Probing the reactivity of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with metal cations and acids in acetonitrile by electrochemistry and UV-Vis spectroscopy

Abstract

2,2-Diphenyl-1-picrylhydrazyl (DPPH) is certainly one of the most widely used free radicals in several applications, because of its high stability. Unfortunately, there are few works dealing with its stability in the presence of many chemical species that coexist during chemical processes. In this work, the stability of DPPH was investigated by electrochemistry and UV-Vis spectroscopy in the presence of some metal cations (Cu²⁺ and Zn²⁺) and acids (HClO₄ and HNO₃) in acetonitrile. In the presence of Cu²⁺, DPPH was oxidized to DPPH⁺ with the formation of an equivalent amount of Cu⁺. With Zn²⁺, DPPH undergoes a slow disproportionation with the formation of Zn(DPPH)⁺ and DPPH⁺, certainly favored by the acidity of the metal cation. This hypothesis was subsequently confirmed by studying the stability of DPPH in the presence of HClO₄. This acid of appreciable strength in acetonitrile (pK_a = 1.83) causes a fast disproportionation of DPPH with the formation of DPPH–H and DPPH⁺. This mechanism was confirmed both by UV-Vis spectroscopy and by electrochemistry, with a stoichiometry corresponding to 2 equivalents of DPPH for about 1 equivalent of HClO₄. In the presence of nitric acid, which is about 10⁷ weaker than HClO₄ in acetonitrile, the disproportionation was much slower. These preliminary results are proof that many chemical species are likely to react with DPPH and indirectly induce sources of bias during its application, especially when evaluating antioxidant properties.