OH-initiated oxidation mechanism and kinetics of organic sunscreen benzophenone-3: A theoretical study

Benzophenone-3 (BP-3), as an important organic UV filter, is widely used in the sunscreen, cosmetic, and personal care products. The chemical reaction mechanism and kinetics of BP-3 degradation initiated by hydroxyl (OH) radical was investigated in the atmosphere based on the density functional theory (DFT). The results showed that the OH radical is more easily added to the C3 position of the aromatic ring (pathway 3), while the H atom abstraction from the OH group on the aromatic ring (pathway 23) is an energetically favorable reaction pathway. At ambient temperature, 298 K, the overall rate constant for the primary reaction is about 1.50 × 10^?10 cm³ mol^?1 s^?1 with the lifetime of 1.92 h. OH addition reactions play the key role in the OH-initiated reaction of BP-3. The study is helpful for better understanding of the removal, transformation, and fate of BP-3 in the atmosphere.     

A comprehensive mechanism for methanol oxidation

A comprehensive detailed chemical kinetic mechanism for methanol oxidation has been developed and validated against multiple experimental data sets. The data are from static-reactor, flow-reactor, shock-tube, and laminar-flame experiments, and cover conditions of temperature from 633–2050 K, pressure from 0.26–20 atm, and equivalence ratio from 0.05–2.6. Methanol oxidation is found to be highly sensitive to the kinetics of the hydroperoxyl radical through a chain-branching reaction sequence involving hydrogen peroxide at low temperatures, and a chain-terminating path at high temperatures. The sensitivity persists at unusually high temperatures due to the fast reaction of CH₂OH+O₂=CH₂O+HO₂ compared to CH₂OH+M=CH₂O+H+M. The branching ratio of CH₃OH+OH=CH₂OH/CH₃O+H₂O was found to be a more important parameter under the higher temperature conditions, due to the rate-controlling nature of the branching reaction of the H-atom formed through CH₃O thermal decomposition. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 805–830, 1998     

Analysis of time-dependent kinetics for the oxidation of methanol on a platinum electrode and reaction mechanism

The reaction kinetics for the oxidation of methanol on a platinum electrode have been examined under precisely controlled conditions. The Tafel relations at constant surface coverages of the strongly adsorbed species show the existence of two potential regions where the predominant reaction path is different. The surface reaction of the strongly adsorbed species with OH(a) is rate determining at E > ca. 0.55 V, while the oxidative adsorption of methanol to form a reactive intermediate becomes the rate-determining step at E < ca. 0.55 V. In the latter potential region, the strongly adsorbed species is not oxidized so that its accumulation on the surface decreases the rate of the oxidative adsorption and thereby the total oxidation rate.     

The kinetics and mechanism of the ferrate(VI) oxidation of hydroxylamines

Aqueous solutions of potassium ferrate(VI) cleanly and rapidly oxidize hydroxylamine to nitrous oxide, N-methylhydroxylamine to nitrosomethane, N-phenylhydroxylamine to nitrosobenzene, and O-methylhydroxylamine to methanol and nitrogen. The kinetics show first-order behavior with respect to each reactant and a two term component representing acid dependent and independent pathways. A general mechanism involving intermediate formation coupled with a two-electron oxidation is proposed.     

The kinetics and mechanism of the oxidation of seleno-DL- methionine by potassium ferrate

The kinetics of the reaction of seleno-DL-methionine with potassium ferrate were investigated under pseudo first-order conditions. The oxidation to the selenoxide is complete within the timeframe of 7.5ms to 2s. The kinetics are first-order in each of the hydrogen ion, selenomethionine and ferrate ion concentrations over the pH range 8.53 to 10.13, but zeroth-order in hydrogen ion concentration at lower pH values. The results are very similar for methionine, except that the overall rate constant is over two orders of magnitude lower. The proposed mechanism involves a rate- determining step between selenomethionine and the protonated ferrate ion.     

Ascorbate-induced oxidation of formate by peroxodisulfate: product yields,kinetics and mechanism

The slow reaction between peroxodisulfate and formate is significantly accelerated by ascorbate at room temperature. The products of this induced oxidation, CO₂ and oxalate (C₂O^2– ₄), were analyzed by several methods and the kinetics of this reaction were measured. The overall mechanism involves free radical species. Ascorbate reacts with peroxodisulfate to initiate production of the sulfate radical ion (SO^– ₄), which reacts with formate to produce carbon dioxide radical ion (CO^– ₂) and sulfate. The carbon dioxide radical reacts with peroxodisulfate to form CO₂ or self-combines to form oxalate. Competition occurring between these two processes determines the overall fate of the carbon dioxide radical species. As pH decreases, protonation of the carbon dioxide radical ion tends to favor production of CO₂.     

