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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.


UV-Vis spectra of Cr(III)/2,3-DHBA mixture at various times after mixing. Conditions:  [Cr(III)]0 = 0.036 M, [2,3-DHBA]0 =  0.011 M, and T = 296 K. Spectrophotometric cell path d = 1 cm. –■– 0:00 hours –– 1:00 hour, –▲– 2:00 hours, –▾– 3:00 hours, –♦– 4:00 hours, –◂– 5:00 hours, –▸– 29:00 hours, –– a few days.
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fig3: UV-Vis spectra of Cr(III)/2,3-DHBA mixture at various times after mixing. Conditions: [Cr(III)]0 = 0.036 M, [2,3-DHBA]0 =  0.011 M, and T = 296 K. Spectrophotometric cell path d = 1 cm. –■– 0:00 hours –– 1:00 hour, –▲– 2:00 hours, –▾– 3:00 hours, –♦– 4:00 hours, –◂– 5:00 hours, –▸– 29:00 hours, –– a few days.

Mentions: All kinetic experiments were conducted at pH values below 4 in the presence of air. The kinetics was followed spectrophotometrically by recording the absorbances at various reaction times after mixing (Figure 3). The absorbances were recorded on a Hitachi Model 100-60 spectrophotometer. They refer to the substitution of water molecules in the [Cr(H2O)6]3+ coordination sphere by 2,3-DHBA and were followed at 573 nm where the biggest absorbance difference between the final product and the initial mixture of [Cr(H2O)6]3+/2,3-DHBA exists.


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)

UV-Vis spectra of Cr(III)/2,3-DHBA mixture at various times after mixing. Conditions:  [Cr(III)]0 = 0.036 M, [2,3-DHBA]0 =  0.011 M, and T = 296 K. Spectrophotometric cell path d = 1 cm. –■– 0:00 hours –– 1:00 hour, –▲– 2:00 hours, –▾– 3:00 hours, –♦– 4:00 hours, –◂– 5:00 hours, –▸– 29:00 hours, –– a few days.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig3: UV-Vis spectra of Cr(III)/2,3-DHBA mixture at various times after mixing. Conditions: [Cr(III)]0 = 0.036 M, [2,3-DHBA]0 =  0.011 M, and T = 296 K. Spectrophotometric cell path d = 1 cm. –■– 0:00 hours –– 1:00 hour, –▲– 2:00 hours, –▾– 3:00 hours, –♦– 4:00 hours, –◂– 5:00 hours, –▸– 29:00 hours, –– a few days.
Mentions: All kinetic experiments were conducted at pH values below 4 in the presence of air. The kinetics was followed spectrophotometrically by recording the absorbances at various reaction times after mixing (Figure 3). The absorbances were recorded on a Hitachi Model 100-60 spectrophotometer. They refer to the substitution of water molecules in the [Cr(H2O)6]3+ coordination sphere by 2,3-DHBA and were followed at 573 nm where the biggest absorbance difference between the final product and the initial mixture of [Cr(H2O)6]3+/2,3-DHBA exists.

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.