Mercedes and road kill. (Building 3D visualization skills)


Today I started the section on 3-dimensional chemistry. Up till now the only stereochemistry we’ve done is cis-trans and E-Z for alkenes, which is at least easily represented flat.

I started off with having them build butane models and hold them so they could se the Newman (Mercedes) projection.

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They rotated this while observing the anti, gauche, staggered, and eclipsed conformations. The more diligent among them dug out their textbook to match conformations with the energy chart.

After they got good at that, they had to practice building models from a given Newman projection, and sketching them in wedge-dash form, and also the reverse. The ability to convert a paper representation to a 3-D mental model, then communicate that on paper, is a critically important skill. I think it can best (only ?) be learned by holding and manipulating the physical models.

The inability to read and represent 3-D structures using the conventions of O-chem leads to a lot of “wrong” answers. This is due to the student’s misunderstanding the given structures, and then incorrectly drawing the products, which may even be correctly conceived. Drawing without using the wedge-dash convention often results in a flat structure. To emphasize the problems with flat structures like this, I’ve taken to calling them road-kill structures: they sort of look right, but not very much.

Tomorrow, conformations of rings, especially cyclohexane. Classically, these are called “boat” and “chair”, but someone pointed out that the boat looks more like a hammock. So hopefully I can get some pix that illustrate axial and equatorial positions. Too bad I didn’t do it this evening before it got dark. It would have been a perfect night for it… Damn lab reports…

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