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

2010 ◽  
Vol 2010 ◽  
pp. 1-8 ◽  
Author(s):  
Athinoula L. Petrou ◽  
Vladimiros Thoma ◽  
Konstantinos Tampouris
Keyword(s):  
Aqueous Solutions ◽  
Activation Parameters ◽  
Entropy Change ◽  
Hydroxyl Group ◽  
Positive Entropy ◽  
Dihydroxybenzoic Acid ◽  
Three Stages ◽  
Ph Decrease ◽  
Two Stages ◽  
Mechanism Of The Reaction

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 anIdmechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters areΔH≠=61.2±3.1 kJmol−1,ΔS≠=−91.1±11.0 JK−1mol−1,ΔH≠=124.5±8.7 kJmol−1, andΔS≠=95.1±29.0 JK−1mol−1. These two stages are proposed to proceed via associative mechanisms. The positive value ofΔS≠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.

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

2008 ◽  
Vol 2008 ◽  
pp. 1-8 ◽  
Author(s):  
Kimon Zavitsanos ◽  
Konstantinos Tampouris ◽  
Athinoula L. Petrou
Keyword(s):  
Aqueous Solutions ◽  
Coordination Sphere ◽  
Activation Parameters ◽  
Experimental Results ◽  
Ligand Concentration ◽  
Water Molecules ◽  
Dihydroxybenzoic Acid ◽  
Proton Release ◽  
Three Stages ◽  
Ph Decrease

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 concentrationk1(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 areΔH2(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.

scholarly journals Kinetics and Mechanisms of the Chromium(III) Reactions with 2,4- and 2,5-Dihydroxybenzoic Acids in Weak Acidic Aqueous Solutions

2010 ◽  
Vol 2010 ◽  
pp. 1-10 ◽  
Author(s):  
Kimon Zavitsanos ◽  
Athinoula L. Petrou
Keyword(s):  
Hydrogen Bonding ◽  
Aqueous Solutions ◽  
Activation Parameters ◽  
Intramolecular Hydrogen Bonding ◽  
Dihydroxybenzoic Acid ◽  
Proton Release ◽  
Second Stage ◽  
Ph Decrease ◽  
Intramolecular Hydrogen ◽  
Two Stages

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 constantk1(obs)=k1[DHBA]+Cand the corresponding activation parameters areΔH1(2,4)≠=49,5 kJ/mol−1,ΔS1(2,4)≠=−103,7J mol−1K−1,ΔH1(2,5)≠=60,3 kJ/mol−1, andΔS1(2,5)≠=−68,0 J mol−1K−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 areΔH2(2,4)≠=45,13 kJ/mol−1,ΔS2(2,4)≠=−185,9 J mol−1K−1,ΔH2(2,5)≠=54,55 kJ/mol−1, andΔS2(2,5)≠=−154,8 J mol−1K−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.

scholarly journals Kinetics and Mechanism of the Reaction between Chromium(III) and 3,4-Dihydroxy-Phenyl-Propenoic Acid (Caffeic Acid) in Weak Acidic Aqueous Solutions

2008 ◽  
Vol 2008 ◽  
pp. 1-7 ◽  
Author(s):  
Vladimiros Thoma ◽  
Konstantinos Tampouris ◽  
Athinoula L. Petrou
Keyword(s):  
Aqueous Solutions ◽  
Metal Ion ◽  
Activation Parameters ◽  
Ligand Molecule ◽  
Propenoic Acid ◽  
Protonation Equilibrium ◽  
Three Stages ◽  
Ph Decrease ◽  
Rapid Protonation ◽  
Mechanism Of The Reaction

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 (Idmechanism) 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 areΔH2(obs)≠= 28.6±2.9 kJmol−1,ΔS2(obs)≠=−220  ±10 JK−1mol−1,ΔH3(obs)≠= 62.9±6.7 kJmol−1andΔS3(obs)≠=−121  ±22 JK−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.

Formation of Dislocation Channels in Neutron Irradiated Molybdenum

1972 ◽  
Vol 30 ◽  
pp. 672-673
Author(s):  
S. Mahajan
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Ambient Temperature ◽  
Room Temperature ◽  
Channel Formation ◽  
Early Stages ◽  
Transmission Electron ◽  
Three Stages ◽  
Two Stages ◽  
Polycrystalline Molybdenum

The evolution of dislocation channels in irradiated metals during deformation can be envisaged to occur in three stages: (i) formation of embryonic cluster free regions, (ii) growth of these regions into microscopically observable channels and (iii) termination of their growth due to the accumulation of dislocation damage. The first two stages are particularly intriguing, and we have attempted to follow the early stages of channel formation in polycrystalline molybdenum, irradiated to 5×1019 n. cm−2 (E > 1 Mev) at the reactor ambient temperature (∼ 60°C), using transmission electron microscopy. The irradiated samples were strained, at room temperature, up to the macroscopic yield point.Figure 1 illustrates the early stages of channel formation. The observations suggest that the cluster free regions, such as A, B and C, form in isolated packets, which could subsequently link-up to evolve a channel.

