Addition-Elimination Reactions 1: The General Mechanism Involving Strong Nucleophiles), students generally are cued to answer the first two questions.

To illustrate the advantage of this type of teaching, consider this example, a reaction from E. J. Corey’s synthesis of porantherine:

post12_imgC

I have used this reaction on exams and in homework problems since I began class-testing Joel’s book. While this reaction of an organolithium reagent and an imine would fall outside of what I could fairly ask my students to evaluate if I used a traditional textbook, however, because it follows the same elementary steps as other reactions students have studied in a mechanistic organization, then students are capable of providing the correct mechanism and/or predicting the product of this reaction. This isn’t a “challenge” problem anymore. The students at my institution (Jefferson Community and Technical College) have performed well on such problems.

This approach can be summarized succinctly. The first year I class-tested Joel’s book, I visited Cambodia over the break between the fall and spring semesters. While I was there, I saw many people wearing T-shirts that had “Same, Same” on the front and “But Different” on the back. After I returned to the U. S. and started teaching Organic II, I had a friend ship me a bundle of these T-shirts so I could give one to each of my students: my repeated use of the expression “Same, same, but different” had become a standing joke in organic lecture.

While organic chemistry, alas, cannot actually be summarized on a T-shirt, the “Same, same, but different” expression does reflect what we want our students to learn: the ability to analyze new reactions by asking themselves, “How is this set of reactants similar to the ones I have studied previously? How will they react? What is different? How will that affect the outcome?”

— Steve Pruett

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