Electrochemical oxidation of gallic acid: A reexamination of the reaction mechanism in aqueous medium

A clear understanding of redox mechanism of antioxidants, like gallic acid (GA) is important to decipher their radical scavenging functioning in the biochemical environment of the body. Herein, electrooxidation process of GA is reexamined in an aqueous medium through a combination of cyclic voltammetric and spectroelec-trochemical studies on gold electrode in wide pH and concentration ranges. These studies reveal redox process of GA as an irreversible two electrons process, resulting in the formation of ortho-quinone, where the second electron transfer is the rate limiting step. The redox process of GA at different pH is presented in the form of a bicubic scheme and the associated thermodynamic parameters are analyzed using Laviron's theory. It allows for the first time the determination of the acidity constant of the corresponding ortho-quinone (pKa = 5.33). The number of exchanged electrons and protons depends on the pH conditions, such that (2e , 2H+) are exchanged at pH < 4.24 and pH > 5.33, while (2e , 1H+) are exchanged at 4.24 < pH < 5.33. A careful analysis using the theoretical treatment proposed by Amatore and Saveant for an ECE-DISP mechanism demonstrates the existence of a DISP2 mechanism leading to the formation of ortho-quinone through disproportionation of the semiquinone radicals, made possible thanks to their stabilization.

Automated summary

The electrooxidation process of gallic acid (GA) in an aqueous medium was reexamined using cyclic voltammetry and spectroelectrochemical studies on a gold electrode. The results revealed that the redox process of GA is an irreversible two-electron process, with the formation of ortho-quinone as the final product. The acidity constant of the ortho-quinone was determined for the first time (pKa = 5.33). The number of exchanged electrons and protons depended on the pH conditions, with (2e, 2H+) exchanged at pH < 4.24 and pH > 5.33, and (2e, 1H+) exchanged at 4.24 < pH < 5.33. The existence of a DISP2 mechanism leading to the formation of ortho-quinone through disproportionation of semiquinone radicals was demonstrated.