The use of the flipped classroom approach in higher education STEM courses has rapidly increased over the past decade, and it appears this type of learning environment will play an important role in improving student success and retention in undergraduate chemistry "gatekeeper" courses. Many adopters of the flipped classroom structure see the greatest benefit originating from the additional time this format provides for the implementation of student-centered learning activities during the classroom period. However, results from recent quasi-experiments suggest that improved course performance for students in flipped classroom environments has a significant contribution from the online preclass activities. In order to compare the impact of the preclass online learning environment to the in-class collaborative activities typically done in a flipped classroom, a randomized controlled trial (RCT) was conducted with student volunteers. A two-day organic chemistry stereochemistry unit was delivered to students who were randomly assigned to "flipped classroom" and "traditional lecture" treatment groups. Performance gains were measured after each phase of the instructional intervention for both treatment groups, and these gains were compared to students from a randomly assigned negative control group. A mixed-methods ANOVA indicates that under these experimental conditions the online learning component appears to account for most of the improvement in posttest scores observed in the flipped classroom treatment. These results suggest optimizing the design of the asynchronous online learning environment will positively impact student performance outcomes. Therefore, this component of the flipped classroom deserves more attention from instructional designers and classroom practitioners.