Oxidation of Caffeine by Permanganate Ion in Perchloric and Sulfuric Acids Solutions: A Comparative Kinetic Study
The kinetics of oxidations of caffeine by permanganate ion in both perchloric and sulfuric acids solutions have been investigated spectrophotometrically at a constant ionic strength of 1.0 mol dm -3 and at 25°C. In both acids, the reaction-time curves were obtained with a sigmoid profile suggesting an autocatalytic effect caused by Mn(II) ions formed as a reaction product. Both catalytic and non-catalytic processes were determined to be first order with respect to the permanganate ion and caffeine concentrations, whereas the orders with respect to [H+] and [Mn(II)] were found to be less than unity. Variation of either ionic strength or dielectric constant of the medium had no significant effect on the oxidation rates. Spectroscopic studies and Michaelis-Menten plots showed no evidence for the formation of intermediate complexes in both acids suggesting that the reactions point towards the outer-sphere pathway. The reactions mechanism adequately describing the kinetic results was proposed. In both acids, the main oxidation products of caffeine were identified as 1,3,7-trimethyluric acid. Under comparable experimental conditions, the oxidation rate of caffeine in perchloric acid was slightly higher than that in sulfuric acid. The constants involved in the different steps of the reactions mechanism have been evaluated. With admiration to the rate-limiting step of these reactions, the activation parameters have been evaluated and discussed.