When to Introduce Conjugate Addition: Sometimes More is More

There were two places in Joel’s text that surprised me: where 1,2 versus 1,4 addition to a conjugated diene appeared—Chapter 11—and where direct versus conjugate nucleophilic addition to polar pi bonds appeared—Chapter 17. Both of these chapters introduce basic concepts and then expand all the way to complex applications, much further than a functional-group organized text would go right away. I was worried that this seemingly vast amount of material would be too much for students to handle, but I was wrong.

Chapter 11, “Electrophilic Addition to Nonpolar Pi Bonds,” begins with an introduction of the topic and ends with conjugate addition of dienes, which initially surprised me. It felt like a lot of information to digest in one chapter. Once I started thinking outside the functional-group organization, however, it of course made complete sense to include conjugate addition to dienes in the same chapter as conjugate addition to alkenes. Indeed, the only reason I had the topic mentally linked to the Diels-Alder reaction was because it was usually in the “diene chapter” in a traditional texts. Bringing in conjugate addition, and introducing kinetic and thermodynamic products, did not make the standard electrophilic topics (addition of HBr, addition of H3O+) more difficult for the students. It actually made for a better experience, providing a bigger picture of reactivity which allowed students to make more connections between ideas (resonance, stability, reactivity). I honestly did not think that I could get my students, at the end of the first semester, to the stage where they could tackle a homework problem like 11.36 (below). But they attacked it, did well, and could explain to each other what the main concepts were. The idea that they were working with the same mechanism as before, simply under a slightly different application, added complexity to their learning and aided understanding.

Even after the success of Chapter 11, Chapter 17, “Nucleophilic Addition to Polar Pi Bonds 1: Addition of Strong Nucleophiles,” still seemed challenging to me. The usefulness of introducing conjugate addition in the same chapter that introduced direct addition was less obvious. Treating the nucleophiles as ones that would add reversibly versus irreversibly seemed like one of those topics that got tacked on at the very end of the second semester, buried within the world of Aldol/Michael/Robinson. I didn’t like those ideas being buried, just as I didn’t like how difficult it was to get students to appreciate the beauty of the Robinson Annulation, but I had accepted it as one of the casualties of late-April/early-May. Last year, to be honest, I held back on conjugate addition in Chapter 17, and held back from the parts of Chapter 18 that dealt with Aldol/Michael/Robinson until after Chapter 21 (“Nucleophilic Addition-Elimination Reactions 2: Weak Nucleophiles”). I put those topics where they were in traditional texts, how I taught it previous years, and it went just as well (or as badly) as previous years. It was clear that the students had lost the mechanistic plot, and could not readily make the connections necessary to understand the material.

This year, I trusted Joel’s text and ran Chapters 17 and 18 as he wrote them. Conjugate addition in Chapter 17 went well; there was a little resistance, and a little initial confusion, but with some pushing (and countless enolate resonance structures) they got there. They were comfortable enough with conjugate versus direct addition—concepts introduced the same chapter as the Wittig reagent—that they did not balk when I introduced Michaels addition. They recognized the Aldol condensation product as one being open to conjugate addition! In this case, as in Chapter 11, more material led to better understanding, since the material builds on mechanistic understanding.

-Michelle Boucher, Utica College

Click here to learn more about Prof. Boucher

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