Visualization of Phase-Transfer Catalysis Through Charge-Transfer Complexes

Unlike the large number of laboratory experiments on phase-transfer catalysis (PTC) that focus on a catalyzed chemical process, the present laboratory emphasizes the mechanism of phase-transfer catalysis itself. Students can follow visually the actual transfer of the reagent by the catalyst, and its consumption in the organic phase, in a step-by-step process that employs a colored charge-transfer (CT) complex as the transferred reacting species. Consumption of the transferred nucleophile is followed by discoloration of the organic phase. This transfer-consumption cycle can be repeated several times by shaking the two separated phases again, allowing students to finally grasp the whole process as a continuous repetition of these cycles. The experiment also links, through visualization, the important synthetic tool of PTC with abstract concepts based on molecular-orbital and hard-soft acid-base theories, invoked in the formation of a CT complex. © 2018 American Chemical Society and Division of Chemical Education, Inc.