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Microdroplet Chemistry

Much excitement has met the news that a number of bulk-phase reactions can be dramatically accelerated in microdroplets (Banerjee et al., Analyst 2017, 142, 1399-1402). These findings have stimulated many analytical studies to learn about reaction intermediates. There is also interest in microdroplet chemical synthesis because it is envisioned as a powerful method for performing reactions that show extremely slow kinetics in the bulk phase. Although a number of alternative methods, such as sonication- and microwave-assisted synthesis, have been undertaken to speed up reactions, microdroplet synthesis is of potential interest particularly because of the gentleness of the process, which can even be environmentally benign by using an aqueous solvent. It has become apparent from our preliminary study that the environment in microdroplets is strikingly different from that of the corresponding bulk phase. How exactly the reaction is facilitated in microdroplets is still not unambiguously known as there are supposed to be many factors that contribute to the reaction rate acceleration. Although the microdroplet evaporation and confinement of reagents could successfully explain the reaction rate enhancement of a bimolecular reaction by the concentration effect, it fails to explain the reaction rate acceleration of a unimolecular reaction, which should not be strongly sensitive to the reagent concentration. Furthermore, the reagent concentration can also be increased in the bulk phase but this is not expected to achieve the dramatic increase of the reaction rate found using microdroplets. Possibly, one of the most important features of microdroplets is the high surface-to-volume ratio providing a unique polar surface environment for a reaction to occur at or near the air–liquid interface. It has also been apparent from our work that a reaction route can also be altered leading to a different product in microdroplet compared to that in the bulk.

We found marked acceleration of some reaction rates (Banerjee et al., Angew. Chem Int. Ed. 2015, 54, 14795-14799) even by a factor of a million when carried out in microdroplets. The mechanism is not presently established but droplet evaporation, droplet confinement of reagents, reactive air-liquid interface, and high electrostatic pressure and reagent orientation on the droplet surface appear to be important factors among others. We suggest that this ‘microdroplet chemistry’ could be a remarkable alternative to accelerate slow and difficult reactions, and in conjunction with mass spectrometry, it may provide a new arena to study chemical and biochemical reactions in a confined environment. This ‘microdroplet chemistry’ is still in its infancy and heightens our interests to apply this method to organic syntheses on the preparative scale.

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