Kinetic and mechanistic study of the reduction of 2,6-dichlorophenol indophenol by dithionites

The reaction of 2,6-dichlorophenolindophenol (DCPI) and dithionites is studied kinetically by applying the stopped-flow technique. Reaction rate constants are given for the pH range 1.30–6.80. The reaction was found to follow first-order kinetics with respect to each of the reactants. For pH 3.97, 5.10 and 6.80, the second-order reaction rate constant was determined by applying four different technique. Mean values of k = 172±5, 200±2 and 276±4 l mol^?1s^?1 are given for pH 3.97, 5.10 and 6.80, respectively. A mechanism is proposed for the reaction, which suggests partial reactions of all possible species of DCPI and dithionites at any pH. An equation for the calculation of k at any pH is derived, which gives k as a function of [H⁺], the partial reaction rate constants and the dissociation constants of DCPI and H₂S₂O₄. Values of reaction rate constants of all possible partial reactions are also presented.     

Ag(I)‐Catalyzed Chlorination of Linezolid during Water Treatment: Kinetics and Mechanism

The kinetic and mechanistic study of Ag(I)‐catalyzed chlorination of linezolid (LNZ) by free available chlorine (FAC) was investigated at environmentally relevant pH 4.0–9.0. Apparent second‐order rate constants decreased with an increase in pH of the reaction mixture. The apparent second‐order rate constant for uncatalyzed reaction, e.g., k″_app = 8.15 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 0.076 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C and for Ag(I) catalyzed reaction total apparent second‐order rate constant, e.g., k″_app = 51.50 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 1.03 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C. The Ag(I) catalyst accelerates the reaction of LNZ with FAC by 10‐fold. A mechanism involving electrophilic halogenation has been proposed based on the kinetic data and LC/ESI/MS spectra. The influence of temperature on the rate of reaction was studied; the rate constants were found to increase with an increase in temperature. The thermodynamic activation parameters E_a, ΔH^#, ΔS^#, and ΔG^# were evaluated for the reaction and discussed. The influence of catalyst, initially added product, dielectric constant, and ionic strength on the rate of reaction was also investigated. The monochlorinated substituted product along with degraded one was formed by the reaction of LNZ with FAC.     

Hydrolysis of Cellobiose in Dilute Sulpuric Acid and Under Hydrothermal Conditions

The sulfuric acid hydrolysis rate of cellobiose between pH 2 and 3 is directly proportional to the acid concentration. In good agreement with other authors, an activation energy of 133 kJ/Mol was found under these acidic conditions. The relation of the reaction rate constants for the glucose formation and glucose degradation (k₁/k₂) shows, in contrast to the hydrolysis of cellulose, little dependence on the temperature. Hydroxymethylfurfural, and to a lesser extent furfural, are glucose degradation products, which are also consumed but at a lower reaction rate than glucose. At pH values between 3 and 4.7 (pure water) strong deviations of the hydrolysis rates were observed. The formation of organic acids decreases the pH but has no influence on the reaction rate. This fact indicates that hydrothermolysis follows a reaction mechanism different from that of acidic hydrolysis.     

Biphasic oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) by N-bromosuccinimide and the formation of chromium(IV) and chromium(V)

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr^III(phen)(H₂O)₂Cl₂]⁺, by N-bromosuccinimide (NBS) is biphasic. The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr^V(phen)Cl₂(O)]Br. The rate constants k_f and k_s, for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k_f and k_s, varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11–6.01) range at 25.0°C. The rate constants k_f and k_s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.     

Effect of the pH value on the thermal stability of alkaline phosphatase

The thermal stability of bovine and chicken intestinal alkaline phosphatase (IAP) was studied at 60°C over a pH range from 6.7 to 9.0. It was shown that the peaks of stability (at pH 7.5) and activity (at pH 9.0) do not coincide. The pH dependence of the elementary rate constants of dissociative thermal inactivation of the IAP dimers was determined: k ₁ (the rate constant of dissociation of active dimer E₂) and k _d (the rate constant of denaturation of the inactive monomers). At pH 7.5, the stability IAP attains its highest level. As the pH increases, k ₁ increases drastically while k _d does so only slightly. A comparison of the rate constants of dissociation k ₁ and association k ₁ showed that the interrelation between these parameters explains why the stability of the active enzyme is lower at acidic and basic pH values. The pH produces a weaker effect on the stability of the inactive monomers. An analysis of the thermal stability of chicken IAP at an optimum pH value and 55–60°C showed that the thermal inactivation is a three-stage process (including dissociation and denaturation) with an induction period. Measuring the induction period makes it possible to determine the minimum number of latent stages preceding the dissociation of E₂: 6, 4, and 3 at 55, 58, and 60°C, respectively.     

