Chemical Intuition and the Reasonableness of Mechanisms

Each year, after I teach my class the overview of the ten most-common elementary steps, I feel a great sense of satisfaction because I begin to see students mastering two critical aspects of elementary steps: Drawing curved arrows in the correct way, and correctly predicting products when they are told which step occurs. But after that, there is still another hill to climb: Getting students to draw multistep mechanisms correctly. I tell my students that for any overall reaction that occurs, I can draw curved arrows in such a way as to show the reaction taking place in a single step, and I even show a couple crazy examples with half a dozen curved arrows on a single structure. But that doesn’t make it right. Rather, each overall reaction proceeds by a relatively well-defined set of elementary steps. I expect my students to know that sequence of steps for each reaction—not because they have memorized the mechanism, but rather because the mechanism makes sense. For this to work, I need my students to understand why steps in one order make sense, but in a different order don’t. In other words, I need my students to gain a certain “chemical intuition” about mechanisms, and this is not something I can/should expect students to pick up on their own—I feel it is up to me to teach certain aspects of it to them.

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Getting Students to See Things Our Way

Research in chemical education has repeatedly trumpeted the message students do not see things the way we see them. However articulate or engaging we are, explanations, demonstrations, and worked examples do not guarantee that students view chemistry the way we do.

For example, when I recently asked students to direct me on how to draw the arrows for an SN2 reaction, I expected them to tell me to first draw an arrow from the nucleophile to the carbon:

And then the arrow that moves the shared pair between the carbon and the leaving group:

However, on multiple occasions this semester, different students directed me to do the reverse. First, they wanted to move electrons to the leaving group:

And then bring in the electrons from the nucleophile:

I still do not understand why they would perceive an SN2 reaction as proceeding in this fashion. They didn’t get it from me—honest!

While this particular example may be as new to you as it was to me, we probably share a list of “things students do that drive me crazy”: strong bases appearing miraculously in acidic solution, protons teleporting from one part of molecule to another, double bonds in benzene circled and labeled “alkene,” etc.

After I started teaching organic chemistry, I catalogued recurring issues and included “preemptive strikes” in a list I would give students entitled “Organic Chemistry Leitmotifs.” Several were humorous attempts at saying, “Don’t do that!”

I am happy to report that students no longer receive that list. One of the strengths of teaching a mechanistically organized course is that it facilitates explanations at critical points that prevent students from falling into misconceptions. For example, Joel and I recently discussed the value of having, in the chapter that introduces aromaticity, an explanation that the double bonds in benzene do not undergo reaction typical of nonaromatic double bonds. I supported Joel’s view that students need this type of information. As an example of other instructors’ interest in making this distinction, I cited a recent Journal of Chemical Education article that describes a demonstration that illustrates this difference in reactivity.

In addition, teaching a mechanistically organized course focuses on the “how” and “why” of organic reactions. This has made a difference. Students now regularly ask questions that allow me to discuss potential pitfalls. They now want to know why a step is unlikely to occur.

In fairness to students—and to offset the use of two exclamation points earlier—I’ll add that students cannot see organic chemistry the way instructors do. We are immersed in the subject and we enjoy it. In a sense, we keep taking the class over and over; certainly, we should be proficient. Our challenge is to bridge the gap between our expert experience and their initiation into the discipline.

— Steve Pruett

Applying MO Theory Toward Reactions… Or Not

There are two fundamentally different applications of molecular orbital (MO) theory in an undergraduate organic chemistry course. One application is toward various aspects of structure and stability of molecular species, including such things as the stabilization that occurs from the formation of a covalent bond, hybridization, rotational characteristics of σ and π bonds, conjugation, and aromaticity. The second application of MO theory is toward the dynamics of reactions, invoking some form of frontier MO theory. What orbitals are involved and how do electrons flow during the course of the reaction? And why should an elementary step take place at all, exhibiting the particular stereochemistry it does?

Over the years, I have learned that most instructors focus on the first of these applications in their courses, but relatively few instructors teach (or spend much time on) frontier MO theory. I think there’s good reason for this: Nearly everything that a student needs to know with regard to electron flow and stereochemistry can be explained using Lewis structures, VSEPR theory, resonance theory, and charge attraction/repulsion. These are things that students already know by the time they begin to study mechanisms.

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What About Transfer Students?

Instructors sometimes express concern about students who transfer from one institution to another in the middle of the two-semester organic sequence. Could starting (or ending) the sequence in a mechanistically organized course while ending (or starting) the sequence with a course organized by functional group cause students difficulty?

Since I teach at a two-year public institution, Jefferson Community and Technical College (JCTC), I deal with more transfer students than Joel does at Elon University, a four-year private institution. My data is based only on my experience and some observations about the two-semester sequence, but I have concluded that a student who transfers into or out of a functional group organization does not face a handicap.

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