Kinetic study of the reaction between Fe2+-oxalate and 2,6-dichlorophenolindophenol

Ascorbic acid present in natural products is often analyzed by its reducing effect on 2,6-dichlorophenolindophenol (DCPI). In case of coexistence of Fe(II) in natural samples and if oxalic acid is used for their extraction and stabilization, a significant interference is observed. The reaction of Fe²⁺ with 2,6-dichlorophenolindophenol (DCPI) in acidic solutions in the presence or absence of oxalate ions was investigated. The reaction was studied with a Stopped-Flow Spectrophotometer (S.F.S) equiped with a data acquisition system, which allows the determination of initial rates and reaction rate constants. Results for the effect of the concentrations of all the reagents of the reaction are presented. During the experimental work all concentrations of the reagents were kept in such levels to make the reaction first-order in respect of DCPI. An interesting finding is, that, although Fe²⁺ and C₂O₄²⁻ ions do not react separately with DCPI (or react very slowly), they react very rapidly in mixtures and show a strong synergetic effect. The experimental data fit with the mechanism of the formation of intermediates FeC₂O₄ and [Fe(C₂O₄)₂]²⁻. The proposed rate law agrees with the above mechanism and the experimental results. The value k_c=(1.6±0.2)×10⁵ M⁻¹·s⁻¹ was calculated for the second-order rate constant of the reaction of FeC₂O₄ and DCPI. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 941–948, 1998     

Rate constants for the reactions of CCl(X2II) with a series of silanes

Rate constants for the gas phase reactions of CCl generated by the flash photolysis of CHBr₂Cl with a series of silanes have been obtained by kinetic absorption spectroscopy. In general, the rate constants are very high, and range from (4.8 ± 0.5) × 10⁸ (SiH₄) to (6.4 ± 0.34) × 10⁹ for Si₂H₆. CCl does not insert into the SiC or primary CH bonds of silanes and its rate of reaction with tertiary SiH bonds is 600 times greater than with tertiary CH bonds. CCl reacts slowly with the SiSi bond. k_H/k_D varies from 1.9 to 1.0 on going from primary to tertiary SiH bonds. The electrophilic character of CCl is manifested, on a per SiH bond basis, by excellent correlations between the rate constants and the hydrilic character of the SiH bond, and between log k and the ionization potential.     

Vibrational rate enhancement in the reaction OH + H2(ν = 1) → H2O + H

The rate constant for the reaction between OH and vibrationally excited H₂, OH + H₂(ν = 1)→H₂O + H, has been measured directly at 298 K. k₀₁ is found to be (7.5±3)×10^?13 cm³/molecules, corresponding to a vibrational rate enhancement of k₀₁/k₀₀ = (1.2 ± 0.4) × 10².     

Über die Si-N-bindung: XLI. Kinetische untersuchungen zur hydrolyse von trimethylsilylurethanen des typs Me3Si(p-XC6H4)NCOOEt

Hydrolysis reactions of silylurethanes Me₃Si(p-XC₆H₄)NCOOEt (I) with X = Cl, H or Me in aqueous buffer solutions, with pH values from 1.94 to 10.00 were studied.The catalytic rate constants for the acid and base catalysed reactions and for the “non-catalysed” reaction k(H₃O⁺), k(CH₃COO^?), k(H₂PO₄^?), k(HPO₄^2?), k(NH₃), k(OH^?) and k₀ were evaluated from the pseudo first-order rate constants k_exp determined by UV spectroscopy.The Brönsted coefficients for the base-catalysed reactions were obtained from the catalytic rate constants found and the known constants of dissociation K(HB⁺).The ρ values of the reactions could be derived from the σ constants given by Jaffé.The kientical results obtained are interpreted mechanistically and are believed to also have model character for other nucleophilic substitution reactions with silicon compounds.     

