At the end of the semester the students are typically burned out, busy with all of their final assignments, and in general do not perform as well on the last unit exam compared to the other three units. At Old Dominion University we teach addition to alkenes and alkynes in the last unit, a very important topic. This unit has historically had the lowest average compared to the other three unit exams. In the past I attributed this decline in scores to a lack of disciplined study and preparation on the students’ part. However, I did not see a decline this past fall semester when using Joel’s text and mechanistic organization for the first time. I attribute this improvement to 1) a focus on learning mechanisms and understanding rather than memorization, 2) the topic being broken up into two smaller chapters, and 3) students being now equipped to handle multistep mechanisms from their knowledge of the 10 elementary steps from Chapter 7.
In the functional group approach, the addition reaction chapter is dense and the content is presented in a way that leads students to memorize the reactions rather than understand the mechanism. For example, there are three ways in which an alkene can be transformed into an alcohol and most texts present these together—because the same functional group results from the addition reaction. However, the regiochemistry and stereochemistry is different for each alcohol product (Markovnikov vs. Anti-Markovnikov, syn vs. anti). In my experience, students have not understood why the alcohol products are different. In order to fully understand this difference, students must understand the mechanism. In reference to this topic on exams, I typically ask students to draw out the mechanism for each reaction. If the student understands the mechanism they understand why each reaction leads to a different alcohol product. Historically, I find that students are unable to correctly draw the mechanism for these reactions when the material is presented based on the functional group approach. When I switched the course organization, however, I noticed a drastic improvement in the responses to this question.
The mechanistic organization allows the addition reactions to be separated into two chapters based on the two types of addition mechanisms—formation of a carbocation (Chapter 11) and formation of a cyclic transition state (Chapter 12). The separation of the addition reactions into two smaller chapters really helped the students compartmentalize the information and approach the reactions step by step.
Addition reactions to alkenes and alkynes are almost always more than one step. In a text book organized by mechanism the students have the necessary background to put together multistep mechanisms because they have been trained to recognize elementary steps. Thus, for the following multistep mechanism problem (11.16 in text), the student only has to learn one new elementary step in this chapter, namely electrophilic addition.
Instead of the typical decline I always see at the end of the semester, this past fall the unit four scores improved from 68% average to a 74% average. The passing rate improved from 47% to 60%.
– Marie Melzer, Old Dominion University
Marie Melzer teaches a mechanistically organized course at Old Dominion University. Click here to learn more about Prof. Melzer.