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1.  Kinetics and Mechanisms of the Chromium(III) Reactions with 2,4- and 2,5-Dihydroxybenzoic Acids in Weak Acidic Aqueous Solutions 
The reactions of 2,4- and 2,5-dihydroxybenzoic acids (dihydroxybenzoic acid, DHBA) with chromium(III) in weak acidic aqueous solutions have been shown to take place in at least two stages. The first stage of the reactions has an observed rate constant k1(obs) = k1[DHBA] + C and the corresponding activation parameters are ΔH1(2,4)≠ = 49, 5 kJ/mol−1, ΔS1(2,4)≠ = −103, 7 J mol−1 K−1, ΔH1(2,5)≠ = 60, 3 kJ/mol−1, and ΔS1(2,5)≠ = −68, 0 J mol−1 K−1. These are composite activation parameters and the breaking of the strong intramolecular hydrogen bonding in the two ligands is suggested to be the first step of the (composite) first stage of the reactions. The second stage is ligand concentration independent and is thus attributed to a chelation process. The corresponding activation parameters are ΔH2(2,4)≠ = 45, 13 kJ/mol−1, ΔS2(2,4)≠ = −185, 9 J mol−1 K−1, ΔH2(2,5)≠ = 54, 55 kJ/mol−1, and ΔS2(2,5)≠ = −154, 8 J mol−1 K−1. The activation parameters support an associative mechanism for the second stage of the reactions. The various substitution processes are accompanied by proton release, resulting in pH decrease.
doi:10.1155/2010/832768
PMCID: PMC2913840  PMID: 20721282
2.  Kinetics and Mechanism of the Reaction between Chromium(III) and 2,3-Dihydroxybenzoic Acid in Weak Acidic Aqueous Solutions 
The reaction between chromium(III) and 2,3-dihydroxybenzoic acid (2,3-DHBA) takes place in at least three stages, involving various intermediates. The ligand (2,3-DHBA)-to-chromium(III) ratio in the final product of the reaction is 1 : 1. The first stage is suggested to be the reaction of [Cr(H2O)5(OH)]2+ with the ligand in weak acidic aqueous solutions that follows an Id mechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are ΔH≠2(obs) = 61.2 ± 3.1 kJmol−1, ΔS≠2(obs) = −91.1 ± 11.0 JK−1mol−1, ΔH≠3(obs) = 124.5 ± 8.7 kJmol−1, and ΔS≠3(obs) = 95.1 ± 29.0 JK−1mol−1. These two stages are proposed to proceed via associative mechanisms. The positive value of ΔS≠3(obs) can be explained by the opening of a four-membered ring (positive entropy change) and the breaking of a hydrogen bond (positive entropy change) at the associative step of the replacement of the carboxyl group by the hydroxyl group at the chromium(III) center (negative entropy change in associative mechanisms). The reactions are accompanied by proton release, as shown by the pH decrease.
doi:10.1155/2010/348692
PMCID: PMC2902047  PMID: 20634986
3.  Reaction of Chromium(III) with 3,4-Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions 
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 k1(obs)′ = k0 + k1(obs)[3, 4-DHBA], and the corresponding activation parameters were calculated as follows: ΔH1(obs)≠ = 51.2 ± 11.5 kJ mol−1, ΔS1(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 ΔH2(obs)≠ = 44.5 ± 5.0 kJ mol−1, ΔS2(obs)≠ = −175.8 ± 70.3 J mol−1 K−1. The observed stages are proposed to proceed via interchange dissociative (Id, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.
doi:10.1155/2008/212461
PMCID: PMC2635946  PMID: 19223961
4.  Kinetics and Mechanism of the Reaction between Chromium(III) and 3,4-Dihydroxy-Phenyl-Propenoic Acid (Caffeic Acid) in Weak Acidic Aqueous Solutions 
Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (Id mechanism) of water molecule from the Cr(H2O)5OH2+ coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters are ΔH2(obs)≠ = 28.6 ± 2.9 kJ mol−1, ΔS2(obs)≠ = −220 ± 10 J K−1mol−1, ΔH3(obs)≠ = 62.9 ± 6.7 kJ mol−1 and ΔS3(obs)≠ = −121 ± 22 J K−1mol−1. The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease.
doi:10.1155/2008/624583
PMCID: PMC2490814  PMID: 18695733

Results 1-4 (4)