Absolute rate constants for the reaction of O(3P) aroms with n-butane and NO(M  Ar) over the temperature range 298–439 K

Absolute rate constants for the reaction of O(³P) atoms with n-butane (k₂) and NO(M  Ar)(k₃) have been determined over the temperature range 298–439 K using a flash photolysis-NO₂ chemiluminescence technique. The Arrhenius expressions obtained were k₂ = 2.5 × 10^?11exp[-(4170 ± 300)/RT] cm³ molecule^?1 s^?1, k₃ = 1.46 × 10^?32 exp[940 ± 200)/ RT] cm⁶ molecule^?2 s^?1, with rate constants at room temperature of k₂ = (2.2 ± 0.4) × 10^?14 cm³ molecule^?1 s^?1 and k₃ = (7.04 ± 0.70)×10^?32 cm⁶ molecule^?2 s^?1. These rate constants are compared and discussed with literature values.     

Proton transfer reactions from H3+ ions to N2, O2, and CO molecules

The rate constants for proton transfer from H₃⁺ ions to N₂, O₂, and CO have been measured as function of hydrogen buffer gas partial pressure. The rate constant for proton transfer from H₃⁺ to N₂ shows a very large pressure dependence, increasing from 1.0 × 10^?9 cm³/s at low H₂ partial pressures to 1.7 × 10^?9 cm³/s at high H₂ partial pressures. The rate constants for proton transfer from H₃⁺ to O₂ and CO are constant with partial pressure of H₂; giving values of 6.4 × 10^?10 cm³/s and 1.7 × 10^?9 cm³/s, respectively. The roles of excess vibrational energy in H₃⁺ ions and of equilibrium between forward and back reaction are discussed. Back reaction is observed only for the reaction of H₃⁺ ions with O₂, and an equilibrium constant of K = 2.0 ± 0.4 at 298 K has been determined. From these data the proton affinity of O₂ is deduced to be 0.47 ± 0.11 kcal/mole higher than that of H₂.     

Temperature dependence of the reaction rate constants for O(3P) atoms with C2H4, C3H6 and NO(M = N2O), determined by a modulation technique

Rate constants for the reaction of O(³P) atoms with C₃H₄, C₃H₆ and NO(M = N₂O) have been measured over the temperature range 300–392°K using a modulation-phase shift technique. The Arrhenius expressions obtained are:C₂H₄, k₂ = 3.37 × 10⁹ exp[?(1270 ± 200)/RT]liter mole^?1 sec^?1,C₃H₆, k₂ = 2.08 × 10⁹ exp[?(0 ± 300)/RT]liter mole^?1 sec^?1,NO(M = N₂O), k₁ = 9.6 × 10⁹ exp[(900 ± 200/RT]liter² mole^?2 sec^?1.These temperature dependencies of k₂ are in good agreement with recent flash photolysis-resonance flourescence measurements, although lower than previous literature values.     

Recombination of 1,1-dimethylpropyl peroxy radicals in polar solvents

The kinetics of 1,1-dimethylpropyl peroxy radicals recombination in polar solvents—water, methanol, and their mixtures—was studied by EPR spectroscopy in combination with the stopped-flow method, and the rate constants of this reaction were determined. Peroxyl radicals were generated by mixing solutions of Ce⁴⁺ sulfate and 1,1-dimethylpropyl hydroperoxide. The observed EPR signal of the peroxyl radical is a singlet with a g-factor of 2.015 ± 0.001, and a line width of ΔH = (1.36 ± 0.02) × 10^?3 T for methanol and ΔH = (9.7 ± 0.2) × 10^?4 T for water. The measured rate constants of (CH₃)₂C(O₂^·)CH₂CH₃ radical recombination at 298 K are 2k_t = (3.9 ± 0.4) × 10⁴ L mol^?1 s^?1 for water and 2k_t = (5.2 ± 0.5) × 10³ L mol^?1 s^?1 for methanol. A linear relationship between ln(2k_t) and the Kirkwood function (ε?1)/(2ε + 1), where e is the dielectric constant of the medium, has been established, indicating an important role of nonspecific solvation in the recombination of tertiary peroxyl radicals.     

