Electron-Transfer Mechanism in Radical-Scavenging Reactions by a Vitamin E Model in a Protic Medium

The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical or galvinoxyl radical by a Vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly acceleratd by the presence of Mg(ClO4)2, in de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH proceeds via an electron transfer from 1H to the radical, followed by a proton toransfer, rather than the one-step hydrogen atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN (0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media