The undergraduate organic chemistry course is a prerequisite course for many students who plan to pursue careers in chemistry and chemical engineering. It also serves those students who wish to pursue professional careers in medicine, dentistry, and veterinary sciences. Previous research on student learning in organic chemistry shows that students struggle to understand ideas such as acid-base reactions and structure-property relationships which are foundational concepts on which more complex concepts are built. Furthermore, the typical organic chemistry course emphasizes students use of the electron-pushing formalism to represent how bonds are formed and broken in chemical reactions. Expert organic chemists use this formalism to represent predicted reaction mechanisms that explain the formation of products. Numerous studies have characterized student difficulties using electron-pushing mechanisms in an expert-like way as well as associating underlying chemical principle with the representations. We suggest that deep understanding of chemical reactions and their underlying chemical principles can be developed by engaging students in causal mechanistic explanation as part of a transformed organic chemistry course that emphasizes students using their knowledge of electrostatics, structure-property relationships, and energy to engage in explanation of chemical phenomena. Our goal is to engage students in as specific type of explanation called in casual mechanistic explanation which includes reasoning about the underlying causal factors in conjunction with the underlying entities and their activities that bring the phenomenon about. The studies reported here use a qualitative approach to elicit student' written explanations and drawn reaction mechanisms for various chemical reactions. Students were sampled at multiple time points over the course of their two-semester organic course to investigate how student reasoning changes overtime. Students participants were enrolled in either the beforementioned transformed organic chemistry course or were enrolled in an untransformed course that we refer to as the traditional context. This traditional context served as a control group for which to compare possible changes in reasoning for students enrolled in the transformed course sequence. Findings suggest that student engagement in causal mechanistic reasoning varies depending on students' general chemistry and organic chemistry course experience as well as the nature of the prompt eliciting the reasoning. Findings also suggest that students are generally capable of drawing mechanistic arrows that would generally be considered correct, however triangulating student reasoning with a detailed analysis of students' drawings, we found that typical organic chemistry assessment items that lack a reasoning component may overestimate student understanding. Our investigations also revealed student difficulties invoking the correct nucleophilic substitution process for a given reaction. Students often invoked an SN1 mechanistic process incorrectly, despite their engagement in casual mechanistic reasoning. Implications of these findings for organic chemistry instruction and assessment are discussed along with implications for future research.
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