A time-resolved lif study of the kinetics of OH(ν = 0) and OH(ν = 1) with HCl and HBr

The kinetics of OH(ν = 0) and OH(ν = 1) have been followed using pulsed photolysis of H₂O or HNO₃ to generate hydroxyl radicals, and time-resolved, laser-induced fluorescence to observe the rates of their subsequent removal in the presence of HCl or HBr. The experiments yield the following rate constants (cm³ molecule^?1 s^?1) at 298 ± 4 K: OH(ν = 0) + HCl: k_o = (6.8 ± 0.25) × 10^?13; OH(ν = 0) + HBr: k_o = (11.2 ± 0.45) × 10^?12; OH(ν = 1) + HCl: k₁ = (9.7 ± 1.0) × 10^?13; OH(gn = 1) + HBr; k₁ = (8.1 ± 1.05) × 10^?12 For OH(ν = 1), the measurements do not distinguish between loss by reaction and relaxation, and the fact that k₁ > k_o for HCl is tentatively attributed to relaxation, probably by near-resonant vibrational—vibrational energy transfer. Clearly, neither of these exothermic, low-activation-energy reactions is enhanced to any great extent, if at all, by vibrational excitation of the OH radical.ft]*|Present address: Battelle/Pacific Northwest Laboratories, P.O. Box 999, Richland, Washington 99352, USA.     

Absolute Rate constants for O + No + N2 → NO2 + N2 from 217–500 K

Flash photolysis of NO coupled with time resolved detection of O via resonance fluorescence has been used to obtain rate constants for the reaction O + NO + N₂ → NO₂ + N₂ at temperatures from 217 to 500 K. The measured rate constants obey the Arrhenius equation k = (15.5 ± 2.0) × 10^?33 exp(1160 ± 70)/1.987 T] cm⁶ molecule^?2 s^?1. An equally acceptable equation describing the temperature dependence of k is k = 3.80 × 10^?27/T^1.82 cm⁶ molecule^?2 s^?1. These results are discussed and compared with previous work.     

Kinetic studies of the photodecomposition of dipyridamole in solution: Interaction with lysophosphatidylcholine and bovine serum albumin

Photoirradiation of dipyridamole (DIP) solution at λ ⪢ 390 nm leads to a reduction of the intensity of the absorption band without change in its appearance. This reduction is due to the breaking of the π-conjugation chain of the DIP molecule. The experimentally measured rate constant k is proportional to the concentration of oxygen and to the ratio of the rates of photoreaction, k_r, and of radiation, k_o. In homogeneous solutions the values of k_r and k_o are three-times greater at pH 5.0 than at pH 7.0. So, the photoreaction is more effective when the DIP molecules are completely protonated. In alcoholic and alkaline solutions (pH 13.5), the deprotonation of DIP molecules is responsible for a significant reduction of k_r. In the presence of microheterogeneous systems, lysophosphatidylcholine (L-PC) micelles and bovine serum albumin (BSA), a significant reduction of k_r is observed. The value of k_r in this case depends on the pH of the solution as well as on the concentration of L-PC and BSA. In the presence of L-PC or BSA, different values of k could be associated to free and bound DIP molecules. A kinetic procedure is proposed which permits the evaluation of the binding constants as well as the kinetic constants. The binding constant of DIP to L-PC micelles is estimated as 1.28 × 10⁴ M⁻¹ and the value of the limiting effective rate constant k is similar to the value obtained in ethanol. This is evidence for the localization of DIP molecules in the polar region of L-PC micelles. The value of k in this case is three-times greater at pH 5.0 than at pH 7.0 as occurs in homogeneous solutions. The binding to BSA was also studied and binding constants of (1.8±0.2) × 10⁴ M⁻¹ and (7.8±0.9) × 10⁴ M⁻¹ were obtained at pH 7.0 and pH 5.0, respectively. In this case the ratio k_{(ph 5.0)}/k_{(ph 7.0)} is also equal to three, the same as in homogeneous solutions or in the presence of L-PC micelles. This implies that DIP molecules maintain their protonation form in the presence of microheterogeneous systems.     

