Second-order Rate Constants For Oxidation Research Articles

Oxidation of iopamidol with ferrate (Fe(VI)): Kinetics and formation of toxic iodinated disinfection by-products

Presence of iodinated X-ray contrast media (ICMs) in source water is of high concern, because of their potential to form highly toxic iodinated disinfection by-products (I-DBPs). This study investigated kinetics, mechanisms and products for oxidation of one ICMs, iopamidol (IPM) by ferrate (Fe(VI)). The obtained apparent second-order rate constants for oxidation of IPM by Fe(VI) ranged from 0.7 M−1 s−1 to 74.6 M−1 s−1 at pH 6.0–10.0, which were highly dependent on pH. It was found that the oxidation of IPM by Fe(VI) led to the formation of highly toxic I-DBPs. Iodoform (IF), iodoacetic acid and triiodoacetic acid formations were observed during the oxidation and IF dominated the formed I-DBPs. The formation of I-DBPs was also governed by pH and the maximum formation of I-DBPs occurred at pH 9.0. Transformation pathways of IPM by Fe(VI) oxidation were speculated to proceed through deiodination, amide hydrolysis and oxidation of amine reactions. The deiodination reaction during the oxidation of IPM by Fe(VI) contributed to the formation of I-DBPs. The formation of I-DBPs during the oxidation of IPM by Fe(VI) was significantly higher than those of iohexol, diatrizoate and iopromide, which was consistent with the lowest molecular orbital energy gap of IPM. Although Fe(VI) is considered as a green oxidant, the formation of highly toxic I-DBPs during the oxidation of IPM should receive great attention.

Kinetic and mechanistic studies of the reactivity of iron(IV) TAMLs toward organic sulfides in water: resolving a fast catalysis versus slower single-turnover reactivity dilemma

TAML complex is oxidized by H2O2 or tBuOOH in water at pH < 10 into the corresponding iron(IV) μ-oxo-bridged dimer 2, which oxidizes readily ring-substituted thioanisoles p-XC6H4SMe (X=H, MeO, Me, Cl, CN) into the corresponding sulfoxides with regeneration of 1. The oxidation studied under pseudo-first-order conditions using the stopped-flow technique by monitoring the fading of the 420-nm band of 2 follows hyperbolic kinetics according to the rate law kobs = ab[p-XC6H4SMe]/(1 + b[p-XC6H4SMe]) at pH 8 and 25 °C. Parameters a, b, and ab all decrease for electron-poorer thioanisoles and the Hammett value ρ ~ 1 has been found for ab, which can be associated with the second-order rate constants for oxidation of thioanisoles by 2. The kinetics of oxidation of p-NO2C6H4SMe by H2O2 catalyzed by 1 has been studied under steady-state conditions. Covering the concentration of 1 in a 100-fold range has revealed that though first-order kinetics in 1 is observed at low catalyst concentrations (below 10−6 M), there is a significant negative deviation from linearity at [1] > 10−6 M. The latter was rationalized by the equilibrium between the monomeric and dimeric FeIV species 2 M ⇌ M–M (Kd), both being able to oxidize p-NO2C6H4SMe with rate constants km and kd which were found to be (13 ± 1) × 104 and (0.32 ± 0.01) × 104 M−1 s−1, respectively. The difference in the rate constants is the key for resolving the dilemma of faster catalysis versus slower single-turnover reactivity of TAML activators in water.

The oxidation of Fe(II) in NaCl–HCO 3− and seawater solutions in the presence of phthalate and salicylate ions: a kinetic model

