Organic chemistry is a required course sequence for many STEM students. However, research indicates that organic chemistry reaction mechanisms are especially challenging for students due to a mixture of underlying conceptual difficulties, the process-oriented thinking inherent to the discipline, and the representations commonly used to depict mechanisms. While student reasoning about many of the reaction types covered in the organic chemistry curriculum has been studied previously, there is minimal research focused specifically on how students think about the mechanisms of addition reactions. Understanding students' conceptions about addition reactions is valuable for both instructors and researchers, as these reactions are among the first for which students must consider different chemical properties to make a decision about alternate reaction pathways. This study provides insight into how students think about these reactions by probing first semester organic chemistry students' thinking using think-aloud interviews as they worked through two addition reactions. To elicit a range of thinking, students worked through the mechanisms using either paper and pencil or an app that dynamically represents the molecules. Generally, students were able to identify the steps of the two addition reactions but did not always successfully apply chemical thinking during the mechanistic steps. Most prominently, both groups of students struggled with the concepts related to carbocation stability, frequently misapplying stabilization via substitution and demonstrating difficulty in identifying the potential for resonance stabilization. Our results suggest that instructors should emphasize the conceptual grounding that directs mechanistic steps, in particular when determining carbocation stability. More generally, our findings suggest that instructors must emphasize the skill of considering and weighing different chemical properties when making decisions about alternative reaction pathways.