08 August, 2018
Professor Himanshu Mishra gave a Special Physical Chemistry Seminar at UC Berkeley: "Understanding Accelerated Chemical Reactions in Electrosprays and Emulsions".
Numerous reports on dramatically accelerated organic reactions in aqueous electrosprays1-6 and emulsions7-8 have appeared in the last decade. While the causative role of “interfaces” has been invoked, rigorous mechanistic insights remain elusive. Specifically, it is unclear if the “interfacial effects” are limited to dangling hydrogen bonds, interfacial tensions, and curvature effects, or gas-phase chemistries and non-equilibrium conditions inside electrosprays, such as Coulomb explosions and the concentration of reactants, play dominant roles. Electrosprays have also been used to investigate thermodynamic properties of the air-water interface, which has led to scientific debate9-13. To clarify those matters, we combined electrospray ionization mass spectrometry (ESIMS), proton nuclear magnetic resonance (1H-NMR), and quantum mechanics to investigate the protonation and oligomerization of isoprene (C5H8) in electrosprays and emulsions of pH-adjusted water. Our experiments and simulations revealed that aqueous electrosprays entailed (gas-phase) reactions with minimally hydrated protons, a scenario unrealizable in emulsions or pristine air-water interfaces at 293 K and 1 atm. In the second part of the seminar, I will share our recent laboratory results on catalyst-free Fries rearrangement of phenylacetate into o- and p-hydroxyaryl ketones in vigorously mixed oil-water emulsions. Typically, this reaction is carried out in organic solvents, catalyzed by Lewis acids, such as HF, AlCl3, BF3, and SnCl4. Even though phenylacetate is immiscible in water (solubility at NTP ~30 mM), we found that the presence of a non-reacting organic phase in emulsions, such as dichloromethane, accelerated the yield of products. I will present our systematic screening of the reaction conditions, including organic solvents, water pH, temperature, and the duration of reactions.