“So I Should Memorize…”

Coming from a student of organic chemistry, the title phrase used to incense me. It wasn’t just that I knew brute-force memorization to be an inferior strategy for learning (a fact I never hesitated to impart on self-proclaimed memorizers). I felt that these students displayed a profound disrespect for the content of the course by subjecting it to the intellectual equivalent of throwing a can against a wall until it busts open. All I’m asking you to do is pick up and study a can opener. So many students of organic chemistry are just shown open cans over and over again and expected to master how a can opener works, I would tell myself. They should consider themselves lucky that I show them the can opener at all.

The selfish bitterness in that attitude is painful in retrospect. I found myself taking a different tack when I was approached by a student after class recently. In this class I had presented the equivalent of the mechanistic can opener: ten elementary steps of polar organic reaction mechanisms shown in one fell swoop as the foundation of all two-electron organic reactivity.

So essentially I should memorize these ten elementary steps?

Breathe. I found myself avoiding my usual anti-memorization crusade: “no, you shouldn’t memorize anything in this course…” Instead, I informed her that the alternative was memorizing reaction mechanisms individually as the course progresses, which will be far more arduous than learning the ten elementary steps now. I told her that these were the key to reaching a higher plane of abstract mechanistic thinking. Against my own expectations of myself, I affirmed memorization.

I think, on some level, I accepted that this student had worked her butt off to even get to my classroom. If memorization (as she called it) was essential to that work, who am I to judge how she learns the essential concepts of the course? Memorize away, my friend. I’m guessing that if you memorize this foundational framework now, you’ll thank me later anyway. On the other hand, if success in the course doesn’t follow, that will be a much more powerful deterrent to memorization than my saying “no, don’t memorize” in an impromptu meeting after class.

Students who connect painful, challenging, or even deliberate learning with “memorization” used to grate on my nerves beyond belief. I couldn’t shake the question of how much my course structure versus those students’ intellectual backgrounds promoted memorization. What was I doing wrong that kept students from wanting to learn rather than memorize what I’m teaching? Now, I don’t worry so much. Students have diverse backgrounds. Not all of them will become chemists, or even want to, or even could want to after experiencing “perfect” teaching. My goal shouldn’t be pumping out a bunch of mini-chemists—that’s impossible! Instead, my goal should be to meet all students where they are intellectually, trying where I can to push the boundaries of their knowledge, skill, and especially curiosity.

The Organic Colossus: Negotiating Structural Complexity

I feel better about recognizing possibilities. But how can you tell what matters? How do you separate what’s important from what isn’t?

I have to admit I was probably a bit too amused when a student said this in office hours for my Organic Chemistry I course recently. She had come to realize the colossal complexity built into even the simplest of organic molecules: the dizzying bouquet of orbitals, filled and unfilled; electron sources and sinks; the zigs and zags of molecular shape; conformational dynamics. She was staring down the organic colossus, standing at a crossroads. And in that respect, she was exactly where I wanted her to be.

In practice, all students of organic chemistry have to make a choice: face down the terrific complexity of organic molecules and bring it under one’s command, or run the opposite direction. Structural complexity is a line in the sand. On one side of that line are memorization, surface-level learning, and just plain giving up. On the other side are reasoning, deep conceptualization, and fearless diving-in. I like making that line known to students: “here’s the complexity of organic chemistry in gory detail. How do we make sense of it? How do we break it up into useful concepts?” I like to think that structural complexity, when shown to students not as something to fear but something to attack with analytical fervor, provides a deep motivating force for learning.

One could argue that “how do we know what matters?” is the central question of Organic Chemistry I as a whole, of course. It’s not a question that can be answered overnight. And no one, even experts, pursues an answer to this question relentlessly all the time. We all favor some topics over others. Some concepts are more natural to us than others. Even so, just cultivating an awareness of the question helps students in a metacognitive way, as they become aware of moments when they’re “phoning it in” (it happens to the best of us, too).

There’s a growing body of work in the chemical education literature about the ways in which students grapple with reaction mechanisms, Lewis structures, and other visually complex aspects of organic chemistry. These papers remind me of well-known work in behavioral economics that focuses on ideas like loss aversion and projection bias, as they’re primarily descriptive studies that focus on the way students think and do not offer solutions (although “further research is needed to…” and “this work has the potential to…” appear as customary bits of optimism). To an expert in organic chemistry, these exposés of student behavior paint a disappointing picture: students spend a lot of time on the wrong side of the battle against structural complexity, using weak strategies or shallow patterns to engage.

Behavioral economics has seeped into the national consciousness and has imbued marketing and finance types with unprecedented practical influence. Could “behavioral organic chemistry” affect teaching practice in a similar way? At the moment, the answer to this question is unclear. “Behavioral organic chemistry” has certainly exposed ubiquitous weaknesses in students’ thinking, but the ways in which teaching or assessment practices should change to fit these observations remain clouded. The facts, theories, and models of organic chemistry should not and indeed cannot change to accommodate students’ shallow thinking. So, faced with the question of what to do with the growing body of evidence documenting errors in students’ thinking, I’m left scratching my head.

