Reaction of Chromium(III) with 3,4‐Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions

Kimon Zavitsanos,Konstantinos Tampouris,Athinoula L Petrou

doi:10.1155/2008/212461

Abstract

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentration k 1(obs)′ = k 0 + k 1(obs)[3, 4-DHBA], and the corresponding activation parameters were calculated as follows: ΔH 1(obs) ≠ = 51.2 ± 11.5 kJ mol−1, ΔS 1(obs) ≠ = −97.3 ± 28.9 J mol−1 K−1 (composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are ΔH 2(obs) ≠ = 44.5 ± 5.0 kJ mol−1, ΔS 2(obs) ≠ = −175.8 ± 70.3 J mol−1 K−1. The observed stages are proposed to proceed via interchange dissociative (I d, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.

Highlights

The ligand 3,4-dihydroxybenzoic acid (3,4-DHBA) (Figure 1), 1, is known to be produced in the reaction of radicals, which are formed in pathophysiological cases, the 4-hydroxybenzoic acid or salicylic acid acting as radical-trapping agent [5]
Aqueous solutions containing 3,4-DHBA (Alfa Aesar) in concentrations ranging from 7.45 × 10−3 to 1.62 × 10−2 M were prepared using dilute (0.1 M) KOH solutions
The experimental results are consistent with a three-step mechanism in which an initial attack between the acid molecule and the Cr(H2O)5OH2+ complex giving a carboxylate bound Cr(III), 2, is followed by two consecutive kinetically indistinguishable nonligand and nonCr(III) dependent steps

Summary

Introduction

The ligand 3,4-dihydroxybenzoic acid (3,4-DHBA) (Figure 1), 1, is known to be produced in the reaction of radicals, which are formed in pathophysiological cases (e.g., ischemic stroke [1, 2], traumatic brain injury [3], and Huntington’s disease [4]), the 4-hydroxybenzoic acid or salicylic acid acting as radical-trapping agent [5]. It is used for assisting the Fenton reaction in effluent treatment [6] and in dechlorination of polychlorinated dioxins [7]. The biological role of Cr(III) remains mostly unclarified, one of the main reasons being the lack of intense characteristics like chargetransfer bands in the spectra with the only exception being the organochromium complexes [18, 19]

Results

Discussion

Conclusion