Kinetic studies on H+- and OH−-catalysed aquation of bis-aspartatochromium(III)

A potential synthetic biochromium source, bis-aspartatochromium(III) ion (where Asp is a tridentate N,O,O′-ligand, bonded via amine nitrogen and carboxylate oxygen atoms) has been obtained and characterized in aqueous solution. Kinetics of partial dechelation of the complex catalysed by H⁺ and OH^? ions has been studied spectrophotometrically within 0.1–1.0 M HClO₄ and 0.1–1.0 M NaOH ranges under first-order conditions. A linear dependence of the k _obs,H on [H⁺] and independence of the k _obs,OH on [OH^?] were established. The derived rate expression and identification of components of the reaction mixture provide evidence for a reaction mechanism, where the key role in the overall process is the formation of an intermediate species with bidentate N,O-bonded Asp via both spontaneous and H⁺(OH^?)-catalysed reaction paths. The intermediate is meta-stable and at pH 5–7 restores the substrate.     

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.     

Kinetics of the reaction of OH with HCl

The rate constant of the reaction OH + HCl → H₂O + Cl was measured in a flow tube over the temperature range 224 to 460°K using resonance fluorescence detection of OH. An Arrhenius expression k₁ = (2.0 ± 0.1) × 10^?12 exp [?(620 ± 20 cal/mole)/RT] was obtained. Stratospheric and reaction kinetic implications are discussed briefly.     

Kinetics of the Base-Catalysed Isomerization of trans-meso-CH3Co(H2O)L2+ (L = 5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)

The isomerization of the complex trans-meso-CH₃Co(H₂O)L²⁺ (L = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) to trans-primary, rac-CH₃Co(H₂O)L²⁺ has been investigated from pH range 7.11 to 8.09 in aqueous solution. The reaction rate law has been determined as: -d[meso-CH₃Co(H₂O)L²⁺]/dt = k_OH [OH^?][meso-CH₃Co(H₂O)L²⁺], where k_OH = 600 ± 10 M^?1s^?1 at 25 °C and μ = 0.5 M. The activation parameters of the reaction were also studied with ΔH^± = 19.1 ± 0.9 Kcal mol^?1 and ΔS^± = 18.0 ± 0.8 cal K^?1mol^?1. A mechanism that involves a secondary NH inversion is proposed.     

Rate constants for the reaction of OH radicals with 2-methyl-2-butene over the temperature range 297–425 K

Absolute rate constants, k₂, for the reaction of OH radicals with 2-methyl-2-butene have been determined over the temperature range 297–425 K using a flash photolysis-resonance fluorescence technique. The Arrhenius expression obtained was k₂ = 3.6 × 10^?11 exp [(450 ± 400)/RT] cm³ molecule^?1 s^?1.     

Interference in the kinetic determination of ascorbic acid by sulphide and sulphite

The reduction of 2,6-dichlorophenolindophenol (DCPI) by sulphides and sulphites has been studied kinetically by the stopped-flow technique. The reaction is first-order with respect to each of the reactants. From the distribution diagrams for the species DH(+)(2), DH and D(-) for DCPI and H(2)Q, HQ(-) and Q(2-) for sulphides or sulphites, a mechanism is proposed which suggests partial reactions of all possible combinations of the reacting species at any pH. An equation for calculation of the second-order reaction rate constants 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 H(2)S or H(2)SO(3). Values of the overall reaction rate constants over a wide pH-range have been determined, together with values of k for all possible partial reactions. For particular pH-values the second-order reaction rate constant was determined by four different methods. Mean values of k = 251 +/- 1 and 240 +/- 1 l.mole(-1).sec(-1) were obtained for pH 3.15 and 4.17, respectively, for the DCPI-Na(2)S reaction and k = 137 +/- 1, 127 +/- 1 and 136 +/- 1 l.mole(-1).sec(-1) for pH 2.02, 4.25 and 5.10, respectively, for the DCPI-Na(2)SO(3) reaction. From the slopes of the linear Arrhenius plots activation energies of 6.6 +/- 0.2 and 4.0 +/- 0.1 kcal/mole for the DCPI-Na(2)S and DCPI-Na(2)SO(3) reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.     

