Kinetics of dissociation of tris‐2,2′‐bipyridyl‐iron(II) cation in aqueous acetic acid solutions

Abstract

AbstractThe kinetics of dissociation of tris‐2,2′‐bipyridyl‐iron(II) complex ion have been examined in aqueous acetic acid solutions. The reaction is first order in the complex ion; the dependence of rate on H+ is somewhat like that observed in aqueous solutions approaching a limiting value at higher H+ concentrations. The influence of solvent composition on the reaction rate under acid‐dependent and acid‐independent conditions shows an initial retardation by acetic acid. The argument of ion‐pair formation based on decrease of dielectric constant proposed to explain the kinetics in other aqueous solvent media was found useless to explain the behavior in acetic acid solutions. Other solvent parameters also did not provide satisfactory correlation with the kinetic results, thus, indicating the operation of more complex microscopic solute‐solvent and solvent‐solvent interactions. While solvent effects play some part in the rate process, the rate of reaction would tend to zero in the absence of H2O and H+. This interesting observation proved useful in proposing a reaction mechanism that is consistent with the rate behavior over the entire range of solvent composition. The activity of water in the reaction medium is controlled by the content of acetic acid which can effect the structure of water through operation of hydrophobic forces and formation of hydrates. While acetic acid cannot possibly fulfill the role of water in occupying the vacated coordination position, the anomalous rise in rate even under some water deficient conditions seems to be related to the coordinating ability of HSO4− derived from H2SO4 present in the solution.