X-ray diffraction method for the investigation of the oxidation kinetics in thin iron films

Summary The time-dependent decrease of the intensity of X-ray diffraction peaks due to the interaction of the investigated metals with gases can be measured in situ with the help of a recently developed ultrahigh-vaccum diffraction chamber. We have used this method to study the oxidation kinetics of about 450 nm thick iron films. The activation energy obtained from the temperature dependence of the kinetics at a relatively high oxygen exposure corresponds to the well-known cation vacancy diffusion while distinctly smaller values are found for short duration of exposure. This result leads to the conclusion that grain boundary diffusion becomes effective in the films at the early stages of the oxidation process.

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Röntgenographisches Verfahren zur Untersuchung der Oxidationskinetik in dünnen Eisenschichten

Zusammenfassung Die zeitliche Änderung der Intensität von Röntgenbeugungsreflexen, die von der Wechselwirkung der untersuchten Metalle mit Gasen herrührt, kann in situ mit Hilfe einer kürzlich entwickelten Ultrahochvakuum-Röntgenbeugungskammer gemessen werden. Wir haben diese Methode zum Studium der Oxidationskinetik ca. 450 nm dicker Eisenfilme herangezogen. Die Aktivierungsenergie, die man aus der Temperaturabhängigkeit der Kinetik bei relativ hohen Einwirkungszeiten des Sauerstoffs erhält, entspricht der wohlbekannten Kationenleerstellendiffusion, während deutlich kleinere Werte für kurze Einwirkungszeiten gefunden werden. Dieses Ergebnis legt den Schluß nahe, daß zu Beginn des Oxidationsprozesses eine Korngrenzendiffusion wirksam wird.

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Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. Thanks are due to Mrs Schuster of the Institute of Material Science I of our University for preparing the SEM micrographs.     

The kinetics and mechanism of the oxidation of DL ethionine and thiourea by potassium ferrate

The kinetics of the reaction between ethionine and thiourea were investigated under pseudo and non-pseudo-first-order conditions. Ethionine was oxidized to the sulfoxide within 500 s and thiourea was oxidized to urea within 10 s. Above a pH of ca. 8.5 the reaction was first-order in the concentration of the organosulfur compound, the hydrogen ions and the ferrate ions, whereas, below this pH, the kinetics were independent of the hydrogen ion concentration. A possible mechanism for both compounds is initial protonation of the ferrate ion followed by the two electron rate-determining step of addition of oxygen to the organosulfur compound. The kinetic parameters for ethionine compare favorably with those for similar compounds, whereas thiourea tends to be more active. The rate constant for the rate-determining step is 4.1 × 10² M^–1 s^–1 for ethionine and 4.1 × 10³ M^–1 s^–1 for thiourea.     

Oxyhalogen-sulfur chemistry: kinetics and mechanism of oxidation of N-acetylthiourea by chlorite and chlorine dioxide

The oxidation reactions of N-acetylthiourea (ACTU) by chlorite and chlorine dioxide were studied in slightly acidic media. The ACTU-ClO(2)(-) reaction has a complex dependence on acid with acid catalysis in pH > 2 followed by acid retardation in higher acid conditions. In excess chlorite conditions the reaction is characterized by a very short induction period followed by a sudden and rapid formation of chlorine dioxide and sulfate. In some ratios of oxidant to reductant mixtures, oligo-oscillatory formation of chlorine dioxide is observed. The stoichiometry of the reaction is 2:1, with a complete desulfurization of the ACTU thiocarbamide to produce the corresponding urea product: 2ClO(2)(-) + CH(3)CONH(NH(2))C=S + H(2)O --> CH(3)CONH(NH(2))C=O + SO(4)(2-) + 2Cl(-) + 2H(+) (A). The reaction of chlorine dioxide and ACTU is extremely rapid and autocatalytic. The stoichiometry of this reaction is 8ClO(2)(aq) + 5CH(3)CONH(NH(2))C=S + 9H(2)O --> 5CH(3)CONH(NH(2))C=O + 5SO(4)(2-) + 8Cl(-) + 18H(+) (B). The ACTU-ClO(2)(-) reaction shows a much stronger HOCl autocatalysis than that which has been observed with other oxychlorine-thiocarbamide reactions. The reaction of chlorine dioxide with ACTU involves the initial formation of an adduct which hydrolyses to eliminate an unstable oxychlorine intermediate HClO(2)(-) which then combines with another ClO(2) molecule to produce and accumulate ClO(2)(-). The oxidation of ACTU involves the successive oxidation of the sulfur center through the sulfenic and sulfinic acids. Oxidation of the sulfinic acid by chlorine dioxide proceeds directly to sulfate bypassing the sulfonic acid. Sulfonic acids are inert to further oxidation and are only oxidized to sulfate via an initial hydrolysis reaction to yield bisulfite, which is then rapidly oxidized. Chlorine dioxide production after the induction period is due to the reaction of the intermediate HOCl species with ClO(2)(-). Oligo-oscillatory behavior arises from the fact that reactions that form ClO(2) are comparable in magnitude to those that consume ClO(2), and hence the assertion of each set of reactions is based on availability of reagents that fuel them. A computer simulation study involving 30 elementary and composite reactions gave a good fit to the induction period observed in the formation of chlorine dioxide and in the autocatalytic consumption of ACTU in its oxidation by ClO(2).     

