Kinetics of oxidation of SCN− by diperiodato cuprate(III) (DPC) in alkaline medium

The kinetics of oxidation of SCN⁻ by DPC has been investigated in alkaline medium. The reaction shows first-order dependence in [SCN⁻]. The pseudo-first-order rate constant (k_obs) changes differently under different [OH⁻]. At low [OH⁻], k_obs decreases when [OH⁻] increases, but when [OH⁻] increases to enough extent, k_obs increases with increase in [OH⁻]. Free radicals were observed in the process of reaction. A plausible mechanism involving Cu(HL)₂ and CuL as active substrates in the reaction has been proposed. The rate equations derived from the mechanism explain all the experimental phenomena satisfactorily. © 1996 John Wiley & Sons, Inc.     

Investigation of the kinetics of aquation of the 1:2 complex between Cr and nitrilotriacetic acid

Aquation of the 1:2 complex between Cr^III and nitrilotriacetic acid (NTA) was monitored using a combination of capillary electrophoresis (CE), ultraviolet–visible (UV–vis) spectrophotometry, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. To our knowledge, this is the first published report of the use of either CE or ATR-FTIR to monitor the kinetics of ligand exchange reactions of Cr^III–aminocarboxylate complexes. The aquation products were identified as the 1:1 Cr^III complex with NTA and “free” NTA. The 1:1 complex dimerized to form a 2:2 complex in a slower subsequent reaction step. Rates of disappearance of the 1:2 complex were first-order under all experimental conditions. First-order rate constants for aquation, k_obs (h⁻¹), measured using all three techniques were similar at equivalent pH conditions, and with values reported previously in the literature. Measured k_obs values exhibited a complicated pH dependence with three distinct regions: (i) at pH < 6.5, k_obs values increased with decreasing pH, (ii) between pH 6.5 and 8.0, k_obs values were relatively constant, and (iii) at 8.0 < pH < 10.0, k_obs increased with increasing pH and then leveled off. A kinetic model incorporating five distinct aquation pathways was successfully employed to model the pH dependence of k_obs from 0.0 < pH < 10.0. These results show that CE and ATR-FTIR can be used as tools for better understanding ligand exchange processes occurring in aqueous solution.     

Reactive kinetics of carbon dioxide loaded aqueous blend of 2-amino-2-ethyl-1,3-propanediol and piperazine using a pressure drop method

In this study, a new pressure drop method has been used to investigate the kinetics of carbon dioxide reaction with aqueous blend of 2-amino-2-ethyl-1,3-propanediol (AEPD) with piperazine (PZ). The blending of a small amount of PZ with AEPD has a significant effect on the observed rate constant, k_obs. It was observed that k_obs values of the blend increased more than twice than the summation of k_obs values of individual alkanolamines. The reaction kinetics in this study were modeled by assuming a termolecular mechanism. The addition of 0.1 mol/L of PZ to 1 mol/L AEPD exhibited an observed rate constant, k_obs of 8824.1 s⁻¹, which is comparable to other alkanolamine mixtures. Hence, PZ/AEPD mixtures can be potentially used for rapid carbon dioxide capture.     

Effect of bromide salts on the acid hydrolysis of anti-bacterial hydrophilic Schiff base amino acid iron(II) complexes

Salt effects on the kinetics of acid hydrolysis of some novel hydrophilic iron(II) complexes have been investigated in aqueous medium. The ligands are derived from the condensation of amino acids (glycine, L-alanine, L-leucine, L-isoleucine, DL-methionine, DL-serine or L-phenylalanine) and sodium 2-hydroxybenzaldehyde-5-sulfonate. The reaction was studied under conditions of pseudo first order kinetics. The general rate equation was suggested as follows: rate = k _obs[complex], where k _obs = k ₂[H⁺]. The reaction rate decreases with an increase of the salt concentration.     

