Making Backward Mechanism Arrows An Endangered Species

I’ve just finished grading the first exam for my Organic II course and I’ve experienced something I never have before. Of the more than 50 students that took my Organic I course last semester, none of them drew a single mechanism arrow in the wrong direction [1]. Yes, some of these students did get parts of mechanism questions wrong, but it wasn’t because they drew arrows incorrectly. The students that did draw arrows incorrectly were students that took Organic I at a different institution or previously used a different textbook. How do I account for these results? I credit Joel Karty’s approach, and his book, because of the excellent job it does stressing the fundamentals in the beginning (first semester) to lay the groundwork for what I just experienced in Organic II (second semester).

Karty’s book conditions students to first look at a molecule’s electronic properties before looking at what functional groups are in the starting materials, conditioning that allows students to then more easily draw new mechanisms. In his book, Joel labels above each reaction arrow the fundamental step that is occurring. This helps students to organize mechanism steps into categories and recognize patterns. I’ve not found any other textbook to do this as consistently. Identifying fundamental steps is further reinforced when students are solving the “Your Turn” problems that appear in each chapter.

I can see this approach working because of the change in nature of discussion in lecture and because of student behavior on exams. This semester it has been a rare occurrence to see students draw a wrong mechanism arrow when working problems in class. Not having to constantly re-explain mechanism arrows means students have more time to concentrate on determining what is the most likely nucleophile and electrophile. I have also noticed that more students draw mechanisms in the margins of their exam when they are unsure of how to answer a question. Organizing the course by mechanism has helped students to recognize that organic chemistry is about patterns and problem solving and not about pure memorization.

In lecture, I have really enjoyed putting on the board reaction schemes that we have not yet covered and watching as students successfully solve the corresponding mechanism. I was floored when a struggling student who barely passed Organic I came to my office last week and drew all the mechanisms of a five-step synthesis problem correctly without my help. I felt like I was in a Twilight Zone episode when she said that this multistep synthesis problem was her favorite problem ever. She had found her form. Organic finally clicked for her and the clear organization of mechanism steps played a big role in this transformation. Goodbye, backwards arrows!

— James Wollack, St. Catherine University

James Wollack teaches at St. Catherine University and is currently class-testing the Preliminary Edition of Joel Karty’s forthcoming textbook. Click here to learn more about Prof. Wollack.


[1] The average on this exam’s mechanism questions was over 80 percent correct, while the average on non-mechanism questions was standard. Overall, this led to the highest class average I have ever recorded on the first exam for Organic II at 72 percent. This was especially significant because this section’s average general chemistry grades were much lower than in the past, and because this exam was mostly organic reactions and less organic concepts like stereochemistry, naming, or equilbria. [Return]

One comment on “Making Backward Mechanism Arrows An Endangered Species

  1. Florencia
    November 18, 2014 at 2:57 pm #

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