This project aims to serve the national interest by better understanding how college instructors teach the topic of reaction mechanisms in organic chemistry courses. Reaction mechanisms are recognized as one of the most difficult topics for students to learn and thus are often a barrier to students’ success in organic chemistry courses. Identifying ways help students better understand organic reaction mechanisms will enhance opportunities for students to advance in chemistry and related studies such as medicine. While prior research has investigated student learning of reaction mechanisms, less is known about how educators teach the topic and their understanding of how to best respond to students’ learning difficulties. Throughout this project, instructors’ approaches to teaching reaction mechanisms will be captured using interviews and classroom observations; thus, both instructors’ beliefs about teaching and learning, and the practices they use in teaching and in responding to student learning will be captured. Importantly, individual and collective approaches will be used to develop resources and tools for educators to reflect upon their teaching and identify opportunities to improve how they teach organic reaction mechanisms.<br/><br/>The overall objective of the project is to explore and document instructors’ topic-specific pedagogical content knowledge (PCK) about reaction mechanisms. The project team will recruit organic chemistry educators with varying levels of experience and career trajectories (e.g., professors of teaching practice, tenure-earning faculty members, tenured faculty members), and from various institution types (e.g., small institutions, two-year institutions, predominately undergraduate institutions, research-intensive institutions) to participate in the project. Through in-depth, semi-structured interviews and multiple classroom observations of reaction mechanism-focused instruction, the project team will characterize educators’ personal (pPCK) and enacted (ePCK) pedagogical content knowledge. The PCK related to reaction mechanisms (both enacted and personal) will be used to create a description of the collective (cPCK) pedagogical content knowledge of reaction mechanisms of participating instructors. This work is expected to advance understandings of the specific strategies educators use to teach students about reaction mechanisms, how instructors address student difficulties and the strategies educators have found to be successful in teaching the topic. This project has the potential to identify the impact of different teaching strategies and enacted practices on students’ understanding, which could inform the design and implementation of professional development opportunities for organic chemistry educators. Finally, the project has the potential to impact the design of studies on instructors’ personal, enacted, and collective pedagogical content knowledge in other Science, Technology, Engineering, and Mathematics (STEM) courses. The NSF IUSE: EDU Program supports research and development projects to improve the effectiveness of STEM education for all students. Through its Engaged Student Learning track, the program supports the creation, exploration, and implementation of promising practices and tools.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.