We started Chapter 9 in class a couple weeks ago, where we learn how to predict the outcome of the SN1/SN2/E1/E2 competition. Similar to how it’s done in most books, we do this by first learning about the major factors that influence the rate of each reaction in this competition. But unlike other books, this is not where students encounter some key, challenging aspects of these reactions for the very first time. Rather, students will have had significant exposure to some of these major factors, like solvent effects, from earlier in the book. Because students revisit these ideas in Chapter 9 as opposed to learning them anew, I can focus much more tightly on the topic at hand: the competing reactions. My students come away with a better handle on things and are able to predict the outcome of this competition more effectively than they have in years prior.
When we discuss solvent effects on the SN1/SN2/E1/E2 competition in Chapter 9, it will actually be the third time that my students will have been exposed to the impacts that protic versus aprotic solvents can have. The topic is introduced in Chapter 2 in the context of solubility and intermolecular forces, which is an area of some comfort for many students entering the course. In general chemistry, students were taught why ionic compounds like NaCl are water-soluble, resulting from the solvation of each ion by water’s dipole. In Chapter 2, students learn that NaCl is insoluble in an aprotic solvent like DMSO, even though DMSO has a significantly larger dipole moment than water. Students come away with an understanding of why protic solvents solvate anions much more strongly than aprotic solvents do and, furthermore, students associate this solvation with being strongly stabilizing.
In Chapter 6, we visit the topic again when I assign end-of-chapter problem 6.82 (below). This problem asks students to think about why acetic acid is much weaker in DMSO than in water (pKa = 12.6 vs. 4.75). Students can rationalize that water, being a protic solvent, solvates the anionic carboxylate product substantially more than DMSO does, which lowers the product energy in the free energy diagram.
Finally, in Chapter 9, students learn that the same solvation phenomenon weakens anionic nucleophiles dramatically more in protic solvents than in aprotic solvents, ultimately causing the reversal of some nucleophilicities. Students see this solvent effect as an extension of their previous two encounters with the topic, so there is no need for me to take a major detour from the main theme of the chapter: predicting the outcome of the competition. Instead, I’m able to treat it more as a refresher, and students take it in stride.
This is very different from my experience teaching solvent effects in the SN1/SN2/E1/E2 competition in the early part of my teaching career. The first exposure that students had to the topic of solvent effects was in the middle of our discussion on predicting the major products of the SN1/SN2/E1/E2 competition. So, while students’ heads were spinning from just having learned the mechanisms for these reactions, and new concepts like nucleophile strength, leaving group ability, and carbocation stability, they were hit with yet another whammy: solvent effects and the reversal of nucleophile strengths. Frustration ran high.
Looking back at how I used to teach this material, it’s clear to me why my students were driven to memorize. With so many key concepts taught at the same time, in the context of new, unfamiliar topics, students didn’t have much of a choice. The way I do it now, introducing some concepts like solvent effects earlier, in the context of familiar topics, students can better grasp the concepts initially and have time to digest before applying them to unfamiliar topics like the SN1/SN2/E1/E2 competition. In fact, this philosophy is no different from the one I use to build up the ideas of mechanisms. Students learn about elementary steps and curved arrow notation in the familiar context of proton transfer reactions in Chapter 6, then build on that understanding when they learn new elementary steps in Chapter 7. They learn about aspects of multistep mechanisms in Chapter 8 before being held accountable for predicting products of those reactions in Chapter 9. It is a philosophy that has proven to be very effective for students.