No More Fearing Alkenes and Alkynes

Since this is the second year I am using Joel’s text, I was sure that I would be comfortable with the syllabus and schedule I set. But I again became nervous as I approached Chapter 11, “Electrophilic Addition to Nonpolar Pi Bonds.” Years of slogging through additions to alkenes and alkynes, working example after example and countless variations of the same mechanism while students thought I was performing magic, left me skittish. Had I left enough time for this chapter? How on earth was anything going to stick this close to Thanksgiving? Sure, it worked great last year, but would it work again?

I started Chapter 11 with the same approach I used last year, telling my students that they were going to help me teach it. I claimed there would be nothing new to them, that everything was just variations on a theme they had already learned with their simple mechanistic steps. I started off with cyclohexene, the hydronium ion, explained that the first step was electrophilic addition, and instructed them to use a mechanism to deduce what the product would be. I broke them up into small groups, and each group produced cyclohexanol as their product! I next put 1-methyl cyclohexanol as the starting alkene on the board and asked them to help me through the mechanism. As a class, they proposed Markovnikov’s Rule and worked through questions of stereochemistry with ease. I kept asking students what happens next, expecting to see blank stares or averted eyes, and I kept getting answers.

The success of this lecture—and Joel’s approach—can for me be summed up with one student comment that earned her a high-five: “After the addition, it’s just a carbocation, right? And so it’s just coordination, or coordination after some rearrangement stuff, right? Nothing hard?”

It shouldn’t be hard, and once students can use the language of organic mechanisms, it isn’t hard. Chapter 11 is the dreaded “alkene” chapter because my thinking is still organized by a functional group approach. But I have to keep reminding myself that for students, this is not the “alkene” chapter where an entirely new mechanism is introduced alongside a new functional group. This chapter is simply concrete examples of reactions that follow mechanisms that start with electrophilic addition (a step they know from Chapter 7) and then use coordination/proton transfer/resonance/re-arrangement (steps covered in-depth in previous chapters). With Joel’s text, and a mechanistic organization, they are finally able to see what I’ve been saying all along: these reactions are all really just variations on themes they already know.

Compared to functional group texts, it does take longer to get to alkenes, alcohols, and synthesis. Once we get there, though, it is easy to pick up speed. Chapter 11 covers hydration and hydrohalogenation of alkenes, hydration of alkynes to ketones, and kinetic and thermodynamic addition to dienes. Putting these topics together into one chapter makes so much sense, and it is amazing how fast all these topics can be thoroughly covered in class when the students arrive at the chapter already understanding the mechanisms. I intended to give Chapter 11 three lectures, and I ended up covering all the material in just over two lectures. We actually had extra time to work a few of Joel’s problems. 11.46, below, was challenging, but not impossible, for the students to complete in ten minutes.

Even though there is a week left of our fall semester, I am already getting excited about the spring. The second semester is where so much of the fun stuff happens, and it is clear that the students are in a place where we can begin to enjoy and appreciate the fun stuff together. So often I am preparing students for the fall final with the terrible feeling that they have only a tenuous grasp on the material and that they might not remember enough for the final, much less the spring. I am not hopelessly naïve, I know that much will be forgotten over winter break, but I am now optimistic that their foundations are solid and that, with some help, we will quickly be back on track. I have told the students time and again that if they understand the mechanisms, they understand the material. With Joel’s text, I have finally been able to get most of them there.

-Michelle Boucher, Utica College

Click here to learn more about Prof. Boucher

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