Kinetics of the association of H+3 with H2 at low temperatures

The rate constant for the formation of H⁺₅ (D⁺₅) at (86 ± 3) °K by the three-body process has been determined (k₃(H) = (2.16 ± 0.10) × 10^?28 × 10^?28 cm⁶/molecule² sec and k₃(D) = (1.47 ± 0.20) × 10^?28 cm⁶/molecule² sec) in a high pressure mass spectrometer. Comparison of this result with published rate data at 300 °K indicates the reaction has an apparent activation energy of ?1.5 kcal/mole.     

Complex formation of iron (III) with chrome azurol s

The complex formation of iron(IIl) with 3”-sulpho-2”,6”-dichloro-3,3'-dimethyl-4'-hydroxy-fuchson-5,5'-dicarboxylic acid (chrome azurol S) was studied by spectrophotometric, conductometric and potentiometric methods. The pure tetrabasic acid of the ligand was prepared from the impure trisodium salt (commercially availalile), and the dissociation constants of the ligand were redetermined. At 20° ± 1° and in the presence of 0.10 M potassium chloride the dissociation constants were: pk₁ < 0.0, pk₂ = 2.25 ± 0.05, pk₃ = 4.71 ± 0.03 and pk₄ = 11.81 ± 0.03.In the pH range 2–4, four complexes were detected (the absolute stability constants at 20° ± 5° and at an ionic strength of 0.10 M are given in parentheses) : a ring-formed dimer complex [Fe(H₂O)₂]₂Ch₂^2- (log k_2,2 = 36.2); a monomer of composition [Fe(H₂O)₄]HCh or [Fe(H₂O)₄]HCh^- (the absolute stability constant was calculated as log k_1,1 = 15.6 for the latter composition); a complex [Fe(H₂O)₄]₂Ch²⁺ (log k_3.1=20.2) and, finally, a complex of composition [Fe(H₂O)₂]H_xCh₂^x-5 (the value of x being unknown). In addition, hydroxo complexes of the dimer were formed at higher pH values.     

Kinetics of the reactions of OH radicals with some oxygenated volatile organic compounds under simulated atmospheric conditions

Some relative rate experiments have been carried out at room temperature and at atmospheric pressure. This concerns the OH-oxidation of some oxygenated volatile organic compounds including methanol (k₁), ethanol (k₂), MTBE (k₃), ethyl acetate (k₄), n-propyl acetate (k₅), isopropyl acetate (k₆), n-butyl acetate (k₇), isobutyl acetate (k₈), and t-butyl acetate (k₉). The experiments were performed in a Teflon-film bag smog chamber. The rate constants obtained are (in cm³ molecule⁻¹ s⁻¹): k₁=(0.90±0.08)×10⁻¹²; k₂=(3.88±0.11)×10⁻¹²; k₃=(2.98±0.06)×10⁻¹²; k₄=(1.73±0.20)×10⁻¹²; k₅=(3.56±0.15)×10⁻¹²; k₆=(3.97±0.18)×10⁻¹²; k₇=(5.78±0.15)×10⁻¹²; k₈=(6.77±0.30)×10⁻¹²; and k₉=(0.56±0.11)×10⁻¹². The agreement between the obtained rate constants and some previously published data has allowed for most of the studied compounds to point out a coherent group of values and to suggest recommended values. Atmospheric implications are also discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 839–847, 1998