Studies on the rate of isotopic oxygen exchange between [Re(py)4O2]+ and solvent water

The rate of oxygen exchange between trans-[Re(py)₄O₂]⁺ and solvent water in pypyH⁺ buffer solution follows simple first-order kinetics and both oxygens are equivalent. The half-life for isotopic oxygen exchange is about 12 h at a pH of 5.0, 25°C, and [py] = 0.10 M. The observed rate constant for exchange increases with acidity, in the pH range 4 to 6, decreases with [py], and is nearly independent of ionic strength. A small but significant increase of k_obs occurs with increasing complex concentration. The rate of exchange follows the rate equation k_obs/2 = k₀ + k₁/[py] with k₀ = 1.4 × 10^?5(2) s^?1 and k₁ = 4.7 × 10^?7(1) M, s^?1 at 25°C. The activation parameters for the reaction at pH = 7.15 (predominately the k₀ term) are: ΔH* = +137.(1) kJ/M and ΔS* = +126.(1) J/MK. The pH effect and complex concentration effect are discussed in mechanistic terms. These results are compared to those found for [Re(en)₂O₂]⁺ and [Re(CN)₄O₂]^3?.     

Experimental versus theoretical evidence for the rate‐limiting steps in uncatalyzed and H+‐ and HO−‐catalyzed hydrolysis of the amide bond

Recent theoretical studies of the alkaline hydrolysis of the amide bond have indicated that the nucleophilic attack of the hydroxide ion at the carbonyl carbon of the amide group is rate limiting. This is shown to be inconsistent with a large amount of experimental observations where the expulsion of the leaving group has been shown to be rate limiting. A kinetic approach has been described, which allows us to diagnose whether the pH‐independent/uncatalyzed hydrolysis of amides involves (a) both the uncatalyzed water reaction (k_w) and H⁺‐ (k_H) and HO^?‐catalyzed (k_OH) water reaction, (b) only the k_w reaction, or (c) only the k + k_OH reaction. The analysis described in this critical review does not favor the recent theoretical claims of the absence of the water reaction in the pH‐independent/uncatalyzed hydrolysis of formamide and urea. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 599–611, 2009     

A shock tube study of CO + OH → CO2 + H and HNCO + OH → products via simultaneous laser absorption measurements of OH and CO2

The rate coefficients for the reactions were determined using mixtures of HNO₃/CO/Ar and HNO₃/HNCO/Ar in incident shock wave experiments. Simultaneous OH and CO₂ absorption time-histories were obtained via cw uv narrow-linewidth absorption at 32606.56 cm⁻¹ (λ = 306.687 nm) and cw infrared narrow-linewidth absorption at 2380.72 cm⁻¹ (λ = 4.2004 μm), respectively. The measurements of k₁ determined from measured CO₂ time-histories are in good agreement with those determined from previous measurements of OH time-histories at this laboratory. The rate coefficient for the overall reaction of HNCO + OH → Products was determined from analysis of OH data traces. The uncertainty in k₂ was found to be +22% −16%. By incorporating data from a previous low-temperature study, the following empirical expression was determined for the bimolecular reaction: over the temperature range 620–1860 K. From analysis of CO₂ data traces, an upper limit on the branching fraction (α = k_2a/k₂) for reaction (2a) of 10% was found, independent of temperature over the range 1250–1860 K. © 1996 John Wiley & Sons, Inc.     

Kinetics of substitution on chloro-1,1,7,7-tetraethyldiethylene-triaminegold(III) in methanol-water mixtures

Summary Rate constants are reported for the replacement of chloride by bromide in the complex [Au(Et₄dien-H)⁺ in methanol-water mixtures. This reaction follows the usual two-term rate law = (k_obs = k₁ + K₂[Br^–1]) for square planar complexes in all the mixtures studied. The logarithms of the rate constants correlate with Grunwald-Winstein solvent Y values and yield slopes of 0.32 (k₁) and 0.15 (k₂). The results suggest that both reaction paths are associative.Dien complexes of gold(III) yield conjugate base species at low pH; Et₄dien-H = Et₂NCH₂CH₂NCH₂CH₂NEt₂.     

ACCELERATED CLEAVAGE OF P-NITROPHENYL PICOLINATE CATALYZED BY COPPER(II) AND ZINC(II) COMPLEXES OF D-GLUCOSAMINE SCHIFF BASE IN MICELLAE SOLUTION

Abstract

The hydrolysis of p-nirrophenyl picolinate (PNPP) catalyzed by the Cu(II) and Zn(H) complexes of d-glucosamine schiff base was investigated kinetically by observing the rates of the release of p-nitrophenol in the buffered micellor solution at different pit and 25°C. The scheme for reaction acting mode involving a ternary complex contain ligand, metal ion and substrate in micelle was establish and the reaction mechanisms were discussed The experimental results indicated that the complexes catalyzed efficiently the hydrolysis of PNPP, especially Zn(II) complex. The rate constant k_N, which shows the catalytic reactivity of complexes, WBI determined to be 0.4251 s^?1 far Cu(II) complex at pH 7 60 in micellar solution. The catalytic reactivity of Zn(II) complex were much larger than Cu(II) complex, the k_N was determined to be 3.1914 s^?1 at same pH value.     

