Every time I teach Section 3.9, which covers rotations about single and double bonds and cis/trans isomerism, I’m reminded of how valuable an exercise it is to have students determine whether a particular double bond can have cis/trans configurations possible. It may not immediately jump out at you as a valuable exercise, but consider this. I had my students answer the following clicker question in class. Section 3.9 was part of the assigned reading for the day, and students were allowed to use their textbook. But only 25% of my students got it right.
When I went over the answer with the class, it was not surprising to learn that students struggled with the second molecule in particular. They had no trouble describing what it takes for cis/trans isomers to exist: the exchange of two substituents bonded to one of the doubly bonded atoms results in a different molecule. But what became clear through that discussion is that students were struggling with the idea of what it means for two molecules to actually be “different.” So that’s what we focused on. We drew the structures of the molecule before and after exchanging the H and F atoms on the right end of the double bond (exchanging the two methyl groups on the left would have been trivial).
Then we built both molecules using the molecular modeling kit and compared them.
Of course when brought the two molecules together without reorienting either, the H and F atoms didn’t match. But when we rotated one of the molecules 180° over the axis of the double bond, everything matched. Therefore, we attempted to achieve a different molecule by swapping the two groups but ended up with an identical molecule, so the conclusion is that cis/trans configurations don’t exist for this double bond.
We then repeated the exercise with the following molecule, for which cis/trans configurations do exist.
In this case, students saw that after we tried every orientation possible for the two molecules, there was no way to get everything in one molecule to match with everything in the other molecule. The two molecules were indeed different.
Students learned two important lessons from this exercise of comparing molecules built with a model kit:
- Do not limit yourself to just the orientations you are given. Try every orientation.
- For two molecules to be identical, every atom in one molecule must line up perfectly (not almost…) with every atom in the second.
Otherwise, the molecules are different.
These lessons are vital for the tasks that students face in Chapters 4 and 5 to determine whether two given molecules are conformers, enantiomers or diastereomers. But I find that it is advantageous to address these things in the context of cis/trans isomerism in Chapter 3 where there is less going on—namely, no single bond rotations and no subtleties of dash/wedge notation. Moreover, the above exercise serves as an early litmus test for students who might struggle with the three-dimensional nature of the tasks to come. Students who find the above exercise challenging have time to seek help—to make themselves better when it comes to determining whether two given molecules are the same or different—before finding themselves in the midst of the more difficult tasks that come in Chapters 4 and 5.