Effect of Micelles on kinetics and mechanism of the oxidation of Serine by acid Permanganate

The oxidation of serine (HORCO₂H) by acid permanganate was investigated both in the absence and presence of sodium dodecyl sulfate (SDS). It has been observed that the presence of surfactant enhanced the reaction rate. The reaction is first order with respect to [Serine] and [MnO₄^?]. The reaction is retarded by the hydrogen ion in the absence of SDS but catalyzed in the presence of SDS. The overall rate expression for the reduction of Mn(VII) may be written as In the presence of SDS of the rate law is The reaction appears to involve a parallel consecutive reaction mechanism in which Mn(IV) appears as the reaction intermediate. ??′_4f signifies the rate constant for the reaction path leading to the formation of Mn(IV) from Mn(VII) as reaction intermediate, whereas ??′_2f signifies the rate constant for the reaction path leading to the reduction of Mn(VII) to Mn(II) without prior formation of Mn(IV). A mechanism satisfying the various kinetic parameters has been proposed.     

Effect of ionic micellar medium on kinetics and mechanism of oxidation of bovine serum albumin: A pulse radiolysis study

Effect of protein–micelle interaction on bovine serum albumin (BSA) oxidation by trichloromethyl peroxyl radical (CCl₃O₂^·) in anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethyl ammonium bromide (CTAB) micellar media has been studied using nanosecond pulse radiolysis technique. Viscosity measurement and light scattering studies have suggested that SDS and CTAB micelles produce BSA–micelle aggregates of different sizes and polydispersity. Oxidation kinetics and transients have been affected both by anionic SDS and cationic CTAB micelles but in a different manner. Tryptophanyl-CCl₃O₂^· adduct radical to tyrosyl radical transformation in BSA has been observed in anionic SDS micelles but not in cationic CTAB micelles. Similar studies have also been done with tryptophan and tyrosine amino acids, which undergo oxidation in BSA. The study suggests that Coulombic and hydrophobic interactions between micelles and protein affect the structure of the protein to shield its functional amino acids, like tryptophan and tyrosine, to neutral oxidizing radical.     

A study of the kinetics and mechanism of oxidation of L-tryptophan by diperiodatonickelate(IV) in aqueous alkaline medium

The kinetics of oxidation of L-tryptophan by diperiodatonickelate(IV) (DPN) in an aqueous alkaline medium at a constant ionic strength of 0.30 mol dm⁻³ was studied spectrophotometrically. The reaction was first order in diperiodatonickelate(IV) and less than first order in tryptophan and the OH⁻ ion. The addition of periodate had no effect on the reaction, and nickel(II) produced did not influence the reaction rate significantly. An increase in ionic strength and decrease in medium permittivity did not affect the reaction rate. A mechanism involving the formation of a complex between L-tryptophan and reactive DPN species was proposed. The constants characterizing the mechanism were evaluated. The activation parameters for the slow reaction step were computed and discussed. The text was submitted by the authors in English.     

A new mechanism for oxidation of epigallocatechin and production of benzotropolone pigments

Enzymatic oxidation of (−)-epigallocatechin gave two new quinone dimers, dehydrotheasinensin C and proepitheaflagallin. Dehydrotheasinensin C has a hydrated cyclohexenetrione structure and its oxidation-reduction dismutation reaction yielded black tea polyphenols, theasinensins C and E, and desgalloyl oolongtheanin. The structure of proepitheaflagallin was determined based on spectroscopic data of its quinoxaline derivatives prepared by condensation with o-phenylenediamine. Proepitheaflagallin was decomposed on heating to give epitheaflagallin and hydroxytheaflavin. The former is a known black tea pigment and the latter is a new pigment with 1′,2′,3′-trihydroxy-3,4-benzotropolone moiety. The results revealed a new mechanism for the production of these pigments from epigallocatechin.