Kinetics and mechanism of oxidation of some nickel(II)–imine–oxime complexes by peroxodisulfate in aqueous acidic solutions

The kinetics of the oxidation of [Ni(II)(H₂L¹)](ClO₄)₂, (H₂L¹ = 3,8-dimethyl-4,7-diaza-3,7-decadiene-2,9-dione dioxime) and [Ni(II)(HL²)]ClO₄, (H₂L² = 3,9-dimethyl-4,8-diaza-3,8-undecadiene-2,10-dione dioxime) by peroxodisulfate anion (PDS) in aqueous media at 298.0 K have been studied. The kinetics of oxidation of both Ni(II) complexes was found to be first order in the complex concentration. The dependence of the pseudo-first-order rate constant, k _obs, for both complexes showed first-order dependence on PDS concentration. The kinetics of oxidation of [Ni(II)(H₂L¹)]²⁺ complex showed a complex dependence on [H⁺] over the pH range of 4.98–7.50, whereas that of [Ni(II)(HL²)]⁺ is independent of pH over the pH range of 5.02–7.76. The value of k _obs, for both complexes, decreased with increasing ionic strength consistent with the involvement of oppositely charged ions in the rate-determining step. The effect of ionic strength is more pronounced for [Ni(II)(H₂L¹)]²⁺–PDS reaction than for [Ni(II)(HL²)]⁺–PDS reaction, confirming the higher charges of the latter.     

A study of the self reaction of CH2ClO2 and CHCl2O2 radicals at 298 K

A low‐pressure discharge‐flow system equipped with laser‐induced fluorescence (LIF) detection of NO₂ and resonance‐fluorescence detection of OH has been employed to study the self reactions CH₂ClO₂ + CH₂ClO₂ → products (1) and CHCl₂O₂ + CHCl₂O₂ → products (2), at T = 298 K and P = 1–3 Torr. Possible secondary reactions involving alkoxy radicals are identified. We report the phenomenological rate constants (k_obs) k_1obs = (4.1 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ k_2obs = (8.6 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and the rate constants derived from modelling the decay profiles for both peroxy radical systems, which takes into account the proposed secondary chemistry involving alkoxy radicals k₁ = (3.3 ± 0.7) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ k₂ = (7.0 ± 1.8) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ A possible mechanism for these self reactions is proposed and QRRK calculations are performed for reactions (1), (2) and the self‐reaction of CH₃O₂, CH₃O₂ + CH₃O₂ → products (3). These calculations, although only semiquantitative, go some way to explaining why both k₁ and k₂ are a factor of ten larger than k₃ and why, as suggested by the products of reaction (1) and (2), it seems that the favored reaction pathway is different from that followed by reaction (3). The atmospheric fate of the chlorinated peroxy species, and hence the impact of their precursors (CH₃Cl and CH₂Cl₂), in the troposphere are briefly discussed. HC(O)Cl is identified as a potentially important reservoir species produced from the photooxidation of these precursors. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 433–444, 1999     

Aspects of Degradation Kinetics of Azithromycin in Aqueous Solution

The chemical stability of azithromycin (AZM) in aqueous solution has been investigated utilizing a stability-indicating LC assay with ultraviolet detection. The degradation kinetics were studied as functions of pH (4–7.2), buffer composition (phosphate, acetate, and citrate), buffer concentration, ionic strength, drug concentration and temperature. The observed rate obtained by measuring the remaining intact AZM was shown to follow pseudo-first-order kinetics. The maximum stability of AZM occured at an approximate pH 6.3 in 0.05 M potassium phosphate. The observed degradation rate increased with ionic strength, buffer concentration and obeyed the Arrhenius equation over the temperature range investigated (70–100 °C). The apparent energy of activation (E _a) for AZM in solution was found to be 96.8 kJ mol^?1 and by application of the Arrhenius equation the stability at 25 °C (k ₂₅) and 40 °C (k ₄₀) had been predicted. Moreover, the degradation rate of AZM was independent on its initial concentration. Trace metal ions are unlikely to be involved in the degradation of AZM in aqueous solution. The major degradation product of AZM in aqueous solution was isolated and identified by LC–MS–MS and ¹H and ¹³C NMR spectra.     

