Chapters 6-10 incrementally ramp up the types of things we hold students accountable for when it comes to reactions. Chapters 6 and 7 introduce students to the 10 most common elementary steps. Chapter 8 deals with constructing multistep mechanisms in reasonable ways. In Chapter 9, students learn how to predict the outcome of SN1/SN2/E1/E2 competition. Then, in Chapter 10, students are presented with nine types of synthetically useful reactions that proceed by nucleophilic substitution and elimination mechanisms. Over the years, I have found that students handle the progression from Chapter 6 through Chapter 9 in stride, but they appear to need some additional “coaching” when it comes to Chapter 10.
Before going into what I’ve learned about coaching students through this transition, let me first tell you why I think students need additional coaching at this point. I think it’s because of the expectation that students have built up about reactions by the time they get to Chapter 10; students expect that they should be able to deduce what is going to happen in the reaction. This should not be surprising. After all, when students are given a predict-the-product problem in Chapter 9, we ask students to slow down and take their time to systematically consider all the factors that control the relative rates of SN1, SN2, E1, and E2 reactions to make their predictions.
In Chapter 10, I noticed that students expect to approach predict-the-product problems similar to how they approached problems in Chapter 9—that is, by slowing down and pondering to deduce what is going to happen. But students must become more efficient with the reactions in Chapter 10, because these are reactions that they will need to begin incorporating into syntheses. Yes, the mechanisms in Chapter 10 are fundamentally very similar to the ones in Chapter 9. But unlike the reactions from Chapter 9, the reactions in Chapter 10 are not particularly variable; for the reactions in Chapter 10, when a compound with a particular functional group is treated with a particular reagent, the same reaction and mechanism take place essentially every time. Therefore, rather than approaching each predict-the-product problem with a deduction mindset, students need to develop a quick recognition of what’s going to happen, and they need to be made aware of this expectation at the outset of Chapter 10.
To help coach students through the necessary change in mindset for Chapter 10, I remind them that the mechanism enables us to understand why a reaction does what it does, and that the mechanisms aren’t changing from Chapter 9 to Chapter 10. But I further stress that with each new reaction we encounter, we also need to have, at the forefronts of our minds, the answers to two very important questions: (1) What changes to the molecule take place in the reaction? and (2) Why is it important to be able to make those changes? For example, when we discuss the reaction of an alcohol with PBr3, the answer to Question 1 is straightforward: OH converts to Br, and the configuration at the attached carbon is reversed. And for Question 2, we talk about the importance of being able to change a poor leaving group into a good one.
Another thing I do to help coach students through the transition to Chapter 10 is ask them, “How do you know what each reaction does?” For example, in class, I’ll ask students, “How do you know that PBr3 converts an alcohol into an alkyl bromide?” Some of the first answers students give me include, “Because the OH group is nucleophilic,” or “Because the first SN2 step makes a good leaving group.” When I hear those answers from students, I’ll reply by saying, “No, that’s why the reaction does what it does. How do you know?” Eventually, a student will answer, “Because it says so in the book?” And after a few other students chuckle a bit, I’ll say “Exactly!” The point I make to students is that these are tried and true reactions, and the mechanisms for them have already been worked out by other people. Therefore, it is the students’ job to understand and draw the mechanisms, not deduce them.
If we fast-forward to an exam situation where students are asked to draw the mechanism and product for a reaction, students should not be taking the time to ponder what’s going to happen. Rather, students should, by that time, immediately know how the reaction will proceed and be able to quickly draw the mechanism. To get to that point of efficiency with any reaction, however, students must study the mechanism when it is first presented to them, ask questions about aspects of the mechanism that might be confusing, and then practice (more than once!) drawing the mechanism to identify places where they tend to make mistakes.
Taking the extra time to coach students through the transition from Chapter 9 to Chapter 10 pays dividends. Preparing students with the right mindset to deal with Chapter 10’s reactions removes some of the anxiety that they otherwise feel, and it also sets students up for success when they encounter other new reactions throughout the rest of the book.
-Joel Karty, Elon University
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