My students learn organic chemistry in a mechanistically organized course and I want to make sure they really understand how the mechanisms apply to reactions that are synthetically useful. There are many approaches that I use to reinforce their learning such as quizzes, practice problems, and SmartWork assignments. I previously talked about why I am a believer in using the lab portion of class to help reinforce the concepts covered in lecture. One important aspect is the reports assigned to each lab. I have found that students benefit from explaining how each mechanism works, using not just arrow pushing diagrams like on a quiz or practice problem, but also by being required to explain each step and aspect of the mechanism in their own words. This exercise forces them to demonstrate how much they truly understand.
In my last post, I discussed using a series of reactions to help students understand the conditions that favor SN1 and SN2 reactions. In the fall, we compare E1 and E2 reactions with a similar approach. Using like substrates with different conditions, we monitor the reaction outcomes by gas chromatography (GC) and NMR. GC is a quick and powerful technique that conveniently separates and analyzes a mixture of alkene isomers. After we look at a typical dehydration of an alcohol for an E1 reaction, we use the alcohol’s corresponding alkyl chloride as our E2 substrate. We do the E2 reaction using both methoxide and tert-butoxide for our bases. The reactions are done on a small enough scale that they can generate enough for an NMR and GC in a short amount of time.
Analyzing the results reinforces the concepts from the text. First, they see that their predicted major products are in fact the major products. They also observe the significant formation of minor products. When looking at organic chemistry, it is easy to narrowly focus on the major products formed in a reaction without giving the side products their due credit. Understanding mechanisms allows us to understand the pathway and regiochemistry that a majority of the molecules will take. It also shows that there are competing pathways and how a change in conditions, such as the choice of base, has such a huge impact on the selectivity of the reaction. Come test time, they will remember the difference between E1 and E2 reaction outcomes and understand the influence that the bulk of the base has on an E2 reaction, not only because I have stressed in it class, but also because they have carried out the reactions themselves!
Analyzing the results is one thing, but I have found that how they report their results can also reinforce learning mechanisms. Throughout the semester, I give my students smaller lab write-ups, each focusing on a different portion of the experiment. They are instructed to write their results as if they were being submitted for publication and I ask them to use the formatting style of scientific journals as the basis for each section of their report. One week they may focus on the background of their experiment, another week may be focused on the experimental methods, or results and spectral analyses. They receive feedback on areas for improvement throughout the semester.
For the final two series of experiments (comparative substitution reactions and comparative elimination reactions), they do a significantly bigger lab report; a full report with all of the sections included. Incorporating the background on these particular reactions and reflecting on their results strengthens what they heard in class and observed through experimentation. I have found that requiring this large amount of writing about organic mechanisms helps students reflect and connect the ideas that we cover in the lecture.
-Nathan Duncan, Maryville College