Study of the kinetics and activation parameters of reduction of Mn(III) to Mn(II) by SO3 2− ion in (MnSiW11O40H2)5- heteropoly ion

In order to gain an insight into the mechanism of reduction of Mn(III) heteropoly ions, and also to establish the conditions for use of some of these ions as oxidizing agent, following measurements have been made. The pseudo-first order rate constants, k_obs, have been determined and specific rate constants, k, were calculated from the plots of k_obs against SO₃ ^2? concentrations. A plot of ln(k₂/T) against inverse temperature gives enthalpy of activation as 10.67 kJ mol^?1 and entropy of activation as ?237.90 J K^?1 mol^?1. Effects of ionic strength and pH have also been studied over a limited range.     

Kinetics of the Decomposition of Ferrocenium Ion and Its Derivatives

The first-order kinetics of the decomposition of ferrocenium ion (Fc⁺) and its substitution derivatives have been studied in aqueous sulfuric acid and in the presence of excess Ce(IV) ion. The observed first-order rate constant (k_obs) is expressed as k_obs = k_d for the acyl-substituted ferrocenium ions and k_obs = k_d+ k_ox[Ce(IV)]_o for the unsubstituted and alkyl-substituted ferrocenium ions. Electron-donating alkyl substituents stabilize the ferrocenium ion whereas electron-withdrawing acyl substituents make it less stable. The order of relative stability toward decomposition is 1,1′-dimethyl Fc⁺ ≥ butyl Fc⁺ > 1,1-dimethylpropyl Fc⁺ > Fc⁺ > > formyl Fc⁺ > acetyl Fc⁺ > > benzoyl Fc⁺. A mechanism to interpret the kinetics is also given.     

Kinetic evidence for the occurrence of general acid-base catalysis in N-methylhydroxylaminolysis of N-ethoxycarbonylphthalimide (NCPH)

Pseudo-first-order rate constants (k_{1 obs}) for the reaction of MeNHOH with NCPH obey the relationship: k_{1 obs}=k^′_b[MeNHOH]_T² where [MeNHOH]_T represents total concentration of N-methylhydroxylamine buffer. The rate constants, k_{1 obs} obtained at different total concentration of acetate buffer ([Buf]_T) in the presence of 0.004 mol dm⁻³ MeNHOH follow the relationship: k_{1 obs}=k_b[Buf]_T. The values of acetate buffer-catalyzed rate constant (k_b) at different pH reveal the occurrence of both general base- and general acid- or general base-specific acid-catalysis in the reaction of MeNHOH with NCPH. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 647–654, 1997.     

Effective scrap iron particles (SIP) pre-treatment for complete mineralization of benzidine based azo dye effluent

This study proposed that hybrid scrap cast iron particles (SIP)-aerobic biodegradation technology could enhance the biodegradability of toxic wastewater. SIP cleaved the azo linkages of Direct Green1 dye to form benzidine, 4-aminophenol, aniline and 1,2,7-triamino-8-hydroxynapthalene-3,6-disulfonic acid. SIP-mediated dye reduction was effective at wide pH range; however, kinetic analysis revealed fastest pseudo-first order dye reduction rate at acidic pH 3 (k_d = 0.549 min⁻¹) followed by pH 9 (k_d = 0.383 min⁻¹) and pH 7 (k_d = 0.318 min⁻¹). The daughter aromatic amines produced were partially adsorbed onto the SIP surface and maximally at neutral pH. The adsorption process followed pseudo-second order adsorption kinetics and Langmuir isotherm. Benzidine was adsorbed more than 4-aminophenol and aniline. BOD₅ of the SIP-treated effluent increased from 0.93 to 12 mg/L showing improved biodegradability. The daughter amines were rapidly mineralized in the aerobic bioreactor within 6 h. Cost-effective SIP pre-treatment could accelerate mineralization and detoxification of recalcitrant wastewater.     

