Electrochemistry as a Tool for Studies of Complex Reaction Mechanisms: The Case of the Atmospheric Aqueous-Phase Aging of Catechols.

Kristijan Vidović,Primož Jovanovič,Irena Grgić,Martin Šala,Ana Kroflič

doi:10.1021/acs.est.9b02456

Abstract

The ultimate goal in the understanding of complex chemical processes is a complete description of the underlying reaction mechanism. In the present study and for this purpose, a novel experimental platform is introduced that builds upon electrochemistry capable of generating reactive intermediate species at the electrode surface. The atmospherically relevant nitration of catechols is taken as a case example. First, we confirm the recently proposed nitration mechanism, advancing the understanding of atmospheric brown carbon formation in the dark. We are able to selectively quantify aromatic isomers, which is beyond the limits of conventional electroanalysis. Second, we identify a new pathway of nitrocatechol hydroxylation, which proceeds simply by oxidation and the addition of water. This pathway can be environmentally significant in the dark aqueous-phase formation of secondary organic aerosols. Third, the developed methodology is capable of selectively detecting a wide range of nitroaromatics; a possible application in environmental monitoring is proposed.

Highlights

Aqueous-phase secondary organic aerosol formation, in which nonradical oxidations rather than radical reactions are presumably paramount, has been attracting increasingly more attention.[1−3] One of the generally established strategies to obtain greater insight into a complex reaction mechanism is to start the reaction from the anticipated intermediate stage and follow the formed reaction product(s).[4−7] Electrochemically assisted electron transfers can be coupled with irreversible homogeneous chemical reactions, which is known as an electron-transfer chemical reaction mechanism (EC mechanism).[8]
Aromatic nitration is essential in natural and biological environments and nitration under mild reaction conditions is advantageous in other applications, the underlying mechanisms have been only rarely studied.[21−23] We developed a novel experimental platform combining the advanced utilization of electrochemistry and state-of-the-art analytical techniques and applied it to explore the underlying mechanism of dark 3MC nitration in the aqueous solution, representative of the atmospheric aqueous phase
At the end of this study, we demonstrate that the unprecedented ability to electrochemically differentiate between aromatic isomers could be further exploited in routine environmental monitoring

Summary

■ INTRODUCTION

Aqueous-phase secondary organic aerosol (aqSOA) formation, in which nonradical oxidations rather than radical reactions are presumably paramount, has been attracting increasingly more attention.[1−3] One of the generally established strategies to obtain greater insight into a complex reaction mechanism is to start the reaction from the anticipated intermediate stage and follow the formed reaction product(s).[4−7] Electrochemically assisted electron transfers can be coupled with irreversible homogeneous chemical reactions, which is known as an electron-transfer chemical reaction mechanism (EC mechanism).[8]. Article homogeneous reaction with nitrite ions (NO2−) in bulk allowed us to confirm the proposed reaction mechanism of dark 3MC nitration in the aqueous phase (Scheme 1). Recent aIn present work, oxidation by HNO2 is prevented and the conversion of 3MC to 3-methyl-o-quinone (3MoQ) is electrochemically assisted, whereas the coupled nitration to 3-methyl-5-nitrocatechol (3M5NC) and 3-methyl-4-nitrocatechol (3M4NC) occurs by NO2− in the bulk.

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES