Focused on the Student, Organized by Mechanism
When an organic reaction is presented to a novice, only the structural differences between the reactants and products are immediately apparent. Students tend to see only what happens, such as the transformation of one functional group into another, changes in connectivity, and aspects of stereochemistry. It should therefore not be surprising that students, when presented with reactions, are tempted to commit the reactions to memory. But there are far too many reactions and accompanying details for memorization to work in organic chemistry.
This is where mechanisms come into play. Mechanisms allow us to understand the sequences of elementary steps—the step-by-step pathways—that convert the reactants to products, so we can see how and why reactions take place as they do. Moreover, the mechanisms that describe the large number of reactions in the course are constructed from just a handful of elementary steps, so mechanisms allow us to see similarities among reactions that are not otherwise apparent. In other words, mechanisms actually simplify organic chemistry. Thus, teaching mechanisms—enabling students to understand and simplify organic chemistry—is an enormous key to success in the course.
At the outset of my teaching career, I fully appreciated the importance of mechanisms, so during my first couple years of teaching, I emphasized mechanisms very heavily. I did so under a functional group organization where reactions are pulled together according to the functional groups that react. That is the organization under which I learned organic chemistry, and it is also the way that most organic chemistry textbooks are organized. Despite my best efforts, the majority of my students struggled with even the basics of mechanisms and, consequently, turned to flash cards as their primary study tool. They tried to memorize their way through the course, which made matters worse.
I began to wonder what impact the organization—an organization according to functional group—had on deterring my students from mechanisms. I had good reason to be concerned, because as I alluded to earlier, functional groups tend to convey the what of chemical reactions, whereas mechanisms convey the how and why. What kinds of mixed messages were my students receiving when I was heavily emphasizing mechanisms, while the organization of the material was giving priority to functional groups? To probe that question, I made a big change to my teaching.
The third year I taught organic chemistry, I rearranged the material to pull together reactions that had the same or similar mechanisms: that is, I taught under a mechanistic organization. I made no other changes that year; the course content, course structure, and my teaching style all remained the same. I even taught out of the same textbook organized according to functional group. But that year I saw dramatic improvements in my students’ mastery of mechanisms. Students had control over the material, which proved to be a tremendous motivator. They were better able to solve different kinds of problems with confidence. Ultimately, I saw significant improvements in student performance, morale, and retention. I became convinced that students benefit remarkably from learning under a mechanistic organization.
My goal in writing this book is to support instructors who are seeking what I was seeking: getting students to use mechanisms to learn organic chemistry in order to achieve better performances and to have better experiences in their organic courses. Using a functional group organization to achieve these outcomes can be an uphill battle because of the high priority that it inherently places on functional groups, which ultimately promotes memorization. This textbook, on the other hand, allows students to receive the same message from both their instructor and their textbook: a clear and consistent message that mechanisms simplify learning organic chemistry and are vital to success in the course.
A Closer Look: Why Is a Mechanistic Organization Better?
Consider what the novice sees when they begin a new functional group chapter in a traditionally organized textbook. In an alcohols chapter, for example, students first learn how to recognize and name alcohols, then they study the physical properties of alcohols. Next, students might spend time on special spectroscopic characteristics of alcohols, after which they learn various routes that can be used to synthesize alcohols from other species. Eventually, students move into the heart of the chapter: new reactions that alcohols undergo and the mechanisms that describe them. Finally, students deal with how those reactions are incorporated into synthesis. Within a particular functional group chapter, students find themselves bouncing among several different themes.
Even within the discussion of new reactions and mechanisms that a particular functional group can undergo, students are typically faced with widely varying reaction types and mechanisms. Take again the example of an alcohols chapter in a book organized according to functional group. Students learn that alcohols can act as an acid or as a base; alcohols can act as nucleophiles to attack a saturated carbon, in a substitution reaction, or to attack the carbon atom of a polar π bond, in a nucleophilic addition reaction; protonated alcohols can act as electrophiles in a substitution or an elimination reaction; and alcohols can undergo oxidation, too.
With the substantial jumping around that takes place within a particular functional group chapter, it is easy to see how students can become overwhelmed. Under a functional group organization, students don’t receive intrinsic and clear guidance as to what they should focus on, not only within a particular functional group chapter but also from one chapter to the next. Without clear guidance, and without substantial time for focus, students often see no choice but to memorize. And they will memorize what they perceive to be most important: predicting products of reactions, typically ignoring (or giving short shrift to) fundamental concepts and mechanisms.
Under the mechanistic organization in this book, students experience a coherent story of chemical reactivity. The story begins with molecular structure and energetics and then guides students into reaction mechanisms through a few transitional chapters. Thereafter, students study how and why reactions take place as they do, focusing on one type of mechanism at a time. Ultimately, students learn how to intuitively use reactions in synthesis. In this manner, students have clear and consistent guidance as to what their focus should be on, both within a single chapter and throughout the entire book.
The patterns we, as experts, see become clear to students when they learn under this mechanistic organization. Consider the following four mechanisms:
The mechanism in Equation P-1 is for Williamson synthesis of an ether; the one in Equation P-2 is for alkylation of a terminal alkyne; the one in Equation P-3 is for alkylation of a ketone; and the one in Equation P-4 is for conversion of a carboxylic acid to a methyl ester. In these four reactions, the reactants are an alcohol, an alkyne, a ketone, and a carboxylic acid. In a functional group organization, these reactions will be taught in four separate chapters that can be located in distinctly different parts of the book. Because all four reaction mechanisms are identical, consisting of a deprotonation followed by an SN2 step, all four reactions are taught together in this book, in the pair of chapters dealing with substitution reactions in synthesis: Chapters 10 and 11.
Seeing these patterns early, students more naturally embrace mechanisms and use them when solving problems. Moreover, as students begin to see such patterns unfold in one chapter, they develop a better toolbox of mechanisms to draw on in subsequent chapters. Ultimately, students gain confidence in using mechanisms to predict what will happen and why. I believe this is vital to their success throughout the course, in later biochemistry courses where organic chemistry is routinely applied, and on admission exams such as the MCAT.
Bowman, B. G.; Karty, J. M.; Gooch, G. Teaching a Modified Hendrickson, Cram, and Hammond Curriculum in Organic Chemistry. J. Chem. Educ. 2007, 84, 1209.
-Joel Karty, Author of Organic Chemistry: Principles and Mechanisms