Oxidation of 4-amino-4 '-methoxydiphenylamine in aqueous and organo-aqueous media

By means of spectrophotometric method, the pK(a) values of 4-amino-4'-metho xydiphenylamine (Variamine Blue, VB) conjugate acids in water, N,N-dimethyl formamide, dimethylsulphoxide have been found. VB electro-oxidation on glas sy carbon rotating disk electrode in aqueous and organo-aqueous media proce eds in two single-electron steps, the second being rate-determining one. As discussed for aqueous-dimethylformamide solutions, the acidity decrease is accompanied by the limiting stage alteration. Chemical oxidation of VB by ammonium persulphate features first order in substrate and first order in o xidant, and leads to the iminiquinone cation formation. Using the SCF MO LC AO MNDO, AM1, PM3 methods, it has been shown that of two possible conjugate acids of VB the form protonated by primary nitrogen has to be oxidized fir st. Hydration consideration reveals the energetic advantage of cation forme d on VB protonation by its primary amino group. Using the above methods, as well as the INDO scheme, within the UHF formalism the spin density distrib ution on atoms in the VB cation radical has been computed, that allows the second oxidative step to be presented in more detail. The mechanism of VB o xidation reaction in aqueous and organo-aqueous media has been proposed.