Most organic professors can agree that we want our students to understand concepts and big pictures rather than memorize a list of facts. When determining the outcome or major product of a reaction, I’ve found that using free energy diagrams is a great way to facilitate concept based understanding over memorization. But despite their long-term helpfulness, free energy diagrams were always challenging for my class. There are two major reasons. One, the student has to know the mechanism and have an understanding of charge stability to draw a correct free energy diagram. Two, no textbook that I had ever used made free energy diagrams a consistent priority. Joel’s textbook addresses both challenges and I attribute my students’ overall improvement in exam scores to the emphasis placed on mechanisms and their connection to the visual aid of a free energy diagram.
Free energy diagrams are first introduced in the acid/base chapter alongside a thorough discussion of charge stability. Students are shown how stability affects acid or base strength and are encouraged to use these diagrams to predict equilibrium outcomes of acid/base reactions. This is a useful aid in teaching a student to understand why one acid is stronger than the other and why a reaction’s equilibrium lies to the left or right.
Most students feel comfortable with the one step acid/base mechanism and learning the SN2 mechanism is usually easy as well. The challenge comes in learning multistep mechanisms and predicting the major product in SN2, SN1, E1, and E2 reactions. However, students are better prepared to handle multistep mechanisms from their exposure to elementary steps presented in Chapter 7 and the approach in Karty to labeling elementary steps gives students confidence. Finally, the diagrams, which the book uses early on and consistently, allow students to see the relative energy of each intermediate and the activation energy of each step and thus helps them truly understand multistep mechanisms.
For example, visualizing that the rate limiting steps of an SN1 and E1 mechanism are the same (see figure below, an overlap of Figure 8-1 and 8-2), helps students recognize why the two mechanisms are in competition with each other and why a mixture of substitution and elimination products usually results.
Determining the major mechanism between SN2, SN1, E2, and E1 is one of the most challenging topics for students. But this semester I found that students were able to make more accurate predictions and I believe this is due to a better understanding of kinetic control through the use of free energy diagrams.
On my third exam I gave a question that involved filling in the transition states and intermediates for a four step mechanism. I have given a similar question in the past for a one step mechanism and student performance had been very poor. To my (pleasant) surprise, not only were the students able to correctly draw the four step mechanism, they were also able to fill in each missing species in the free energy diagram and correctly identify the rate determining step.
I am currently teaching electrophilic addition reactions to conjugated dienes (in Chapter 11). This topic usually gives students trouble because they do not have a full understanding of kinetic vs. thermodynamic control. The Karty text introduces this topic in Chapter 9. So, when it came back up in Chapter 11, the students were easily able to distinguish the kinetic product vs. the thermodynamic product. The regular use of free energy diagrams helped greatly in explaining the regiochemistry of Markovnikov’s rule in 11.3 (Figure 11-4 below).
This semester’s exam scores have improved 5-10 points on average (3 exams given, ~200 students) over last year’s scores. Through free energy diagrams and the mechanistic approach, I have happily watched as my students transition from memorization to actually understanding the reasoning behind the rules.
– Marie Melzer, Old Dominion University
Marie Melzer teaches a mechanistically organized course at Old Dominion University. Click here to learn more about Prof. Melzer.