Water serves as an inert environment for the dispersion and application of many kinds of herbicides. Viologen compounds, a type of widely used but highly toxic herbicide, are stable in bulk water, whose half-life can be up to 23 weeks in natural water, imposing a severe health risk to mammals. In this study, we present the striking results of the spontaneous and ultrafast reduction-induced degradation of three viologen compounds in water microdroplets and provide the concentration, time, temperature dependence, mechanism, and scale-up of the reactions. We postulate that the electrons existing at the air−water interface of the microdroplets due to the unique redox potential therein initiate the reduction, from which further degradation occurs. The host−guest complexation between cucurbit[7]uril and viologens only slightly changes the redox potential of viologens in the bulk but completely inhibits the reactions in microdroplets, adding to the uniqueness of the redox potentials at the air−water interfaces of microdroplets. Taken together, microdroplets might have been functioning as naturally occurring ubiquitous tiny electrochemical cells for a plethora of unique redox reactions that were thought to be impossible in the bulk water.
Significance
Water microdroplets can accelerate chemical reactions by orders of magnitude compared to the same reactions in bulk water and/or trigger spontaneous reactions that do not occur in bulk solution. Among the properties of water microdroplets, the unique redox ability resulting from the spontaneous dissociation of OH
−
into a released electron and •OH at the air–water interfaces is especially intriguing. At the air–water interface, OH
−
exhibits a strong reducing potential, and the resulting •OH is highly oxidative, making water microdroplets a unity of opposites. We report the reduction of pyridine into pyridyl anions (C
5
H
5
N
−
) and the oxidation of pyridine into hydroxypyridine, which extends what we know about the redox power of water microdroplets.
Oxidative C–H/N–H cross-coupling has emerged
as an
atom-economical method for the construction of C–N bonds. Conventional
oxidative C–H/N–H coupling requires at least one of
the following: high temperatures, strong oxidizers, transition metal
catalysts, organic solvents, light, and electrochemical cells. In
this study, by merely spraying the water solutions of the substrates
into microdroplets at room temperature, we show a series of oxidative
C–H/N–H coupling products that are strikingly produced
in a spontaneous and ultrafast manner. The reactions are accelerated
by six orders of magnitude compared to the same reactions in the bulk.
It has been previously proposed by fluorescence microscopy and theory
that the spontaneously generated electric field at the microdroplets
peripheries can be in the ∼109 V/m range. Based
on mass spectrometric analysis of key radical intermediates, we opine
that the ultrahigh electric field catalytically oxidizes the substrates
by removing an electron, which further promotes C/N coupling. Taken
together, we anticipate that microdroplet chemistry will be an avenue
rich in green opportunities of constructing C-heteroatom bonds.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.