When students ask me why learning organic chemistry is such hard work, I often begin by telling them that it’s just so different from general chemistry. At the beginning of the course, there’s plenty of overlap since students in organic chemistry must have a strong foundation in the structure, bonding, and properties of molecules. Eventually, though, we begin looking at organic molecules with a new level of detail and in a way that’s deeply visual when compared to their treatment in general chemistry. In my experience, this is the moment when students begin getting very confused and when I begin getting very excited.
Today was that day for me—I started Chapter 4 and taught my students about conformational isomers. Like Karty’s text, I began our class discussion by analyzing the structure of ethane via Newman projections. With a careful explanation of what the Newman projection represents from the line structure and the help of a model kit, I noticed that my students caught on pretty quickly. To assess, and further push, their understanding, I asked them to draw the seven conformational isomers of butane (rotating about the central carbon-carbon bond in 60o increments). Rather than drawing them in their notes, however, I asked students to sketch out the Newman projections on sticky notes. Then, they worked together to place the sticky notes onto a potential energy diagram to map the relative energies. The benefit to the sticky notes is that students can adjust their work by simply repositioning the paper.
As anticipated, the majority of students initially assumed that butane is identical to ethane: that all of the staggered conformers are equal in energy, that all of the eclipsed conformers are equal in energy, and that all of the eclipsed conformers are less stable than the staggered ones. But one or two students questioned the terminal methyl groups. Wouldn’t they impact the energy? As students questioned their assumptions, I handed out ball-and-stick models of butane. The audible “click” of the terminal methyl groups bumping into each other as they rotated about the central carbon-carbon bond helped students deepen their understanding. Below is a picture of the end result: sticky notes of Newman projections adjusted to correctly depict the relative conformer energies.
I like this activity because of its hands-on nature, both with the sticky notes and the model kits. I think it helps to bridge the gap between the models we use to illustrate bond rotation on paper versus what’s actually happening at the molecular level. Furthermore, the kinesthetic nature of standing up to position the sticky notes enlivens my students and breaks up the monotony of a long class period. What strategies are you using to help students connect 3D structures with illustrations on paper? I’d love to hear about other active learning strategies that you’re implementing in the classroom to help students better understand isomerism.
-Grace Ferris, Lesley University
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