Mechanisms can greatly simplify organic chemistry thereby allowing us to draw connections between reactions that might otherwise appear to be unrelated. With some reactions, however, I have found that the way in which the mechanism is presented can have a dramatic effect on whether a student successfully makes these connections. Consider, for example, the halogenation of an alpha carbon. The mechanism under basic conditions is often presented as in Scheme A here:
I taught this mechanism this way for several years, but I noticed that many of my students struggled with it. I recently made some changes to how I present this mechanism—in class and in my textbook—that seem to have benefited students significantly. What makes this mechanism difficult for some students to handle? And what were the changes I made?
One of the difficulties of this mechanism stems from the fact that in the second step the alpha C atom of the enolate anion gains the bond rather than the O atom. The O atom has the negative charge, however, so it sticks out like a sore thumb as being electron rich, making it incredibly tempting for students to draw a curved arrow directly from O– to the electrophile.
Another problem with this presentation becomes apparent when trying to identify (and teach) the elementary steps taking place. I see the first step as a proton transfer and the second as an SN2 step; steps that students learn early on. However, the curved arrows in each step above don’t look like the proton transfer and SN2 steps that students are accustomed to seeing:
The source of these problems is that the enolate anion has resonance, and only the strongest resonance contributor is shown. However, it is the minor resonance contributor—in which the negative charge appears on the alpha C atom—that better describes the enolate anion’s behavior in the second step. Recognizing this, I began teaching the mechanism as in Scheme B here, where I show both resonance structures:
Now students can more clearly see the nucleophilic character of the alpha C, and, more to the point, the curved arrow notation looks a lot more familiar. At the same time, students are reminded of the importance of the contribution by the resonance structure in which the negative charge appears on O.
I apply the same strategy to other mechanisms involving species that exhibit resonance, and I find it to be particularly helpful to students when introducing alpha halogenation under acidic conditions. Typically, the mechanism (after enolization) is drawn as in Scheme C here:
We see that the second step is a proton transfer, but what is the first step? It could be viewed as an electrophilic addition to the C=C double bond (with an additional curved arrow from O). But, by incorporating the resonance structures the same way as I do under basic conditions, students can see that the step is very reminiscent of an SN2 step.
I like this because of the familiarity it adds to the mechanism. Additionally, I think it gives students comfort to know that the mechanism under acidic conditions consists of the same steps that constitute the mechanism under basic conditions (just in a different order). Seeing these patterns are the kinds of things that help students make connections among various reactions.
— Joel Karty