We started resonance structures in class today. It wasn’t until after class had ended that I realized how many great teaching moments we had, and I attribute those teaching moments to the way resonance is presented in my textbook. Resonance is introduced in Chapter 1, and students are taught to draw resonance structures by first identifying one of four features:
Once we identify which feature is present, we add the curved arrows that are prescribed specifically for that feature. Then, to draw the new resonance structure, we rearrange valence electrons according to the directions given by the curved arrow notation.
The first example we worked with in class today was this species:
We began by identifying Feature 1:
Next, we added these curved arrows:
And finally, we moved the electrons accordingly to arrive at the other resonance structure:
The teaching moments came when students started asking the following questions:
Q1: Why is the double bond at the top not involved in resonance?
Q2-3: Why can’t we draw the following two curved arrow notations?
I found myself answering these questions in essentially the same way: Each feature tells us that there’s another resonance structure, and the curved arrow notation that belongs to each feature tells us how to move valence electrons so that we end up with another valid Lewis structure. Therefore, if electrons are not part of one of those features, we shouldn’t attempt to move those electrons. If we do, we’ll end up with a structure that is invalid. Furthermore, if we identify a particular feature and then apply a curved arrow notation that is different from the curved arrow notation that belongs to that feature, we will also likely end up with a structure that is invalid.
I proceeded to demonstrate my points by answering the three questions from earlier:
Q1: Why is the double bond at the top not involved in resonance?
A1: The double bond is not part of a feature for resonance. The atom with a lone pair is not connected to the double bond; it’s too far away.
Q2: Why can’t we draw the following curved arrow notation?
A2: If we move the electrons according to the curved arrow notation above, we will end up with the structure below, in which carbon has exceeded its octet.
Q3: Why can’t we draw the following curved arrow notation?
A3: If we move the electrons according to the curved arrow notation above, we will end up with the structure below, in which carbon has exceeded its octet.
These teaching moments are not limited to just drawing resonance structures; they are far-reaching. First, the skills that students apply to drawing resonance structures (arrow pushing, octets, etc.) also apply to drawing mechanisms. Therefore, identifying and correcting mistakes like the ones above sets students up for success when we start talking about reactions in a few weeks. Second, the way I went about answering their questions above models how students can improve their self-learning skills in organic chemistry. After students ask a question that begins with “Why can’t…?” or “Why doesn’t…?”, they should ask themselves, “What if…?” Then, they should draw the outcome and ask, “What’s wrong with this picture?” I have found that when students go the extra step to simply draw the outcome, they can quite often answer the question on their own.
And all of this was on the second day of class!
-Joel Karty, Elon University
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