Recent efforts in organic chemistry education research focus on investigating activities and strategies designed to elicit students' mechanistic reasoning. This study investigates how a scaffolded case comparison activity implemented in an introductory organic chemistry course elicits and supports students' mechanistic reasoning in an authentic classroom setting. The activity included an adaptation of a previously reported reasoning scaffold to support small-group student discussions comparing organic reactions. We analyzed students' written responses to the in-class activity using Hammer's resources framework and Toulmin's argumentation model, interwoven to create an anti-deficit approach to exploring students' developing reasoning. The analysis of students' written artifacts sought to identify ways in which a scaffolded case comparison implemented in a collaborative class setting may support students' engagement in complex reasoning and argumentation development. We found that the in-class activity elicited students' writing about various aspects of mechanistic reasoning, including identifying explicit and implicit properties, dynamic reasoning, and multivariate reasoning. These findings indicate that the activity can engage students in complex mechanistic reasoning aspects in the classroom setting. Furthermore, this study extends the literature by detailing the nuances of students' developing causal reasoning with energetic and electrostatic accounts as shown in their writing. The results highlight students' emerging causal reasoning with varying levels of complexity and conceptual integration. This study provides direct implications for instructors seeking to implement similar classroom activities. The findings indicate directions for future research on the development of instructional activities and tools that further support students' developing causal reasoning, such as adapting existing scaffolding structures to support argumentation development and the integration of challenging concepts such as energetics.