Potentiometric study of the kinetics and equilibria of formation of chloroglycinate palladium(II) complexes from bis-glycinato palladium(II) complexes

The equilibria and kinetics of the reaction of Pd(gly)₂ complexes with hydrogen ions and chloride ions has been studied by a potentiometric method. The underlying idea of the method is the measurement of solution pH as a function of reaction time t using a glass electrode. The solutions used had the following initial compositions: xM Pd(gly)₂, xM Hgly, and 1 M NaCl with x = 1 × 10^?4, 5 × 10^?4, and 1 × 10^?3; initial pH₀ was from ～3.5 to ～4.4. The experimentally determined pH versus t dependences and the rate equation for a pseudo-second-order reaction were used to determine the equilibrium constant of formation of Pd(gly)(Hgly)Cl complexes from Pd(gly)₂ complexes and the observed rate constant for this reaction, k _obs. The dependence of k _obs on the pH of the acid solutions studied was assigned to a change in the sequence of the reactions of addition of a hydrogen ion and a chloride ion to the complex Pd(gly)₂.     

Equilibrium and kinetic studies of the Fe(III) complex formation with glycinehydroxamic acid

Spectrophotometric methods were utilized for stability constant determinations of the Fe(III) interaction with glycinehydroxamic acid (GX) at I = 0.15 M NACl and T = 25°C. Program SQUAD II was used to assess the absorbance data in the wavelength range 300–520 nm. Four constants were determined for 1:1:1, 1:1:0, 2:1:1 or 3:1:3 and 2:1:0 complex species in the pH range 1.0–7.5. The kinetics of the interactions of Fe(III) with GX were also studied in the pH range 1.0–3.0 by the stopped flow method. The observed rate constant at a given pH was k_obs = _A + BT_GX. The parameters A and B are functions of pH in the range 1.7–3.0 and only A is a function of pH in the range 1.0–1.7. The mechanism of complex formation was discussed in the light of the experimental results and the equilibrium study. It has been concluded that FeOH²⁺ is the reactive species in the complex formation of FeGXH³⁺ species while Fe(OH)₂⁺ is the reactive species in the complex formation of FeGX²⁺ species.     

Studies in nickel(IV) chemistry. Part 6. Kinetics of electron transfer from phenylhydrazine to tris-(dimethylglyoximato) nickelate(IV) in aqueous medium

The kinetics of electron transfer from phenylhydrazine(S) to tris-(dimethylglyoximato) nickelate(IV), Ni(dmg) (dmg^2? = dimethylglyoximate dianion), have been studied in aqueous medium in the range of 6.21 ? pH ? 12.2. The kinetics exhibit a pseudo-first-order disappearance of Ni(dmg) when excess S is present. The pseudo-first-order rate constants k_obs are almost linearly dependent on [S]₀ for varying concentrations of the reductant. At constant [S]₀, the k_obs?pH profile is U shaped. The k_obs values register a decrease as the [H⁺] is increased in the pH range of ∽12.2–9.5, remain almost constant (minimum) in the range of ∽9–8, and then again linearly increase as [H⁺] is increased in the pH range of ∽7–6.21. Results are interpreted in terms of a probable mechanism involving outer-sphere electron transfer from the phenylhydrazine and phenylhydrazinium cation species to the unprotonated and one-protonated species of the Ni(IV) complex. The reduction rate appears to be dependent on the nature of the species (unprotonated and one-protonated) of the oxidant Ni(IV) complex. The phenylhydrazinium cation reduces the Ni(IV) complex at least one order of magnitude faster than does the neutral reductant species. The major product of the oxidation of phenylhydrazine by the Ni(IV) complex is 4-hydroxyazobenzene with a small amount of phenol.     

