Study on kinetics and mechanism of reaction of iron (III) with salicylaldoxime and <i>ortho</i>-hydroxyacetophenoneoxime in HCIO₄-NaCIO₄ media

Abstract

The kinetics of the reaction of iron (lll) with the title ligands leading to 1:1 chelate formation have been studied at different temperatures (25°-35°C) and ionic strength <i>l </i> = 1.0 mol dm^-3 (NaClO₄ + HCIO₄). A dual path mechanism involving both Fe(aq)³⁺ and Fe(OH)(aq)²⁺ and undissociated ligand (LH₂) is consistent with the experimental observations where [H⁺]&gt;&gt;T_Fe&gt;&gt;T_L (where T_Fe and T_L stand for the total concentration of iron and ligand respectively). The results conform to: <i>k</i>_obs/B= k₁[H⁺] + <i>k</i>₂<i>K</i>_h where B=T_Fe/([H⁺]+<i>K</i>_h)+1/Q; <i>K</i>_h = hydrolysis constant of Fe(aq)³⁺; <i>k</i>₁ and <i>k</i>₂ are the second order forward rate constants of Fe(aq)³⁺ and Fe(OH)(aq)²⁺ respectively and Q is the equilibrium constant of the reaction, Fe(aq)³⁺LH₂ <img src='http://www.niscair.res.in/jinfo/harpoon.gif' border=0> Fe(LH)²⁺+H⁺. At higher acidities, dissociation of the complex has been followed and the rate constants determined from the principle of reversibility are in good agreement with those obtained from the studies of formation of the complex at lower acidities. Activation parameters (<img src='/image/spc_char/delta.gif' border=0><i>H</i>^≠ and <img src='/image/spc_char/delta.gif' border=0><i>S</i>^≠) for each of the steps have been determined. Fe(OH)(aq)²⁺appears to react in a dissociative fashion (Eigen-Tamm mechanism) and its characteristic water exchange rate has been obtained as a rough estimate. On the other hand, Fe(aq)³⁺ appears to react through an associative interchange (I_a) mechanism.