Correlation of voltammetric behavior of α-hydroxy and α-methoxy quinones with the change of acidity level in acetonitrile

A family of quinones containing an alpha-hydroxy group, anilinoperezones (2-(1,5-dimethyl-4-hexenyl)-3-hydroxy-5-methyl-6-[4'(R-2-phenyl) amine]-1,4-benzoquinones, PQOHs), was studied electrochemically and compared with its methylated derivatives (2(1,5-dimethyl-4-hexenyl)-3-methoxy-5-methyl-6-[4'-(R-2-phenyl) amine]-1,4-benzoquinones, PQOMes). The supporting electrolyte was acetonitrile + 0.1 mol L-1 Et4NBF4 in the absence and the presence of different proton sources (benzoic acid, perchloric acid) and base (sodium benzoate and tetraethylammonium phenolate, TEAPhen). The comparative study was carried out to determine the influence of the internal proton donors and external proton sources in the electrodic mechanism of reduction, related mainly to the first electron transfer. PQOHs are reduced more easily than PQOMes, which exhibit electrochemical behavior typical of quinones; for PQOHs, however, a lower stoichiometry of electron transfer is observed, caused by the presence of a self-protonation mechanism. The addition of a proton donor modifies only the stoichiometry of the PQOH electroreduction process, without changing the reduction potential. On the contrary, the presence of a basic additive, such as TEAPhen, removes the hydroxylic proton and, consequently, makes the reduction more difficult, as expected. (C) 2004 Elsevier B.V. All rights reserved.