Study of equilibria and kinetics of the acid catalysed interaction of iron(III) with salicylaldehyde ando-hydroxyacetophenone

Abstract

The equilibria and kinetics of the reaction of FeIII with salicylaldehyde ando-hydroxyacetophenone, leading to 1∶1 chelate formation, have been studied at different temperatures (25–35°C) and ionic strength, I = 1.0 mol dm−3 (NaClO4+HClO4). A dual path mechanism involving both Fe aq 3+ and Fe(OH) aq 2+ species and undissociated free ligand (LH) is consistent with the experimental observations where [H+]≫[Fe]T≫[L]T (where [Fe]T and [L]T stand for total concentrations of iron and ligand respectively). The results conform to kobs/B = k1[H+]+k2Kh where B = [Fe]T/(Kh+[H+])+1/Q; Kh = hydrolysis constant of Fe aq 3+ ; k1, k2 are the forward second order rate constants of Fe aq 3+ and Fe(OH) aq 2+ , respectively, and Q is the equilibrium constant of the reaction, Fe3++LH⇋FeL2++H+. Thermodynamic parameters for each of the steps have been determined. Fe(OH) aq 2+ appears to react in a dissociative fashion (Eigen-Tamm mechanism), whilst Fe aq 3+ appears to react through the associative inter-change (Ia) mechanism. The equilibrium constants (Q) obtained spectrophotometrically are compared with those obtained from kinetic studies.