Electrospray ionization mass spectrometry is an ideal tool to study structure-reactivity relationships in electrochemical oxidation

Electrochemistry (EC) is an important and fascinating field in chemistry studying chemical reactions driven by electric current or, conversely, the generation of electric current from chemical reactions. Its hyphenation to mass spectrometry (MS), an ideal tool for the identification of reaction products in various systems, constitutes a valuable hybrid method for real time prediction and monitoring of electrochemical transformations. For the first time, we show how electrospray ionization - high-resolution mass spectrometry (ESI-HRMS) supports a systematic study on oxidation of benzenes of different hydroxylation grades, i.e. phenol and hydroxyphenols, in an electrochemical flow-through μ-prep cell. Besides giving valuable insights into the specifics of such systems in comparison to closed/static electrochemical cells, potentially non-favorable observations in coupling of EC and ESI are discussed. A correlation coefficient of ∼1 between the root square of the scan rate and the current suggested the kinetics to be controlled by diffusion. Pulse optimization further showed that the extent of oxidation product formation depends on the electrical potential, while MS analysis of oxidation of positional isomers of dihydroxybenzenes enabled a systematic assessment of oxidation susceptibility and associated reaction mechanisms. While the general product profile of a compound was rather robust, their intensity including the detected degree of polymerization depended on the type of supporting electrolyte, working electrode, and pH, with oligomers up to a degree of polymerization of ten (DP 10). Finally, we observed that the product profile of the electrochemical conversion resembled compound degradation in the environment, providing evidence that EC-MS hyphenation is a valuable tool for the elucidation and prediction of such processes.