|Summary:||The kinetics of the oxidation of formic acid by permanganate in aqueous perchloric acid solution, i.e., 2MnO¯₄ + 3HCOOH + 2H⁺ → 2MnO₂ + 3CO₂ + 4H₂O were studied in the temperature range 15 to 35°C. The variables examined included the reactant and hydrogen ion concentrations, ionic strength, the presence of various metal ions, and solvent and reactant deuterium isotope effects.
The reaction appears to proceed through two independent paths in which the rate-determining steps are bimolecular reactions of permanganate with formic acid and with formate ion, respectively. The kinetics are thus of the form
-d [MnO₄¯] / dt = [M nO₄¯] [HCOOH] (kA + kBKi / [H⁺] )
where kA and kg are rate constants of the two bimolecular reactions involving formic acid and formate ion, respectively, and Ki is the ionization constant of formic acid. The Arrhenius expressions for the rate constants were found to be kA = 1.1 x 10⁹exp. (-16400/RT) 1.mole¯¹sec¯¹ and kB =
7.8 x 10⁹exp. (-13000/RT) 1.mole¯¹sec¯¹.
The formate ion reaction exhibits a large deuterium (HCOO¯:DCOO¯) isotope effect which suggests cleavage of the C-H bond in the rate-determining step. The absence of a corresponding isotope effect in the formic acid reaction suggests that it proceeds by a different mechanism.
Fe⁺⁺⁺(but not Ag⁺, Cu⁺⁺, Co⁺⁺ or Na⁺) was found to catalyze the reaction, possibly by a mechanism involving a FeMnO₄⁺⁺ complex.
Previous investigations of the formic acid-permanganate system have been confined to lower acidities than the present one, and only the formate ion contribution to the reaction had been detected. === Science, Faculty of === Chemistry, Department of === Graduate|