Kinetics and mechanism of the defluorination of 8-fluoropurine nucleosides in basic and acidic media

For investigating the stability of C(8)-fluorine bond in 8-fluoropurine nucleosides some protected 8-fluoroguanosine, 8-fluoroinosine and 8-fluoroadenosine derivatives were prepared by direct fluorination of acetyl-protected purine nucleosides with elemental fluorine in solvents such as chloroform, acetonitrile and nitromethane. Fluorination reactions conducted in chloroform medium gave better yields of 8-fluoropurines. The fluorination yields were slightly lower when acetonitrile or nitromethane was used as solvent, but the product purification was found to be much easier. When the synthesized, protected fluoronucleosides were subjected to standard basic (NH₃ in methanol or 2-propanol) and acidic (HCl in methanol) deprotection conditions relevant to nucleoside chemistry, an efficient defluorination reaction took place. The kinetics of these defluorination reactions were conveniently followed, under pseudo-first-order reaction conditions, using ¹⁹F NMR spectroscopy. ¹H NMR, LC-MS and mass spectroscopy identified the products of the kinetic reaction mixtures. The defluorination reaction rate constants (k_obs) in basic media depended upon the electron density at C(8) while the k_obs data in acidic medium were determined by the pK_a of N⁷. An addition-elimination based mechanism (S_NAr) has been proposed for the defluorination reactions of these 8-fluoropurine nucleosides.     

A study of some reactions of acetylacetonatobis(ethylenediamine)cobalt(III) ion in sodium hydroxide solution

The reactions of acetylacetonato cobalt (III) ion in sodium hydroxide solutions have been studied spectrophotometrically over a range of temperatures and hydroxide ion concentrations. The activation enthalpy, ΔH^≠ was 70.6 kJ mol^?1 and the activation entropy, ΔS^≠ was ? 119 JK^?1mol^?1, with a rate law of k_obs = k₂ [OH^?]². A mechanism involving initial de-chelation of the acetylacetone ligand is suggested. The rate of exchange of methyl hydrogen of the acetylacetone ligand was studied, using proton nuclear magnetic resonance. The rate law was k_obs = k [OH^?]. Initial de-chelation is also suggested as a mechanism for this process. The ¹³C nuclear magnetic resonance spectrum of the complex is reported.     

Kinetic study on acid‐base catalyzed hydrolysis of azimsulfuron,a sulfonylurea herbicide

Pseudo‐first‐order rate constants (k_obs) for hydrolysis of a sulfonylurea herbicide, azimsulfuron, AZIM®, {N‐[[(4,6‐dimethoxy‐2‐pyrimidinyl)amino]carbony]‐1‐methyl‐4‐(2‐methyl‐2H‐tetrazol‐5‐yl)‐1H‐pyrazole‐5‐sulfonamide} (AZS) follow an empirical relationship: k_obs =α₁ + α₂[⁻OH] + α₃[⁻OH]² within the [NaOH] range of 0.1–2.0 M at different temperatures ranging from 40 to 55°C. The contribution of α₃[⁻OH]² term is small compared with α₂[⁻OH] term and this turns out to be zero at 60°C. Pseudo‐first‐order rate constants (k_obs) for hydrolysis of AZS within the [H⁺] range from 2.5 × 10⁻⁶ to 1.4 M follow the relationship: k_obs = (α₁K _a + B₁[H⁺] + B₂[H⁺]²)/([H⁺] + K_a) where pK_a = 4.37 at 50°C. The value of B₁ is nearly 25 times larger than that of α₁. The rate of alkaline hydrolysis of AZIM is weakly sensitive to ionic strength. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 253–260, 1999     

Electron transfer mechanism for the oxidation of ternary N-(2-acetamido)iminodiacetatocobaltate(II) complexes involving succinate and maleate as secondary ligands by periodate

