When I teach nucleophilic substitution and elimination reactions, I find that students typically have very little trouble drawing each mechanism and predicting the products, so long as they are specifically told which reaction. But many students find one aspect very challenging: predicting the winner of an SN1/SN2/E1/E2 competition. In my first few years of teaching, I dreaded giving and grading my second exam, the exam that covered this reaction competition. Many good students would receive a very low grade, thus creating poor class morale for the rest of the semester. The SN1/SN2/E1/E2-competition exam was the exam where many pre-health students decided that their fate was sealed. In the years since then I have turned things around considerably. What did I learn? And what did I change?
This most important thing I learned was why students were struggling with predicting which reaction dominates—students were memorizing! I initially thought it was strange that students were memorizing because I felt that the textbook I was using clearly explained the various factors that favor one reaction over another; and I heavily reinforced those ideas in class. But then it became apparent to me why students still resorted to memorization. After treating each of those factors separately, the textbook I was using brought them together to summarize and make generalizations about the competition, as well as to provide exceptions. For example, if (1) the attacking species is both a strong nucleophile and a strong base, and (2) the substrate is primary, then SN2 will give the major product and E2 will give the minor product. An exception is when the attacking species is the tert-butoxide anion, in which case E2 will give the major product and SN2 will give the minor. As another example, if (1) the attacking species is a strong nucleophile but a weak base, and (2) the substrate is secondary, then both SN1 and SN2 can occur. In an aprotic solvent, however, we can expect SN2 to dominate. But if the substrate is benzylic, the solvent is protic, and the leaving group is very good, then we can expect SN1 to dominate.
Not surprisingly, when a student sees that 15 pages of chemistry has been distilled down to a single page (or a single table) of summary/generalizations, it is incredibly attractive to try to memorize them. There are two major problems that arise. One is that, by doing so, students circumvent the actual chemistry, so there ends up being no context for why a particular reaction dominates. This makes it easy for a student to forget a critical piece of information or become confused, especially given the large number of different combinations in which the factors can contribute. The second problem is that, in addition to memorizing the general rules, students must also remember which scenarios require additional information, such as when to consider solvent, the bulkiness of the attacking species, or the benzylic nature of the substrate. In other words, students feel that they must memorize exceptions in addition to the rules.
I’ve since had a huge effect on my students’ success with the SN1/SN2/E1/E2 competition by providing a system for deciding the winner of the competition, which consists of the following steps:
(1) Determine if the leaving group on the substrate is at least as good as F−. If so, then
(2) Examine the type of carbon to which the leaving group is attached.
- If the carbon is primary, then rule out SN1 and E1 unless the carbocation can be resonance-stabilized.
- If the carbon is tertiary, then rule out SN2.
(3) Construct the following table, and for each factor, add a check mark to the column for the reaction that is favored.
|Strength of attacking species|
|Concentration of attacking species|
|Type of carbon atom|
|Type of Solvent|
(4) The column with the most check marks is the winner.
(5) If there is a tie between substitution and elimination, heat favors elimination.
This system is also incorporated into my new textbook for two reasons. First, it gives students something to hang their hat on. Yes, students must still remember how to execute these steps, but the system for doing so does not change from one scenario to the next. The second great benefit is that when going through these steps, students will frequently be reminded why. When deciding whether a leaving group is at least as good as F−, for example, they must apply arguments of charge stability. When the carbon is tertiary, which rules out SN2, students will be reminded that it is due to steric hindrance. And it is this type of reinforcement that sets students up for being in command of other reactions they will encounter in the course.
— Joel Karty