The oxidation of Fe(II) by molecular oxygen in NaCl–HCO 3 − and natural seawater solutions and in the presence of phthalic and salicylic acid has been studied at different pH F and Fe(II)-organic compounds ratios. These two compounds have been selected as simple organic model compounds in order to describe the effect of organic ligands on the oxidation rates of Fe(II) in the environment. A kinetic model including the speciation of Fe(II) and the organic species interacting with the major ions in solution was developed to describe the oxidation rate in any media. The model successfully describes Fe(II) behavior in both NaCl-bicarbonate solutions and seawater in the absence and in the presence of the selected compounds. The second-order rate constants for oxidation of the different species by oxygen are the same for all the experimental conditions studied; the speciation of the species accounts for the differences observed for each selected media. The Fe(OH) 2 species is the most significant contributor to the overall oxidation rate in 0.002 m NaHCO 3 solution, concentration found in natural seawater. Phthalate ion decreases the Fe(II) oxidation rate in the selected media, while salicylate ion increases oxidation as a result of the formation of the Fe(II)–salicylate complex. The oxidation rate for the Fe(II)-phthalate is close to zero accounting for the lower rate than in pure Fe(II) solution. Salicylate ion forms a 1:1 complex with an oxidation rate k L=6.62 10 5±300 M −1 min −1. The higher rate is probably related to the formation of a stable Fe(III)–salicylate complex.

Oxidation of deoxy myoglobin by [Fe(CN) 6] 3−

The ability of myoglobin (Mb) to reversibly bind O 2 and other ligands has been well characterized. Mb also participates with a variety of redox metals to form metmyoglobin ( metMb). By using an anaerobic stopped-flow device we have measured outer-sphere oxidation by [Fe(CN) 6] 3− of native sperm whale myoglobin, recombinant wild-type Mb, and a series of mutant Mb proteins in which the distal His-64 was changed to Gly, Phe, Leu or Val. Second-order rate constants for oxidation of mutant proteins are 10–15 times greater than for recombinant or native ( k ox ~ 10 6 M − s −). We attribute the reduced rate of oxidation of wild-type protein to a higher reorganization energy imposed by the presence of the unique water/His-64/heme interaction, which is absent in the mutant proteins.

Positive Halogens from Halides and Hydrogen Peroxide with Organotellurium Catalysts

The oxidations of sodium bromide, sodium chloride, and sodium iodide to positive halogen with hydrogen peroxide in two-phase systems of dichloromethane and pH 6 phosphate buffer were catalyzed by organotellurium catalysts 1−3. The positive halogens were trapped by cyclohexene for bromine and chlorine to give mixtures of the 1,2-dihalocyclohexane (4) and 2-halocyclohexanol (5). For the bromination (4a)/hydrobromination (5a) of cyclohexene, unoptimized turnover numbers of 1010 mol of product per mole of catalyst for 1, 960 for 2, and 820 for 3 were measured with 4a/5a ratios of 55:45, 53:47, and 52:48, respectively. In cyclohexane, the turnover number for 1 was 150 and the 4a/5a ratio was 68:32. In the uncatalyzed process and in the reaction of aqueous bromine with cyclohexene, the 4a/5a ratio is 55:45 in dichloromethane and 67:33 in cyclohexane. The relative rates of catalysis for equimolar amounts of 1−3 were nearly identical to the relative second-order rate constants for oxidation of the organotellurium...

Kinetics of reactions of chlorine dioxide (OCIO) in water—II. Quantitative structure-activity relationships for phenolic compounds

Chlorine dioxide (OCIO) rapidly oxidizes substituted phenoxide anions in aqueous solution and second-order rate constants for this reaction have recently been reported for 25 compounds. Analysis of these data for correlations reveals several quantitative structure-activity relationships (QSARs) that can be used to predict rate constants for the reaction of additional phenolic compounds with chlorine dioxide. The rate constants for oxidation of phenoxide anions give good Hammett correlations to σ − constants and Marcus correlations using half-wave oxidation potentials. Ortho- as well as meta- and para-substituted phenoxides have been included in both types of correlations and the only outliers are compounds that exhibit strong intramolecular hydrogen bonding. The results of the Marcus correlation analysis are consistent with the expected mechanism of reaction where the rate-limiting step is single electron-transfer from the phenoxide anion to chlorine dioxide. The second-order rate constants for oxidation of the undissociated forms of substituted phenols are about six orders of magnitude smaller than the corresponding values for phenoxide anions, indicating that only the reaction of phenoxide anions will be significant under the conditions of water treatment with chlorine dioxide. The rate constants for undissociated phenols also have much greater relative uncertainties than those for the phenoxide anions, and give much less satisfactory correlations.

