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.

In my own classroom, I spend a good deal of time on the application of MO theory toward molecular structure and stability, but I don’t spend any time on frontier MO theory until I get to Diels-Alder reactions in the second semester. By that time, I feel that students have matured enough and have enough experience under their belts to deal with the added complexity of these new concepts. But before then, I feel that it’s necessary to stick with what is already familiar to students. I explain the dynamics of SN2 reactions, for example, as a result of an electron-rich nucleophile being attracted to an electron-poor carbon atom of the substrate. I explain the stereospecificity of the reaction as a result of electrostatic and steric issues: frontside attack is disfavored by steric and electrostatic repulsion between the incoming nucleophile and the departing leaving group, and we see the opposite with backside attack. I use similar arguments to explain why E2 reactions are favored when the H and leaving group being eliminated are anti to each other. And students “get” these explanations; so to me, it doesn’t seem necessary to teach frontier MO theory, an entirely new concept, to get the same ideas across.

Although I choose not to teach frontier MO theory to my own students in the first semester, I recognize the importance (and power) of its application toward all reaction types. What I have done in my textbook, therefore, is to include a short “interchapter” on the topic, located just after Chapter 7 (the chapter that presents an overview of the 10 most common elementary steps). Therefore, once students have been exposed to the basics of each elementary step (including specific examples and the idea of electron flow from a “rich” site to a “poor” site), an instructor has the option to teach the same elementary steps from the perspective of frontier MO theory. Teaching it this way, as opposed to splitting it up over multiple reaction chapters, offers the advantage of having students see more of the big picture of frontier MO theory—especially along the lines of similarities and differences from one elementary step to another. Moreover, treating this material in an interchapter offers the advantage of having it be entirely optional as the material in the main chapters does not depend on it.

— Joel Karty

2 thoughts on “Applying MO Theory Toward Reactions… Or Not

  1. Thanks for this post. Teaching ‘around’ textbook mistakes on MO theory has been a thorn in my side for over 25 years. So a reflection on the role of so-called MO theory by authors of organic chemistry textbooks is welcome and long overdue.

    I agree that there is a time and place for teaching MO theory. Unfortunately, once one figures out when and where to talk about MOs, one still has to get the story right and this is where most books fail.

    For example, many books display a sigma ‘bonding orbital’ for H2 (and other molecules) and then describe this picture as a ‘valence bond’ model. This is incorrect. Polyatomic orbitals like bonding and antibonding orbitals simply do not appear in valence bond models.

    Another gripe: many books show ‘symmetric’ orbital mixing diagrams in which the antibonding MOs are destabilized to the same extent that bonding MOs are stabilized. This makes sense in a model like Huckel MO that relies on a ‘zero overlap’ approximation, but it is not a feature of MO theory in general and it robs MO theory of some of its predictive power.

    Omissions are just as important as errors. To make sense of MO theory (or VB theory for that matter), one needs to grasp two things: 1) orbital overlap and 2) orbital energy gaps. Overlap is the more difficult concept to teach/grasp and most books are either misleading or superficial in their treatment.

    Opportunities for engaging with quantum models, VB or MO, are scattered throughout an introductory course, but they usually slide by without getting noticed. Conformational preferences, the effect of alkyl substituents on alkene energy, the (much larger) effect of alkyl substituents on carbocation energy, the substituent effects of various EDG/EWG on adjacent pi systems, and so on, can all be tackled during the first semester using overlap + energy gaps. Topics in geometry, polarity, energy, and spectroscopy can also be addressed during the first semester.

    It is a fair question whether a quantum-based approach is unnecessarily complicated, but if one’s long-range goal is to have students take quantum models on board as useful intellectual tools, more frequent, rather than less frequent, contact with quantum-based ideas is necessary.

    1. Professor Shusterterman brings up some excellent points with regard to the accuracy of what we teach surrounding quantum models: If we’re going to teach certain aspects of MO and VB theory, shouldn’t they at least be accurate?

      I, for one, am torn on this matter, at least in some respects. When I teach orbital mixing to my sophomores, I show the energy diagrams as being symmetric—in what I draw, the energy by which a bonding MO is stabilized is roughly the same as the energy by which the corresponding antibonding MO is destabilized. It eats at me to some extent, though, because I know that it is an oversimplification; I feel like I am “lying” to my students.

      When I do so, however, I am consoled by two things. One is that the difference between knowing the truth versus not knowing the truth in this case doesn’t impact my students’ ability to achieve the outcomes I lay out for organic chemistry. As such, it’s difficult for me to justify presenting this aspect of quantum mechanics to my organic students, 80% of whom will not go on to take an upper-level physical chemistry or inorganic chemistry course. The second thing that consoles me is that my students who do go on to take one of those courses will learn the truth there, where it really matters. I occasionally teach our Physical Chemistry 2 course (quantum mechanics) here at Elon, and when we discuss these orbital mixing diagrams, I find myself saying: “Remember when you all learned this in organic? Well, you were lied to…. Here’s the way it really is.”

      This is not the only instance where we oversimplify something in an introductory course, only to correct it in a subsequent course. We sweep a few things under the rug, for example, when we talk about px and py orbitals rather than p+1 and p-1 orbitals, only to disclose the real truth in a formal quantum mechanics course. And when we teach students that the formation of a covalent bond is driven by the simultaneous attraction of electrons to two nuclei instead of one (i.e., a lowering of the potential energy), well, that’s not true either. The real answer is that the kinetic energy is lowered.

      I think that what it boils down to is, in a case-by-case basis, what “can of worms” is potentially opened when students are presented the real truth rather than a simplified model. For the orbital mixing diagram question, I feel that presenting the real truth in a sophomore organic chemistry course has more potential for confusing students rather than enlightening. And even for students who can grasp these higher-level concepts, I worry that students will be distracted by the “trees,” taking too much focus away from the “forest.”

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