Learning to reason through organic reaction mechanisms is challenging for students because of the volume of reactions covered in introductory organic chemistry and the complexity of conceptual knowledge and reasoning skills required to develop meaningful understanding. However, understanding reaction mechanisms is valuable for students because they are useful for predicting and explaining reaction outcomes. To identify the features students find pertinent when explaining reaction mechanisms, we have collected students’ written descriptions of an acid-catalysed amide hydrolysis reaction. Students’ writing was produced during the implementation of Writing-to-Learn assignments in a second semester organic chemistry laboratory course. We analysed students’ written responses using an analytical framework for recognizing students’ mechanistic reasoning, originally developed with attention to the philosophy of science literature. The analysis sought to identify the presence of specific features necessary for mechanistic reasoning belonging to four broad categories: (1) describing an overview of the reaction, (2) detailing the setup conditions required for the mechanism to occur, (3) describing the changes that take place over the course of the mechanism, and (4) identifying the properties of reacting species. This work provides a qualitative description of the variety of ways in which students included these features necessary for mechanistic reasoning in their writing. We additionally analysed instances of co-occurrence for these features in students’ writing to make inferences about students’ mechanistic reasoning, defined here as the use of chemical properties to justify how electrons, atoms, and molecules are reorganized over the course of a reaction. Feature co-occurrences were quantified using the lift metric to measure the degree of their mutual dependence. The quantitative lift results provide empirical support for the hierarchical nature of students’ mechanistic descriptions and indicate the variation in students’ descriptions of mechanistic change in conjunction with appeals to chemistry concepts. This research applies a framework for identifying the features present in students’ written mechanistic descriptions, and illustrates the use of an association metric to make inferences about students’ mechanistic reasoning. The findings reveal the capacity of implementing and analysing writing to make inferences about students’ mechanistic reasoning.

An understanding of acid–base reactions is necessary for success in chemistry courses and relevant to careers outside of chemistry, yet research has demonstrated that students often struggle with learning acid–base reaction mechanisms in organic chemistry. One response to this challenge is the development of educational applications to support instruction and learning. The development of these supports also creates an opportunity to probe students’ thinking about organic chemistry reaction mechanisms using multiple modalities—i.e., using an app interface or the traditional paper–pencil. This study used think-aloud interviews conducted with undergraduate students in their first semester of organic chemistry to understand how they worked through two acid–base reactions using either paper–pencil or an app. Analysis of the interviews indicates that students from both groups recognize the steps of acid–base reactions, but do not always apply the underlying concepts, such as assessment of pK_a values or resonance, when determining how a reaction will proceed. The modality seemed to somewhat influence students’ thinking, as the app prevented students from making chemically unreasonable mistakes. However, some students relied on the cues it provided, which could potentially be problematic when they are required to respond to assessments that do not provide these cues. Our results suggest that instructors should emphasize the conceptual grounding for the steps that govern acid–base reactions to promote chemical thinking about the relationships between the reaction components and how those influence reaction outcomes, as well as support students to think critically about the chemical information contained within the modalities they are using.