Taking the Fear Out of Lengthy Mechanisms: A Good Type of Problem from Chapter 7

Even if a student intends to devote a great amount of time and effort to studying mechanisms and to using mechanisms to understand reactions, their efforts can be easily thwarted by the sheer intimidation of relatively long mechanisms. Years ago, when I was still teaching under a functional group organization, I would hear gasps and groans from my students—even my better students—each time I worked through a mechanism having as few as six, five, or four steps. Nowadays, learning under a mechanistic organization, my students take these kinds of “lengthy” mechanisms in stride, and I think a lot of it has to do with the preparation they receive from one of my favorite types of problems in Chapter 7. A good example is Problem 7.30, which deals with a Fischer esterification reaction:

Although the mechanism is still presented in its entirety in this problem, there are a number of factors that diminish intimidation:

1. Students are told that the reaction will be covered in Chapter 21, reassuring them that they will not be held accountable (yet) for nuances of the reaction or for aspects such as predicting products or using the reaction in synthesis.
2. The problem is written in such a way that each step of the mechanism can be treated independently of the other steps. So, at this stage, students do not need to see the “big picture” of the mechanism to solve the problem (though they can certainly follow the changes in each step to see how the mechanism accounts for morphing the initial reactants into the final products). Moreover, in this low-risk environment, students get to practice their newly learned skills of identifying aspects of what drives each elementary step (part a of the problem) and drawing curved arrows (part b).
3. From part c, students are reassured that each step is one of the ten most common elementary steps they learned in Chapter 7. This not only reinforces material from Chapter 7, but also reminds students that the bulk of the mechanisms they will encounter are constructed from the same ten elementary steps. And that sends a loud and clear message to students that mechanisms—even really lengthy ones—help to simplify organic chemistry a great deal.

-Joel Karty