Spatial Visualization in Chemistry

I’ve started reading Multiple Representations in Chemistry, a compendium of articles exploring the challenging link between Johnstone’s Triangle and how chemists use different visualizations (even for the same molecule). Over the years, I’ve been paying attention to these aspects of my introductory chemistry courses (G-Chem and non-majors Chem), for example when we discuss chemical reactions, but I had not pondered many of the issues that arise in O-Chem. Chapter 1 (“Learning at the Sub-micro Level: Structural Representations”) provides several interesting examples. I’ve picked out three for today’s blog post.

 

Let’s look at chirality. This is not often emphasized in the G-Chem syllabus, and I’ve only explicitly covered it a few times when we used a textbook that included it in the chapter that included molecular shape and hybridization. I do however cover it regularly in my non-majors class (where we spend four weeks on organic chemistry at a very simple level). Here’s a Figure showing the visualization task.

 

Did you find the task easy? I didn’t. And I’ve been teaching for a long time. Although I’m probably figured this out much faster than most students (simply from experience). The folks who studied this task report that “most students employ a strategy of rotation to align a corresponding bond in each structure, thus reducing the problem to a 2D consideration…” Another interesting tidbit is that “students found mental rotation around the vertical bond easier than about the other bonds.” Thinking about my own comfortability, I found that I tend to draw one of the thin lines either in a vertical or horizontal position because that seems to be the easiest for me to visualize. If they’re both diagonal, I have more difficulty. I find this interesting, never having considered it before.

 

The second example is one that I haven’t used in G-Chem, and since I don’t teach O-Chem, I haven’t used it in class either – although I’m very comfortable using it as a visual aid for myself. I’ve also used it when trying to explain dihedral angles to my research students. Here’s the Figure below from a task asking students to name molecules, one with a typical “structural” drawing, and the other with a Newman projection.

 

The study showed that most students had no trouble naming the structural drawing on the left (using the standard rules one learns in O-Chem), but many had trouble with the Newman projection on the right. Now it seems to me that all one needs to do is redraw the Newman projection into the corresponding structural drawing. I don’t know if the students did this (the text doesn’t say and I didn’t look up the original paper) but apparently after being asked in two further exams, students still found it problematic. Did they not learn it after having trouble the first time? I don’t know. It does say that the students complained that the question was “not fair”.

 

Here’s the final example I will highlight today. This involves drawing the product of a standard S_N2 nucleophilic substitution. The only difference in the two questions is whether the hydroxide is shown on the left or the right of the reactant organic molecule. Look at the dominant answer provided by students in both cases.

 

 

If you teach O-Chem, you might slap your head in dismay after all that time you spent discussing inversion of stereochemistry and “back-side” attack. But if you’ve seen this error before, chances are you’re incorporating examples in class so that students don’t keep making this sort of mistake.

 

Reading all this made me think about Resonance structures. When we get to this point in Lewis structures, I normally make a song-and-dance in class about how to think about this in terms of the delocalization of electrons. We talk about different representations and the limitations of models and different ways of drawing this structure on a two-dimensional flat surface. Or I should say I talk about them in class. (This post briefly mentions how I discuss it.) Now I’m not sure whether the student really understand what I’m talking about. (We went over this in my G-Chem class last week.) I think I should think about writing up some supplementary information for my students, since this concept likely remains fuzzy for the students.