A framework for probing student understanding of organic mechanisms
In a recent study, researchers from the US investigated students’ written descriptions of an organic mechanism. They found that students were able to say what was happening but were not always able to explain why. The study also demonstrated that the method used to investigate students’ descriptions – writing-to-learn (WTL) – can assist teachers in revealing their own students’ mechanistic reasoning.
The researchers wanted to probe students’ descriptions so that they could better understand how students conceptualise the electron pushing formalism. Chemists use electron pushing, or ‘curly arrows’, to explain or predict the outcome of reactions. Electron pushing has been described as a language students must understand before they can engage in mechanistic problem solving. An awareness of what the ‘curly arrow’ means to students helps to ensure that they connect them to the correct fundamental concepts.
Writing to learn
To get students to describe a reaction, the researchers used a WTL assignment centred on the notorious drug thalidomide. The WTL pedagogy involves students producing a written account of their understanding of a topic or concept. Although similar to an essay, WTL activities are principally about students exploring ideas, clarifying thoughts on a concept, and learning. They are not assignments to be graded.
The WTL prompt sheet in this study asked students to describe the history of thalidomide and its current uses. It also asked students to discuss the acid-catalysed amide hydrolysis mechanism of the drug’s synthesis, and the two resulting products. The research team gave the assignment to 543 students at a large, research-intensive university within a second-semester organic chemistry laboratory course.
The team only evaluated the discussion of the reaction mechanism. They broadly categorised features of responses necessary for mechanistic reasoning into ones that: described the target phenomenon, specified set-up conditions, identified activities and, finally, identified properties of entities.
Student understanding revealed
A frequency analysis according to these categories revealed that few students recognised how important the reaction medium or conditions were in determining the course of the reaction. Additionally, students identified surface features, such as charges, more readily than fundamental properties, such as nucleophilicity and electrophilicity. A more sophisticated analysis detected an understanding of bond breaking and making whenever students explicitly described electron movement. However, this understanding was often absent when electron movement was implicitly described, such as when citing pronation or deprotonation. Students were able to describe what was happening in the reaction but were not always able to explain why it happens. This is similar to being able to draw a correct mechanism without understanding why it occurs.
- Explicitly emphasise the connections between the electron pushing formalism and the fundamental chemistry concepts they are built on. For example, while students in this study often explained protonation and deprotonation, they referenced and understood the more fundamental concepts of nucleophilicity and electrophilicity less well.
- Be aware of a specific example of this problem: it is often difficult to know whether students understand the implicit electron movement involved when they use the phrase ‘a nucleophile “attacks” an electrophile’.
- Use the four categories for descriptions used in this study as a scaffold to support students’ mechanistic reasoning. For example, ask students to identify the reaction conditions, or to describe the relevant properties of molecules driving particular mechanistic steps, in addition to asking for the electron pushing diagram for a reaction mechanism.
- Incorporate WTL into your practice to give yourself a new way of assessing your students’ mechanistic reasoning and to provide your students with more variety than the usual ‘draw a mechanism’-type question when practising mechanistic reasoning.
- Download the freely available WTL assignment used in this study from Appendix 1. It may be appropriate to use in your own context. If not, the paper provides sufficient detail for you to design your own. It even provides some sample student answers that you could use to train your own students in how to start writing about reaction mechanisms.
F M Watts et al., Chem. Educ. Res. Pract., 2020, DOI: 10.1039/c9rp00185a