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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) (a) and 2,3-DHBA (b). Conditions: (a) Chromium(III) Spectrum: [Cr(III)] = 0.050 M,  T=298 K. Spectrophotometric cell path d = 1 cm. (b) 2,3-DHBA Spectra: [2,3-DHBA] = 0.040 M,  T =  298 K. –■– 2,3-DHBA reduced, –– 2,3-DHBA oxidized (minutes), –▲– 2,3-DHBA oxidized (hours), –▾– 2,3-DHBA oxidized (a few days). Spectrophotometric cell path d = 1 cm.
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fig2: UV-Vis spectra of Cr(III) (a) and 2,3-DHBA (b). Conditions: (a) Chromium(III) Spectrum: [Cr(III)] = 0.050 M, T=298 K. Spectrophotometric cell path d = 1 cm. (b) 2,3-DHBA Spectra: [2,3-DHBA] = 0.040 M, T = 298 K. –■– 2,3-DHBA reduced, –– 2,3-DHBA oxidized (minutes), –▲– 2,3-DHBA oxidized (hours), –▾– 2,3-DHBA oxidized (a few days). Spectrophotometric cell path d = 1 cm.

Mentions: The reactants used were of analytical grade. 2,3-DHBA (Ferak Berlin) was used as received and was dissolved in dilute (0.1 M) solution of KOH (Merck) for pH adjustment for the ligand to be dissolved, in concentrations ranging from 3.9 × 10−3 to 7.8 × 10−3 M. Stock solutions of Cr(III) were prepared from Cr(NO3)3 · 9H2O (Fluka). The chromium(III) concentrations ranged between (5–14) × 10−2 M. The ionic strength was adjusted using KNO3. The solutions of 2,3-DHBA were used soon after their preparation in order to avoid transformation and decomposition reactions. The addition of the Cr(III) solution kept the pH below 4 due to its acidic hydrolysis: (1)Cr(H2O)63+⇌Cr(H2O)5(OH)2++H+ The UV-Vis spectrum of the chromium(III) solution and of the ligand along with spectra of its oxidation products at various times after dissolution are given in Figure 2.


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) (a) and 2,3-DHBA (b). Conditions: (a) Chromium(III) Spectrum: [Cr(III)] = 0.050 M,  T=298 K. Spectrophotometric cell path d = 1 cm. (b) 2,3-DHBA Spectra: [2,3-DHBA] = 0.040 M,  T =  298 K. –■– 2,3-DHBA reduced, –– 2,3-DHBA oxidized (minutes), –▲– 2,3-DHBA oxidized (hours), –▾– 2,3-DHBA oxidized (a few days). Spectrophotometric cell path d = 1 cm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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fig2: UV-Vis spectra of Cr(III) (a) and 2,3-DHBA (b). Conditions: (a) Chromium(III) Spectrum: [Cr(III)] = 0.050 M, T=298 K. Spectrophotometric cell path d = 1 cm. (b) 2,3-DHBA Spectra: [2,3-DHBA] = 0.040 M, T = 298 K. –■– 2,3-DHBA reduced, –– 2,3-DHBA oxidized (minutes), –▲– 2,3-DHBA oxidized (hours), –▾– 2,3-DHBA oxidized (a few days). Spectrophotometric cell path d = 1 cm.
Mentions: The reactants used were of analytical grade. 2,3-DHBA (Ferak Berlin) was used as received and was dissolved in dilute (0.1 M) solution of KOH (Merck) for pH adjustment for the ligand to be dissolved, in concentrations ranging from 3.9 × 10−3 to 7.8 × 10−3 M. Stock solutions of Cr(III) were prepared from Cr(NO3)3 · 9H2O (Fluka). The chromium(III) concentrations ranged between (5–14) × 10−2 M. The ionic strength was adjusted using KNO3. The solutions of 2,3-DHBA were used soon after their preparation in order to avoid transformation and decomposition reactions. The addition of the Cr(III) solution kept the pH below 4 due to its acidic hydrolysis: (1)Cr(H2O)63+⇌Cr(H2O)5(OH)2++H+ The UV-Vis spectrum of the chromium(III) solution and of the ligand along with spectra of its oxidation products at various times after dissolution are given in Figure 2.

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.