Variable scan rate cyclic voltammetry and theoretical studies on tocopherol (vitamin E) model compounds

Variable scan rate (0.1-500 V s^-1) cyclic voltammetry experiments were performed on a series of model tocopherol (vitamin E) compounds with differing degrees of methyl substitution around the aromatic (phenolic) ring. α-Tocopherol, with a fully methylated aromatic ring, produced stable phenoxonium cations upon oxidation in CH₃CN, and was modeled via an ECE mechanism (where "E" represents an electron transfer and "C" a chemical step). Compounds with less methyl substitution around the aromatic ring were more reactive following oxidation, and formed additional oxidation products (hemiketals and p-quinones), and were modeled according to a more complicated ECECC mechanism. The equilibrium and rate constants associated with the chemical steps were estimated by digital simulations of the variable scan rate data over a range of temperatures (T = 253-313 K) in acetonitrile containing 0.5 M Bu₄NPF₆ as the supporting electrolyte. The relative lifetimes of the phenoxonium cations of tocol and the tocopherols were compared with theoretical results obtained from molecular orbital calculations.