Oxidation of capsaicin in acetonitrile in dry and wet conditions

An electrochemical study of the phenol capsaicin (CAPH), the active ingredient in chilli pepper, was performed in dried and wet acetonitrile on a glassy carbon electrode. Under dried conditions, two oxidation peaks at ca. 0.7 vs. (Fc/Fc⁺)/V (labelled E₁) and 1.0 vs. (Fc/Fc⁺)/V (labelled E₂) and two reduction peaks at ca. 0.1 and −0.5 vs. (Fc/Fc⁺)/V when the scan direction was reversed were observed. Rotating disk electrode experiments indicated that the two oxidation processes occur by same number of electrons and it is proposed they occur in two one-electron steps. As water was added to the acetonitrile, hydrogen bonding interactions between the water and the phenolic groups led to the E₁ and E₂ potentials shifting progressively more negatively. The shift in E₂ as water was added was greater than the shift in E₁, so that after the addition of approximately 0.2 M H₂O, only one voltammetric wave was observed (labelled as E₁′) corresponding to a two-electron oxidation, that continued to shift more negatively as more water was added. Under very wet conditions ([H₂O] > 1 M), only one chemically irreversible oxidation peak was observed (E₁′) at ca. > 0.5 vs. (Fc/Fc⁺)/V with one reduction peak at ca. −0.1 vs. (Fc/Fc⁺)/V upon reversal of the scan direction. Controlled-potential electrolysis under wet conditions indicated a total of two-electrons per molecule were transferred with the overall mechanism interpreted as a −2e^–/−H⁺ oxidation, followed by a hydrolysis reaction and loss of a methoxy group to form a 1,2-benzoquinone moiety.