Organic chemistry is a complex discipline that explores chemical reactivity at the molecular level. As such, it is often a required course for many pre-professional majors (medical, veterinary, dental, nursing, etc.) as well as for chemistry and chemical engineering majors due to the fact that understanding how chemical reactions occur at the molecular level is foundational for understanding the complexities of living organisms. We suggest that students can develop a robust understanding of chemical reactivity by engaging in mechanistic reasoning. Mechanistic reasoning works to explain a given phenomenon by linking the causal and mechanistic components that gave rise to that phenomenon. Causal elements explain why a given phenomenon occurred; mechanistic elements account for a step-wise series of events that occur at the particulate level. We have designed an activity that attempts to elicit student use of mechanistic reasoning to explain a chemical reaction. Statements that draw on chemical principles that explain why chemical species interact in a particular way (“the why”) and statements that give a detailed account of electron movement to form products (“the how”) are evidence of engaging in this type of reasoning. Helping students develop mechanistic reasoning is a primary goal of a transformed undergraduate organic chemistry course that is currently being piloted at Michigan State University called Organic Chemistry, Life, the Universe, and Everything (OCLUE). Preliminary results on students’ ability to articulate a causal mechanistic explanation in undergraduate organic chemistry will be presented.