Students are known to have various difficulties in dealing with organic reaction mechanisms. A systematic classification of these difficulties appears necessary to design appropriate support. This paper presents insights into whether and how Bloom's revised taxonomy can be used to classify student difficulties in dealing with organic reaction mechanisms. We conducted an interview study with 12 undergraduate chemistry students using problem-solving tasks on nucleophilic substitution and elimination reactions to provide examples to test the classification. In our attempt at systemization, student difficulties are perceived as unachieved learning objectives. The classification reveals that student difficulties pertain to different cognitive process and knowledge dimensions. Specific major difficulties occurred within each cell of Bloom's revised taxonomy and for individual students. Our analysis suggests that general support for dealing with reaction mechanisms might be less beneficial for some students and that more adapted support is needed. Our approach of using Bloom's revised taxonomy to classify student difficulties might also benefit other domains to better understand student difficulties and evaluate appropriate support.

Reaction mechanisms are known to be a great challenge for students enrolled in organic chemistry courses. Students often have difficulties in both understanding the representation and inferring the appropriate chemical concepts. By means of cognitive task analysis, undergraduate students' verbal explanations to a series of case comparisons on nucleophilic substitution reactions were analyzed to infer which chemical concepts were used and how different concepts were related in students' argumentation. These categorized concepts were transformed into weighted networks to capture the prevalence and centrality of individual concepts across students and tasks. Comparing these student networks to sample solutions provided insights into deviances in students' explanations about the consideration or not-consideration of specific concepts. While specific concepts seem to be commonly used by students (e.g., electronegativity), some concepts seem to be triggered by specific features of the task representation. While a lack of or a misunderstanding of concepts is often a problem, the present analysis illustrates that the selection of appropriate concepts relevant for the task at hand is also a major difficulty for students. Implications for teaching and options for supporting students in the process of inferring and selecting relevant concepts are discussed.

Understanding and using domain-specific representations is a key challenge in science disciplines. Tutorial videos can provide a possible approach to support students in working with these representations. Incorporating multimedia learning principles, aspects of cognitive load theory, and combining a specific visual display with a well-structured scientific explanation increase the accessibility of this learning resource for students. Regarding the visual display, specific highlighting techniques can guide students’ attention to relevant parts of a representation. Our empirical findings, based on designing tutorial videos to support students’ understanding of reaction mechanisms in organic chemistry, suggest that students with low prior knowledge particularly benefit from this kind of support. In terms of increasing students’ learning gains, the verbal explanation presented in the video also has a great influence. Purposefully scripting these verbal explanations allows specific student needs to be addressed, for instance in terms of compensating low prior knowledge (e.g., by sequencing or enriching different levels of the explanation) or language capabilities (e.g., in case of second language learners). Furthermore, these explanations are a learning resource in themselves, as such a tutorial renders the structure of an expected explanation transparent, and thus accessible to students.