Kinetics and mechanism of disproportionation and ferricyanide oxidation of the 10-methylacridinium cation in aqueous base

The kinetics of disproportionation and ferricyanide ion oxidation of the 10-methylacridinium cation have been measured spectrophotometrically over the pH range 9-14 in.20% CH 3 CN - 80% H 2 O (v/v) and ionic strength 1.0 at 25 °C. Disproportionation is kinetically second-order in total acridine species. The pH-rate profile is consistent with the rate-determining reaction of one acridinium cation with the pseudobase alkoxide anion derived from a second acridinium cation. Ferricyanide ion oxidation is kinetically first-order in each of ferricyanide ion and total acridine species. The pH-rate profile requires three distinct pathways for the ferricyanide ion oxidation of the 10-methylacridinium cation. For pH 12.8 the predominant oxidation pathway involves reaction of ferricyanide ion with the pseudobase alkoxide ion. Between pH 9.7 and 12.8, the major oxidation pathway involves initial disproportionation of the acridinium cation followed by ferricyanide ion oxidation of the 9,10-dihydro-10-methylacridine product. This latter route accounts for a maximum of 69% of the total ferricyanide ion oxidation at pH 11.1.