The catalytic oxidation of dithiols by a semisynthetic enzyme

The semisynthetic enzyme flavopapain (1C), obtained from the alkylation of Cys-25 of papain with 8..cap alpha..-(bromo-acetyl)-10-methylsioalloxazine (1B), was found to be an effective catalysts for the oxidation of dithiols to disulfides. The k/sub 2//K/sub s/ values for the oxidation of d,l-dihydrolipoamide and d,l-dihydrolipoic acid determined from anaerobic single-reaction stopped-flow kinetics were 4400 and 3400 M/sup -1/s/sup -1/, respectively. These values were, respectively, 126 and 200 times larger than the second-order rate constants for oxidation of d,l-dihydrolipoamide and d,l-dihydrolipoic acid by the model flavin 8-acetyl-10-methylisoalloxazine (1A). Under aerobic turnover conditions using the synthetic dye MTT as an electron acceptor, the k/sub cat/ and K/sub m/ values for the oxidation of d,l-dihydrolipoamide by 1C were in approximate agreement with the k/sub 2/ and K/sub s/ values, indicating that the rate-limiting step of the catalytic cycle is substrate oxidation rather than oxidation of dihydroflavopapain. When compared with flavopapains 2C and 3C (obtained as above but by modification with 7..cap alpha..- and 6..cap alpha..-(bromoacetyl)-10-methylisoalloxazine (2B and 3B, respectively)), flavopapain 1C is the most efficient catalyst. The circular dichroic spectra of flavopapains 1C, 2C, and 3C were recorded, and the dissociation constants of the sulfite addition complexes of 1C and 2C were determined.

The role of peroxide in haem degradation. A study of the oxidation of ferrihaems by hydrogen peroxide.

The oxidation of ferrihaems by H2O2 was studied as a model for haem catabolism. Rates of ferrihaem oxidation were evaluated by using a new computer-based method that measures the loss in catalytic activity of the ferrihaem during oxidation. For protoferrihaem, deuteroferrihaem, coproferrihaem and mesoferrihaem, oxidation proceeded via the monomeric species and no dimer contribution was detectable. The pH-dependence of oxidation was studied in the range 6.5--11. Within experimental error, the data were compatible with an inverse linear dependence on [H+]. This was interpreted in terms of attack by HO2- on monomeric ferrihaem. The specific second-order rate constants for oxidation of monomeric species by HO2- were of the same order of magnitude for all the ferrihaems, and were in the sequence coproferrihaem greater than protoferrihaem greater than mesoferrihaem congruent to deuteroferrihaem. A model is suggested involving formation of a ferrihaem monomerperoxide complex, which may either dissociate with the formation of a peroxidatic intermediate or be involved in an intramolecular oxidation of the ferrihaem. Haem catabolism may occur via the same or a similar intermediate.

Amine oxidation. Part X. The oxidation of some 4-substituted NN-dimethylbenzylamines and a selection of other amines with 1-chlorobenzotriazole

1-Chlorobenzotriazole in benzene converts substituted NN-dimethylbenzylamines into NN-disubstituted aminoalkylbenzotriazoles and a product containing positive chlorine. Hydrolytic work-up gives substituted benzaldehydes and benzylmethylamines. Kinetic studies show that the consumption of 1-chlorobenzotriazole, corrected for the positive chlorine in the product, obeys the rate law –d[chlorobenzotriazole]/dt=k2′[amine][chlorobenzotriazole]. Correlation of the logarithm of the second-order rate constants for oxidation of the NN-dimethylbenzylamines using the Hammett and Bronsted relationships gives ρ–0.71 and α 0.63 respectively. Comparative kinetic measurements with NN-dimethylbenzylamine and NN-dimethyl[α-2H2]benzylamine give k2′H/k2′D = 1·3. Product studies reveal information about the factors that control the direction of the oxidative dealkylation. The identity of the active chlorine product although not conclusively established is assigned as the NN-dimethylbenzylchloroammonium ion.In the suggested mechanism the predominant role of 1-chlorobenzotriazole involves an initial one-electron abstraction from the amine followed by a product-determining deprotonation of the resulting aminium radical. Chlorine transfer is a minor route.Information obtained from the reaction of a selection of other amines supports the dual nature of these oxidations.