There are no two ways about it—solving synthesis problems is one of the more challenging tasks that students face in organic chemistry. One reason for the difficulty is the sheer number of reactions we deal with throughout the year, perhaps a few hundred in total. Invariably my students ask me if they need to know all of them for synthesis, including the ones we learned last semester. In the past, my typical answer has been, “Yes, of course!” But the reality, at least in my opinion, is that some of the reactions we teach in undergraduate organic chemistry are much more useful for synthesis than others.
With that being the case, it would really benefit students to know which reactions those are, so that students could better focus their time mastering those reactions—especially when it comes to incorporating those reactions effectively in synthesis. Rather than just telling students which reactions to focus their time on, it occurred to me that students would benefit by uncovering those reactions for themselves. So, this year I designed an assignment to do just that.
As I began doing last year (see my previous post), during our coverage of Chapter 13 on multistep syntheses, I again showed students how to make use of the reaction summary tables in Appendix C (reactions that alter the carbon skeleton) and Appendix D (functional group transformations) to search for a reaction when a particular molecular change is called for in a synthesis. In addition, this year I also asked students to track, over a several-week span, how many times each reaction is used when solving synthesis problems.
The idea is simple: the more frequently a student invokes a particular reaction, the more useful the reaction is for that student. For convenience, I asked students to simply place a check mark next to an entry in the reaction tables in the appendices each time the reaction was used.
This assignment proved rather effective. Frequently in office hours, when students found themselves stumped trying to solve a synthesis problem, they would turn to Appendix C or D to find the relevant reaction. Having found it, the student would then place a check mark next to that reaction and often say something to the effect of, “I guess I need to study that reaction more!” Over time, the check marks would accumulate, sending a strong message to students as to which reactions seem to be more useful. Here’s an example of one student’s work:
For this student, oxymercuration-reduction and hydroboration-oxidation seem to be particularly useful reactions, whereas halohydrin formation is not as useful.
From a teaching standpoint, this new exercise has been very beneficial, as I have watched students more quickly gain confidence and independence working synthesis problems. As a result, I feel more comfortable pushing students to even greater heights in synthesis.