Accelerating the "On Water" Reaction: By Organic-Water Interface or By Hydrodynamic Effects?

A series of organic reactions proceed dramatically faster in a heterogeneous mixture of the reactants and water than in a homogeneous mixture. Currently it is unclear whether the rate acceleration is due to the free OH groups at the organic-water interface, or due to the hydrodynamic effects caused by vigorous stirring, vortexing, or ultrasonication. Herein we produced static droplets in microfluidic devices to answer this question. In the work, a series of organic droplets containing diethyl azodicarboxylate (DEAD) and quadricyclane surrounded by water were produced, which were transferred to and confined in glass capillaries to minimize the hydrodynamic effects. The cycloaddition process of DEAD with quadricyclane was recorded by a CCD camera. The results showed the reaction proceeded in three steps, and the organic-water interface alone was catalytically efficient enough to enhance the reaction rate to the same level as in the bulk emulsion reaction, indicating that the hydrodynamic effects were negligible.