Hydrolysis of PNPP Catalyzed by Cu (II), Ni (II) Schiff Base Complexes in CTAB Micellar Solution

The kinetics of hydrolysis of p‐nitrophenyl picolinate(PNPP) catalyzed by metallomicelles formed from Cu (II), Ni (II) Schiff base complexes (CuL, NiL) and CTAB micelle were investigated in the pH range of 6.0–9.0 at 30°C. For the Cu (II) Schiff base complex CuL, the apparent rate constants (k _obsd) of PNPP hydrolysis initially increased with the increasing pH of reaction media, then fell off. For the Ni (II) Schiff base complex NiL, the k _obsd always increased with the increasing pH. The kinetic and thermodynamic parameters were calculated. The hydrolysis rate of PNPP catalyzed by Cu (II) complex was much larger than that by Ni (II) complex in CTAB micellar solution. The catalytic mechanism of the PNPP hydrolysis was discussed in detail, and the possibly active specie for the catalytic hydrolysis of PNPP was the monohydroxo metal complex.     

Kinetics of substitution of cis-diaqua-bis[2-(m-tolylazo)pyridine]-ruthenium(II) ion with pyridine-2-aldoxime in EtOH-H2O mixtures

Summary Kinetic and mechanistic studies on the anation of cis-[Ru(tap)₂(H₂O)₂]₂₊ (where tap = 2-(m-tolylazo)pyridine) by pyridine-2-aldoxime (LH) have been made spectrophotometrically at different temperatures (35–50° C) in aqueous medium and various EtOH-H₂O mixtures. The following rate law has been established at pH 5.6: k _obs = k ₁ k ₂[LH]/(k ₋₁ + k ₂[LH]) where k ₁ is the H₂O dissociation rate constant of the substrate complex and k _-1 and k ₂ are the aquation and ligand capturing rate constants of the pentacoordinate intermediate, [Ru(tap)₂(H₂O)]²⁺. The rate constants are independent of ionic strength. The reaction rate increases with increasing pH. Activation parameters (ΔH‡ and ΔS‡) have been calculated for media of four different dielectric constants and compared with other substitution reactions in aqueous medium. A dissociative mechanism is proposed.     

Kinetic Study of Radical Polymerization VIII. A Comprehensive Study of Solution Copolymerization of Vinyl Acetate and Methyl Acrylate by 1H‐NMR Spectroscopy

Radical copolymerization reaction of vinyl acetate (VA) and methyl acrylate (MA) was performed in a solution of benzene‐d₆ using benzoyl peroxide (BPO) as the initiator at 60°C. Kinetic studies of this copolymerization reaction were investigated by on‐line ¹H‐NMR spectroscopy. Individual monomer conversions vs. reaction time, which was followed by this technique, were used to calculate the overall monomer conversion, as well as the monomer mixture and the copolymer compositions as a function of time. Monomer reactivity ratios were calculated by various linear and nonlinear terminal models and also by simplified penultimate model with r _2(VA)=0 at low and medium/high conversions. Overall rate coefficient of copolymerization was calculated from the overall monomer conversion vs. time data and k _p  . k _t ^?0.5 was then estimated. It was observed that k _p  . k _t ^?0.5 increases with increasing the mole fraction of MA in the initial feed, indicating the increase in the polymerization rate with increasing MA concentration in the initial monomer mixture. The effect of mole fraction of MA in the initial monomer mixture on the drifts in the monomer mixture and copolymer compositions with reaction progress was also evaluated experimentally and theoretically.     

Mechanism and reaction rate of the karl-fischer titration reaction: Part I. Potentiometric measurements

The reaction rate of the coulometric variant of the Karl-Fischer titration reaction (in which electrolytically generated triiodide is used as oxidant instead of iodine) has been measured in methanol. The reaction is first order in water, sulfur dioxide and triiodide, respectively. For pH<5 the reaction rate constant decreases logarithmically with decreasing pH. Addition of pyridine solely influences the pH (by fixing it to a value of about 6) and has no direct influence on the reaction rate. A linear relation exists between the reaction rate constant and the reciprocal value of the iodide concentration, from which we can calculate the individual reaction rates for the oxidation by iodine and triiodide, respectively. While the reaction rate constant for triiodide is relatively small (k₃≈350 l² mol^?2s^?1), the reaction rate constant for iodine is much larger (k₃≈1.5×10⁷ l² mol^?2 s^?1.     

