Stepwise or Concerted Mechanisms? Rethinking of the Classical Carbocation Rearrangement Reaction

The carbocation rearrangement reaction is an important element in organic chemistry education. Classical textbooks have long taught the stepwise and concerted mechanisms of carbocation rearrangement. However, our students have questioned the classical mechanisms when they learned this topic. Based on the constructivist learning theory and student-centered teaching philosophy, we adopted a problem-based learning method and used pinacol rearrangement as a model to guide upper-division undergraduates' quantitative reflection about the rearrangement mechanism using density functional theory calculations. Our students defined the critical value ΔGsc to quantitatively analyze the reaction mechanism by themselves and found that the two mechanisms coexist and are quantifiable. This approach provided an alternative perspective on the rearrangement reaction to students and a deeper understanding than those of classic textbooks. We hope this approach encourages students to further use core ideas of structures and bonding to rationalize complex chemical reaction phenomena in their future courses.