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Kinetics and Mechanism of the Reaction between Chromium(III) and 2,3-Dihydroxybenzoic Acid in Weak Acidic Aqueous Solutions.

Petrou AL, Thoma V, Tampouris K - Bioinorg Chem Appl (2010)

Bottom Line: The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are DeltaH( not equal) (2(obs)) = 61.2 +/- 3.1 kJmol(-1), DeltaS( not equal) (2(obs)) = -91.1 +/- 11.0 JK(-1)mol(-1), DeltaH( not equal) (3(obs)) = 124.5 +/- 8.7 kJmol(-1), and DeltaS( not equal) (3(obs)) = 95.1 +/- 29.0 JK(-1)mol(-1).These two stages are proposed to proceed via associative mechanisms.The reactions are accompanied by proton release, as shown by the pH decrease.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Inorganic Chemistry, University of Athens, Panepistimioupolis, 15771 Athens, Greece.

ABSTRACT
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(H(2)O)(5)(OH)](2+) with the ligand in weak acidic aqueous solutions that follows an I(d) mechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are DeltaH( not equal) (2(obs)) = 61.2 +/- 3.1 kJmol(-1), DeltaS( not equal) (2(obs)) = -91.1 +/- 11.0 JK(-1)mol(-1), DeltaH( not equal) (3(obs)) = 124.5 +/- 8.7 kJmol(-1), and DeltaS( not equal) (3(obs)) = 95.1 +/- 29.0 JK(-1)mol(-1). These two stages are proposed to proceed via associative mechanisms. The positive value of DeltaS( not equal) (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.

No MeSH data available.


A possible mechanism of the reaction between chromium(III) and 2,3-dihydroxybenzoic acid in weak acidic aqueous solutions.
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sch1: A possible mechanism of the reaction between chromium(III) and 2,3-dihydroxybenzoic acid in weak acidic aqueous solutions.

Mentions: The form of the metal ion that reacts with the ligand 2,3-DHBA is [Cr(H2O)5OH]2+ since it is well known that the hydroxy complex [Cr(H2O)5(OH)]2+ is highly more reactive than [Cr(H2O)6]3+. The Ka for the reaction (4)Cr(H2O)63+⇌Cr(H2O)5(OH)2++H+ is about 10−4 [11, 12]. At pH lower than 4 the chromium(III) complex exists mainly in the hexa-aqua monomeric form. Its spectrum shows maxima at 575 nm and 410 nm (at the visible region). An amount of [Cr(H2O)5(OH)]2+ is though always present under the above conditions. In the experiments reported here that are conducted over the pH range 3-4 the reaction should be considered as taking place first with [Cr(H2O)5(OH)]2+ rather than with [Cr(H2O)6]3+, since a fast first step is taking place and it is very well known that Cr(H2O)63+ is very substitution inert. In the ligand molecule there are two intramolecular hydrogen bonds, one between adjacent hydroxyl groups and the other between the hydroxyl group and the carbonyl oxygen atom [13] (Scheme 1).


Kinetics and Mechanism of the Reaction between Chromium(III) and 2,3-Dihydroxybenzoic Acid in Weak Acidic Aqueous Solutions.

Petrou AL, Thoma V, Tampouris K - Bioinorg Chem Appl (2010)

A possible mechanism of the reaction between chromium(III) and 2,3-dihydroxybenzoic acid in weak acidic aqueous solutions.
© Copyright Policy - open-access
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2902047&req=5

sch1: A possible mechanism of the reaction between chromium(III) and 2,3-dihydroxybenzoic acid in weak acidic aqueous solutions.
Mentions: The form of the metal ion that reacts with the ligand 2,3-DHBA is [Cr(H2O)5OH]2+ since it is well known that the hydroxy complex [Cr(H2O)5(OH)]2+ is highly more reactive than [Cr(H2O)6]3+. The Ka for the reaction (4)Cr(H2O)63+⇌Cr(H2O)5(OH)2++H+ is about 10−4 [11, 12]. At pH lower than 4 the chromium(III) complex exists mainly in the hexa-aqua monomeric form. Its spectrum shows maxima at 575 nm and 410 nm (at the visible region). An amount of [Cr(H2O)5(OH)]2+ is though always present under the above conditions. In the experiments reported here that are conducted over the pH range 3-4 the reaction should be considered as taking place first with [Cr(H2O)5(OH)]2+ rather than with [Cr(H2O)6]3+, since a fast first step is taking place and it is very well known that Cr(H2O)63+ is very substitution inert. In the ligand molecule there are two intramolecular hydrogen bonds, one between adjacent hydroxyl groups and the other between the hydroxyl group and the carbonyl oxygen atom [13] (Scheme 1).

Bottom Line: The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are DeltaH( not equal) (2(obs)) = 61.2 +/- 3.1 kJmol(-1), DeltaS( not equal) (2(obs)) = -91.1 +/- 11.0 JK(-1)mol(-1), DeltaH( not equal) (3(obs)) = 124.5 +/- 8.7 kJmol(-1), and DeltaS( not equal) (3(obs)) = 95.1 +/- 29.0 JK(-1)mol(-1).These two stages are proposed to proceed via associative mechanisms.The reactions are accompanied by proton release, as shown by the pH decrease.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Inorganic Chemistry, University of Athens, Panepistimioupolis, 15771 Athens, Greece.

ABSTRACT
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(H(2)O)(5)(OH)](2+) with the ligand in weak acidic aqueous solutions that follows an I(d) mechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are DeltaH( not equal) (2(obs)) = 61.2 +/- 3.1 kJmol(-1), DeltaS( not equal) (2(obs)) = -91.1 +/- 11.0 JK(-1)mol(-1), DeltaH( not equal) (3(obs)) = 124.5 +/- 8.7 kJmol(-1), and DeltaS( not equal) (3(obs)) = 95.1 +/- 29.0 JK(-1)mol(-1). These two stages are proposed to proceed via associative mechanisms. The positive value of DeltaS( not equal) (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.

No MeSH data available.