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.     

Heterogeneous reaction kinetics between mono epoxy–terminated polydimethylsiloxane and gelatin: Effect of material ratio and temperature

Heterogeneous reaction kinetics between mono epoxy–terminated polydimethylsiloxane (PDMS-E) macromonomer and gelatin was studied. The purpose of this work was to establish the relationship between the reaction conversion rate and the reaction rate, and to confirm the optimization range of the reaction conditions, including material ratio and temperature. The Van Slyke method was used to monitor the reactions by measuring the conversion rate of amino groups in gelatin. The rate constant for the reaction of the amino group was calculated as k. The results showed that k followed the order k_0.8 > k_0.6 > k_0.4 > k_1.0 > k_0.2 when PDMS-E/gelatin material ratios were tuned from 0:1.0 to 1.0:1.0 at an interval of 0.2. The reaction conversion rate of amino groups reached its peak value at the ratio of 0.8:1.0. The temperature-dependent rate constant showed that k followed the order k₅₅ > k₅₀ > k₄₅ > k₄₀, and the reaction conversion rate of amino groups reached its peak value at 50°C. The reaction was accompanied by an increase in the endothermic process and a decrease in entropy.     

Rate constant and products of the BrO + BrO reaction at 298 K

The overall rate coefficient k of the self recombination of BrO radicals has been measured at 298 K with use of the discharge flow/mass spectrometry technique. The rate coefficient k₂ for the reaction channel forming Br₂ has been also determined. The results are: k = (3.2 ± 0.5) × 10^?12 and k₂ = (4.7 ± 1.5) × 10^?13 (in cm³ molecule^?1 s^?1). These results are discussed with respect to previous literature data.     

The Reductions of Bis(terpyridyl) and Ethylenediaminetetraacetato Complexes of Cobalt(III) by Ascorbic Acid

The reductions of Co(terpy)₂³⁺ and Co(edta)^? complexes by ascorbic acid have been subjected to a detailed kinetic study in the range of pH =1–10.9. For each complex the rate law of the reaction is interpreted as a rate determining reaction between Co(III) complex and the ascorbic acid in the form of HA^? (k₁) and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Co(III) complex. With given Ka₁ and Ka₂, the rate constants are k₁ = 0.25 and 9.87 × 10^?5 M^?1s^?1, k₂ = 1.28 × 10⁶ and 18.7 M^?1s^?1 for Co(terpy₎2³⁺and Co(edta)^? complexes, respectively, at T = 25 °C and μ = 0.50M (terpy)and 1.0 M (edta) HClO₄/LiClO₄. The mechanism of the reaction is discussed on the basis of Marcus theory for outer sphere electron transfer process. Spin change and charge effect, duly considered, account for the non‐adiabatic behavior in the reduction of Co(edta)^? complex.     

Ac polarography and faradaic impedance of strongly adsorbed electroactive species: Part IV. Experimental study of the faradaic impedance of the redox systems benzo(c)cinnoline-dihydrobenzo(c) cinnoline

The faradaic impedance of the surface redox system benzo(c)cinnoline-dihydrobenzo(c)-cinnoline is studied experimentally in aqueous medium between pH 5 and 13. The variations of the impedance components are in good accord with the theoretical predictions. A V-shaped curve is found for log k_s=f(pH) (k_s=rate constant of the surface electrochemical reaction). It is estimated that the determination of rate constant values up to 2×10⁴ s^?1 on a mercury electrode is possible by this method.     

Kinetic and mechanistic studies on the complexation and anation of dioxotetracyanomolybdate (IV) with 2,2′-bipyridyl in aqueous solution

Dioxotetracyanomolybdate(IV) has been found to form a 1 : 2 complex with 2,2′-bipyridyl. The kinetics of the reaction has been studied over the pH range 5.3–8.7 by visible spectrophotometry under pseudo conditions. The effect of the 2,2′-bipyridyl and dioxotetracyanomolybdate(IV), temperature, ionic strength, and pH on the reaction rate was determined. The reaction follows first-order kinetics with respect to dioxotetracyanomolybdate(IV) ion and fractional-order kinetics with respect to 2,2′-bipyridyl. Values for the outer-sphere complex formation constant (K_os2) and rate constants (k₂) were also calculated from the kinetic data. It was found that rate of the reaction increases with the decreasing pH. The following rate equation based on the outersphere complexation equilibrium preceding the associative interchange has been derived. On the basis of the observed results probable mechanism has been proposed. © 1996 John Wiley & Sons, Inc.