One way forward, which has been embraced by Cooper, Underwood, and colleagues, is to focus on assessment. If a student is reaching a right answer for the wrong reasons, isn’t the question to blame? Issues of practicality and scale aside, I think this approach has promise. Asking the right questions is key. I also hope that future studies can capture the evolution of students’ thinking from shallow to deep. I don’t know of any studies that have followed students of chemistry from undergraduate through graduate school, but my own experience is that I did not achieve mastery until about halfway through graduate school—after an overwhelming deluge of information was forced upon my mind, and I had to push back. I would love to see an ethnographic study that documents this process for graduate students in chemistry.

Orbital Theory Doesn’t Have to Suck

When I was in graduate school, I was mystified by the ways in which older graduate students in my synthetic organic chemistry group would invoke orbital interactions with an ease that I had never seen. Physical organic chemistry seemed baked into their brains, as if they were all operating on some kind of mystical shared understanding of organic structure. I used to see scientists in this light often when I was a kid. They knew something that made them clever, and I wanted in!

Organic chemists catch quite a bit of flak for the way we use orbital theory. It seems as though all we do is draw blobs around lone pairs and bonds in Lewis structures (taking care to distinguish between sigma and pi bonds, of course) and call them real. But are they? Or do the physical chemists who yell that “real” molecular orbitals are always delocalized actually have a point?

Let’s just state it plainly: molecular orbital theory is difficult for students. It overlays the weirdness of quantum mechanics on chemical structures, which are expressed in a classical language of their own that’s hard enough to get a grip on. Plus, the upgrade in explanatory power that comes with replacing classical ideas with molecular orbital theory is typically not that great, especially in organic chemistry. Hell, deeply understanding electronegativity plus the language of Lewis structures and curved arrows can get you at least a B in most sophomore organic chemistry courses. As a result, adoption of molecular orbital theory by students is anemic at best. I think this, plus the anxiety over “what will the physical chemists think?!” described above, has caused organic chemistry instructors to shy away from embracing orbital theory in sophomore courses. This approach is misguided for two reasons: (1) molecular orbital theory does provide massive explanatory power when applied the right way, and (2) organic chemists’ “blobs” are just as legitimate as canonical molecular orbitals.

Organickers often use what we might call localized molecular orbitals, which sit on one or between two atoms. The beauty of this system—and the thing you quickly figure out watching an expert use it—is that you can draw orbital shapes right over bonds and lone pairs in structures. The whole system is based a deep faith in the Lewis model of chemical bonding and its artistic products, Lewis structures. There is a deep logic to orbital polarizations and energies within this system consistent with classical notions of where electrons are and are not.

If we replace the philosophical question “are they real” with the more practical question “do they work,” the answer is “absolutely!” This is where natural bond orbital (NBO) theory comes in. The NBO theorist affirms the organicker’s desire to use localized molecular orbitals. “Yes we can!” she says, “subject to a few conditions.” The bottom line is that NBO theory can be used to represent any molecular wave function, as long as we allow:

  • Mixing of atomic orbitals in any proportions (continuous hybridization)
  • Orbital occupancy of any value between 0.0 and 2.0 (trippy, but work with me here)
  • Mixing of hybrids in any proportions to make natural bond orbitals
  • Mixing of natural bond orbitals to make natural localized MOs (real delocalization effects, important to resonance-active molecules)
  • Mixing of NLMOs to make the overall wave function (again, straightforward…and necessary to satisfy the Schrödinger equation)

The second and fourth points are related: when NBO occupancies differ a great deal from 2.0 or 0.0, that’s the electron delocalization alarm going off. However, the beauty of NBO theory is that it helps one see that most molecules that we would expect to be very “Lewis friendly,” with wave functions adequately described by a set of localized MOs with integer occupancies, are in fact that simple! And for molecules that aren’t that simple, (a) it’s easy enough to switch into delocalized-MO mode and (b) strange occupancies point to election donation (1.7 instead of 2.0, say) or acceptance (0.3 instead of 0.0), which often aligns with our chemical intuition.

For the organic chemist, or even for the hobbyist who just wants to understand molecular orbital theory, my opinion is that NBO theory is a kinder, gentler theory. That doesn’t make it inferior to canonical MO theory in any way, shape, or form. On the contrary, in fact—the intuitive nature of NBO theory is its primary advantage.

Natural bond orbital theory needs its own Frank Lambert. Lambert is a known character to those of us who have taught introductory chemistry for a long time. The man made it his life’s mission to expunge the characterization of entropy as “disorder” from textbooks and teaching praxis and replace it with a more intuitive and broadly applicable description of entropy as “energy dispersal.” He runs an entire web domain devoted to the subject (secondlaw.oxy.edu, formerly secondlaw.com), has published numerous journal articles on the topic, and has ceaselessly advocated for it, for example by keeping track of descriptions of entropy in textbooks. Because of the sheer rarity of a single person moving the needle in such a dramatic way, Lambert is something of a legend among chemical educators. As the major advocates of natural bond orbital theory (Weinhold and Landis) are beginning to advance in years, I wonder if this area will gain its own knight in shining armor?

Welcome to Dispatches from Molecule World!

Every professional chemist has a little place that s/he retreats to when thinking about chemistry—a comforting submicroscopic place full of bouncing atoms and molecules. I call mine “molecule world.” As the weather’s turned cold here in Atlanta, I’ve decided to get back into the blogging game with Dispatches from Molecule World, a blog on chemistry and chemical education.

Stay tuned for commentary on recent papers or meetings in chemical education, chemistry-related posts (the first series will be on natural bond orbital theory), and who knows what else. Home brewing, chess, running, and dad life will also probably appear from time to time.