Each year, as my students complete Chapter 8, I find tremendous value in assigning a handful of written mechanism problems that not only challenge students, but also reinforce important lessons about mechanisms that we learned throughout Chapters 7 and 8. This year, I assigned the following two mechanism problems (not found in Chapter 7 or 8), and the impact they had on my students was remarkable, setting up several “teaching moments” in my office hours.
I want to share some of those teaching moments by presenting the issues that students brought to me, and how I helped those students work through the issues with the help of the textbook.
Student Issue 1: Struggling with how to begin.
Teaching Moment 1: I asked students what differences they saw between the structure of the reactant and the structure of the product, and further asked them which elementary steps from Chapter 7 have the potential to accomplish those changes. That was key to getting students to start making progress.
Student Issue 2: Focusing on the triple bonds in the product appearing in different locations than in the reactant.
Teaching Moment 2: I asked students which elementary steps from Chapter 7 exhibit triple bonds moving. The answer is none, so I asked the students to focus on other differences in structure between the reactant and the product.
Student Issue 3: Focusing on the methyl group in the reactant appearing in a different location in the product.
Teaching Moment 3: I asked students which elementary steps from Chapter 7 exhibit a methyl group moving across four carbon atoms. The answer is none (the closest one is a 1,2-methyl shift), so I asked students to focus on other differences still.
Student Issue 4: Struggling to see that the entire mechanism consists of only proton transfers.
Teaching Moment 4: The issue stems from the fact that the given molecules were drawn as line structures. Therefore, I asked students to draw the complete Lewis structures of the reactant and product molecules. Immediately, students realized that C-1 must gain two protons and C-5 must lose two protons. And when I asked which elementary step accomplishes the gaining or losing of protons, students quickly settled on proton transfer steps.
Student Issue 5: Struggling with whether to protonate or deprotonate in the first step.
Teaching Moment 5: Even after students realized that the entire mechanism can be constructed just by stringing together proton transfer steps, some students still attempted to add a proton to the organic compound in the first step of Problem 1a, or to remove a proton in the first step of Problem 1b. I responded in one of two ways. As one response, I reinforced the idea that it makes more sense to remove a proton first under basic conditions and to add a proton first under acidic conditions—an idea that students first encountered with tautomerization mechanisms in Section 7.9. As the other response, I pointed out the violation of the compatibility rules that students learned in Section 8.6a: strong acids should not appear in a mechanism under basic conditions, and strong bases should not appear in a mechanism under acidic conditions. Protonating first in Problem 1a would produce a carbocation, which is a strong acid, whereas deprotonating first in Problem 1b would produce a carbanion, which is a strong base.
Student Issue 6: Struggling with whether to protonate or deprotonate in the second step.
Teaching Moment 6: For Problem 1a above, students accomplished the first step by deprotonating C-5 to produce a carbanion. To complete the mechanism, another proton would have to be removed from C-5 and two protons would have to be added to C-1. Some students struggled with which step to carry out next. In response, I asked those students to show the outcome of deprotonation in the second step, which resulted in a -2 charge. Invariably, students agreed that a -2 charge is too unstable, and then rationalized that it made more sense for the second step to be protonation. Similarly, for Problem 1b, protonating in the second step would result in a +2 charge, so students rationalized that it made more sense to deprotonate in the second step.
Student Issue 7: Rationalizing why the carbon that needs to pick up or lose a proton in the second step is actually basic or acidic.
Teaching Moment 7: When a student deprotonates C-5 in the first step of Problem 1a, the negative charge appears on C-5 in the product. For the second step, C-1 must pick up a proton. However, C-5 jumps out as the basic site, not C-1. For students struggling with this dilemma, I ask them, “What is special about a species that has an atom with a lone pair attached to a double bond?” This jogs their memory about the features for resonance that we learned in Section 1.11, and students will promptly move the electrons to arrive at the resonance structure that has the lone pair and the negative charge on C-1. Similarly, for Problem 1b, after a proton is added to C-1 in the first step, students find it helpful to work with the other resonance structure to remove a proton in the second step.
Once students successfully complete the first two steps of either Problem 1a or 1b, they quickly realize that it’s just a matter of repeating those two steps in the same order to arrive at the final product.
Rarely will you find problems that offer so many teaching moments. But I have found the two problems above to be gems. And more than simply offering numerous teaching moments, these problems also offer the perfect timing. That’s because I assign the problems toward the end of Chapter 8, when we are just transitioning into multistep mechanisms. Therefore, students come away with valuable insights into approaching new reactions and new mechanisms—insights they will rely on regularly in Chapter 9 and beyond. Furthermore, students end up buying into mechanisms even more—the idea that mechanisms are not only powerful tools to help understand reactions, but are also very manageable and worth their while to learn.
As I said, I have found these particular mechanism problems to be incredible gems. If you try them out with your students, I hope you do, too!
-Joel Karty