The kinetics of oxidation of the ternary complexes [Co^II(ADA)(Su)(H₂O)]^2? and [Co^II(ADA)(Ma)(H₂O)]^2? (ADA?=?N-(2-acetamido)iminodiacetate, Su?=?succinate and Ma?=?maleate) by periodate have been investigated spectrophotometrically at 580?nm under pseudo-first-order conditions in aqueous medium over 30?C50?°C range, pH 3.72?C4.99, and I?=?0.2?mol?dm^?3. The kinetics of the oxidation of [Co^II(ADA)(Su)(H₂O)]^2? obeyed the rate law d[Co^III]/dt?=?[Co^II(ADA)(Su)(H₂O)]^2?[H₅IO₆] {k ₄ K ₅?+?(k ₅ K ₆ K ₂/[H⁺)}, and the kinetics oxidation of [Co^II(ADA)(Ma)(H₂O)]^2? obeyed the rate law d[Co^III]/dt?=?k ₁ K ₂[Co^II] _T [I^VII] _T /{1?+?([H⁺]/K ₇)?+?K ₂[I^VII] _T }. The pseudo-first-order rate constant, k _obs, increased with increasing pH, indicating that the hydroxo form of maleate complex, [Co^II(ADA)(Ma)(OH)]^3?, is the reactive species. The initial Co(III) products were slowly converted to the final products, fitting an inner-sphere mechanism. Thermodynamic activation parameters were calculated using the transition state theory equation. The initial cobalt(II) complexes were characterized by physicochemical and spectroscopic methods.     

The oxidation of carbohydrazide by the 12-tungstocobaltate(III) ion in acidic medium: Kinetics and mechanism

The redox reaction between the 12-tungstocobaltate(III) ion and carbohydrazide is first order with respect to both the oxidant and the substrate. The observed pseudo first-order rate constant, k_obs, is retarded by increasing the concentrations of H⁺ and alkali metal ion (Li⁺, Na⁺ and K⁺). There is a linear correlation between the k_obs and the concentrations of carbohydrazide and H⁺ ion, but the plots of k_obs against the concentrations of the alkali metal ions is non-linear. However, the same data is applicable to the Davies equation for the effect of the ionic strength on the k_obs.     

Effects of CH3OH‐H2O and CH3OH solvents on rate of reaction of phthalimide with piperidine

Pseudo‐first‐order rate constants (k_obs) for the cleavage of phthalimide in the presence of piperidine (Pip) vary linearly with the total concentration of Pip ([Pip]_T) at a constant content of methanol in mixed aqueous solvents containing 2% v/v acetonitrile. Such linear variation of k_obs against [Pip]_T exists within the methanol content range 10%–∼80% v/v. The change in k_obs with the change in [Pip]_T at 98% v/v CH₃OH in mixed methanol‐acetonitrile solvent shows the relationship: k_obs = k[Pip]_T + k[Pip], where respective k and k represent apparent second‐order and third‐order rate constants for nucleophilic and general base‐catalyzed piperidinolysis of phthalimide. The values of k_obs, obtained within [Pip]_T range 0.02–0.40 M at 0.03 M NaOH and 20 as well as 50% v/v CH₃OH reveal the relationship: k_obs = k₀/(1 + {k_n[Pip]/k_OX[⁻OX]_T}), where k₀ is the pseudo‐first‐order rate constant for hydrolysis of phthalimide, k_n and k_OX represent nucleophilic second‐order rate constants for the reaction of Pip with phthalimide and for the XO⁻‐catalyzed cyclization of N‐piperidinylphthalamide to phthalimide, respectively, and [⁻OX]_T = [NaOH] + [⁻OX_re], where [⁻OX_re] = [⁻OH_re] + [CH₃O_re⁻]. The reversible reactions of Pip with H₂O and CH₃OH produce ⁻OH_re and CH₃O_re⁻ ions. The effects of mixed methanol‐water solvents on the rates of piperidinolysis of PTH reveal a nonlinear decrease in k with the increase in the content of methanol. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 29–40, 2001