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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.