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