Kinetics and mechanism of the reaction of iron(III) with 6-methyl-2,4-heptanedione and 3,5-heptanedione

1990 ◽  
Vol 55 (8) ◽  
pp. 1984-1990 ◽  
Author(s):  
José M. Hernando ◽  
Olimpio Montero ◽  
Carlos Blanco
Keyword(s):  
Aqueous Solution ◽  
Metal Ion ◽  
Activation Parameters ◽  
Enol Form ◽  
Kinetics And Mechanism ◽  
Kinetic Constants ◽  
Steric Factors ◽  
Kinetics Of ◽  
Mechanism Of The Reaction

The kinetics of the reactions of iron(III) with 6-methyl-2,4-heptanedione and 3,5-heptanedione to form the corresponding monocomplexes have been studied spectrophotometrically in the range 5 °C to 16 °C at I 25 mol l-1 in aqueous solution. In the proposed mechanism for the two complexes, the enol form reacts with the metal ion by parallel acid-independent and inverse-acid paths. The kinetic constants for both pathways have been calculated at five temperatures. Activation parameters have also been calculated. The results are consistent with an associative activation for Fe(H2O)63+ and dissociative activation for Fe(H2O)5(OH)2+. The differences in the results for the complexes of heptanediones studied are interpreted in terms of steric factors.

The kinetics, isotope effects, and mechanism of the reaction of 2,2-di(4-nitrophenyl)-1,1,1-trifluoroethane with alkoxide bases in alcohol solvents

1985 ◽  
Vol 63 (3) ◽  
pp. 576-580 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Wlodzimierz Galezowski ◽  
Kenneth T. Leffek ◽  
Urszula Maciejewska
Keyword(s):  
Rate Constants ◽  
Isotope Effects ◽  
Activation Parameters ◽  
Tert Butanol ◽  
Deuterium Isotope Effects ◽  
Alcohol Solvents ◽  
Multistep Reaction ◽  
Mechanism Of The Reaction

The reaction between 2,2-di(4-nitrophenyl)-1,1,1-trifluoroethane and the alkoxide bases ŌCH3, ŌC2H5, ŌnC4H9, ŌCH(CH3)2, and ŌC(CH3)3 in their corresponding alcohol solvents is a multistep reaction with several intermediates: 2,2-di(4-nitrophenyl)-1,1-difluoro-1-alkoxyethane (A), 2,2-di(4-nitrophenyl)-1-fluoro-1-alkoxyethene (B), 2,2-di(4-nitrophenyl)-1,1-dialkoxyethene (C), 2,2-di(4-nitrophenyl)-1,1-difluoroethene (D), and 4,4′-dinitrobenzophene (E). Rate constants and activation parameters have been measured for the appearance of the two stable products B and C. The kinetic deuterium isotope effects for the appearance of B fell in the range of kH/kD = 1 to 2 at 25 °C for the primary and secondary alkoxides, whereas kH/kD = 5.4 at 30 °C for the appearance of D with tert-butoxide. Exchange experiments showed that H/D exchange took place between the substrate and solvent to the extent of 100% with methoxide, 50% with ethoxide and isopropoxide, and 0% with tert-butoxide. It is concluded the HF elimination from the substrate follows an (ElcB)R mechanism with methoxide/methanol, changing to (ElcB)I or E2 with tert-butoxide/tert-butanol.

Mechanistic Study on Gold(I)-Catalyzed Unsaturated Spiroketalization Reaction

2022 ◽  
Vol 19 ◽  
Author(s):  
Kamlesh Sharma
Keyword(s):  
Activation Barrier ◽  
Ring Formation ◽  
Hydroxyl Group ◽  
Mechanistic Study ◽  
Protecting Group ◽  
Wide Range ◽  
Oxocarbenium Ion ◽  
Metal Catalyzed ◽  
Insight Into ◽  
Mechanism Of The Reaction

Abstract: The mechanism of metal-catalyzed spiroketalization of propargyl acetonide is explored by employing DFT with the B3LYP/6-31+G(d) method. Acetonide is used as a regioselective regulator in the formation of monounsaturated spiroketal. The energies of transition states, intermediates, reactants and products are calculated to provide new insight into the mechanism of the reaction. The energetic features, validation of the observed trends in regioselectivity are conferred in terms of electronic indices via FMO analysis. The presence of acetonide facilitates a stepwise spiroketalization as it masks the competing nucleophile, and thus hydroxyl group present, exclusively acts as a nucleophile. The vinyl gold intermediate 3 is formed from 2 via activation barrier TS1. This is the first ring formation, which is 6-exo-dig cyclization. The intermediate 3 is converted into allenyl ether 4, which isomerizes to the intermediate oxocarbenium ion 5 via activation barrier TS2. The intermediate 5 cyclizes to 6 via TS3. This is the second ring formation. The intermediate 6 on protodeauration turns into 6,6-monounsaturated spiroketal 7. It is concluded that acetonide as a protecting group serves the purpose, and thus a wide range of spiroketals can be prepared, regioselectivity.

Agent-based economic models and econometrics

2012 ◽  
Vol 27 (2) ◽  
pp. 187-219 ◽  
Author(s):  
Shu-Heng Chen ◽  
Chia-Ling Chang ◽  
Ye-Rong Du
Keyword(s):  
Financial Markets ◽  
Agent Based Modeling ◽  
Computational Finance ◽  
Agent Based ◽  
Second Stage ◽  
The Past ◽  
Stage Development ◽  
Three Stages ◽  
The One ◽  
Two Stages

AbstractThis paper reviews the development of agent-based (computational) economics (ACE) from an econometrics viewpoint. The review comprises three stages, characterizing the past, the present, and the future of this development. The first two stages can be interpreted as an attempt to build the econometric foundation of ACE, and, through that, enrich its empirical content. The second stage may then invoke a reverse reflection on the possible agent-based foundation of econometrics. While ACE modeling has been applied to different branches of economics, the one, and probably the only one, which is able to provide evidence of this three-stage development is finance or financial economics. We will, therefore, focus our review only on the literature of agent-based computational finance, or, more specifically, the agent-based modeling of financial markets.

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