Kinetic and mechanistic studies of oxidation of alanine and phenylalanine by sodium N-chlorobenzene sulfonamide (chloramine-B) in hydrochloric acid medium

The kinetics of oxidation of alanine and phenylalanine by sodium N-chlorobenzene sulfonamide (CAB) has been investigated at 30°C in two ranges of acid concentrations. The reactions follow identical kinetics for both amino acids. At low acid concentration (0.03–0.10M), simultaneous catalysis by H⁺ and Cl^? ions is noted. The rate shows a first-order dependence on [CAB], but is independent of [substrate]. A variation of the ionic strength or the dielectric constant of the medium or the presence of the added reaction product benzene sulfonamide (BSA) has no pronounced effect on the rate. At [HCl] > 0.2M, the rate is independent of [H⁺], but shows a first-order dependence on [CAB] and a fractional-order dependence on [amino acid]. The addition of BSA or Cl^? ions, or a change in the ionic strength of the medium has no influence on the rate. Upon decreasing the dielectric constant of the medium, the rate increased, indicating positive ion–dipole interaction in the rate-determining step. The reaction was studied at different temperatures, and activation parameters have been computed. Rate laws in agreement with experimental results have been derived. Suitable mechanisms to account for the observed kinetics are proposed. The rate constants obtained from the derived rate laws as [H⁺], [Cl^?], and [substrate] vary are in excellent agreement with the observed rate constants, thus justifying the proposed rate laws and hence the suggested mechanistic schemes.     

Kinetics of the photocatalytic degradation of methylene blue on titanium dioxide

The first step of the photocatalytic degradation of methylene blue (MB) on anatase is photocatalytic reduction with subsequent decomposition of the dye itself and its leucobase. At low catalyst concentrations (≤2 g/L), the dye decomposition rate constant increases with increasing anatase concentration. A plateau appears for anatase concentrations above 2 g/L. Under steady-state conditions, the reaction kinetics is described by the Michaelis–Menten equation if the catalyst concentration is significantly greater than the MB concentration, which permits us to determine the kinetic parameters of the degradation process.     

Mechanism of oxidation of aromatic amines by peroxomonosulphate – a kinetic study

Oxidation of aromatic amines (substrate) by peroxomonosulphate (PMS) in phthalate buffers at various temperatures revealed first-order dependence on [PMS] as well as [Substrate]. Rate of oxidation was found to increase with an increase in pH as well as dielectric constant (D) of the medium. The most plausible mechanism involving PMS^? and substrate in rate limiting step was proposed. Hammett's plot of log-rate constant versus σ deviated from linearity. Deviations were explained due to para resonance interaction energies. © 1995 John Wiley & Sons, Inc.     

Catalytic action of CTAB/KBr/C9OH micellar media on the basic hydrolysis of crystal violet

The kinetics of basic hydrolysis of crystal violet (CV) in CTAB/KBr/C₉OH micellar media was investigated under pseudo-first-order conditions. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of CV at 590?nm. It was observed that the pseudo-first-order rate constant increases with increase in C₀. The enhancement of reaction rate with C₀ is explained on the basis of dependence of reaction rate on micellar morphology. Further, the viscosity and DLS analysis supports nonanol-induced morphological transitions. Fluorescence spectroscopy has been used to understand dye–micelles interactions. The enhancement of fluorescence intensity of CV with C₀ suggests an increase in dye–micelles interaction with C_0. The concentration of surfactant and salt had a marked effect on reaction rate. The inhibition of reaction rate at high concentration of surfactant and salt is due to the ionic competition of OH^? and Br^? ions for the reaction center. The influence of [OH^?] on CV hydrolysis was also investigated. The results show that the pseudo-first-order rate constant, k’, increases linearly with hydroxide ion concentration, indicating first-order dependence on [OH^?].     

碱性介质中二羟基二过碘酸合镍(Ⅳ)配离子氧化乙二胺的动力学及机理

在恒定离子强度(μ=0.40mol/L)的碱性介质中,于20～35℃用分光光度法研究了二羟基二过碘酸合镍(Ⅳ)配离子(DDN)氧化乙二胺的动力学。结果表明,反应对DDN为一级,对乙二胺为正分数级。准一级速率常数kobs随〔OH^-]增加而增加,1/k_obs对[IO₄^-]有线性关系,表明二羟基一过碘酸合镍(Ⅳ)配离子(DMN)是氧化剂的活性物种。无盐效应未检出自由基存在,据此提出了包括DDN和DMN存在前期平衡和内层一步双电子转移的氧化反应机理。     

Deoximation by cetyltrimethylammonium dichromate: A kinetic study

The deoximation kinetics of some oximes was studied by using cetyltrimethylammonium dichromate (CTADC) in dichloromethane in the presence of acetic acid and a cationic surfactant. The rate of reaction is highly sensitive to the change in [CTADC], [oxime], [acid], [surfactant], polarity of the solvents, and reaction temperature. The reaction is found to be catalyzed by acid with an appreciable uncatalytic rate. The reaction is first order with respect to substrate. With increase in CTADC concentration, rate of the reaction increases with a fractional order dependency with respect to oxidant. Consistent to the observation, a mechanism has been proposed in which the substrate forms a complex with CTADC in the rate determining step followed by decomposition with a fast process to yield corresponding carbonyl compounds. The structure of the substituents has also a significant effect on the rate constant. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 482–488, 2011     

The sorption properties of rutile and its catalytic activity in the destruction of dyes

The photocatalytic destruction of cationic and anionic dyes on dispersed spectrally pure rutile in the presence of oxygen was studied. The rate constant for these reactions does not correlate with the rate constant for the photobleaching of these dyes in homogeneous solutions in the absence of a catalyst, the rate constants for adsorption, and the rate of establishment of sorption equilibrium in rutile-dye systems, but increases as the sorption value grows. The photocatalytic process substantially disturbs sorption equilibrium in most of the systems studied. The conclusion is drawn that both weakly and strongly sorbed substrate molecules can participate in the photocatalytic process.     

A kinetic study of oxidation of 1,4‐dioxane by diperiodatonickelate(IV) in aqueous alkaline medium

The kinetics of oxidation of 1,4‐Dioxane (Dio) by Diperiodatonickelate (IV) (DPN) in aqueous alkaline medium at a constant ionic strength of 1.5 mol dm⁻³ was studied spectrophotometrically. The reaction shows first‐order kinetics in [DPN] and less than unit order dependence each in [Dio] and [OH⁻]. Addition of products, Ni(II) and periodate have no significant effect on the reaction rate. An increase in ionic strength and decrease in dielectric constant of the medium increases the rate. A mechanism based on experimental results, involving two paths, one [Dio] dependent and the other [Dio] independent is proposed. The constants involved in the mechanism are evaluated. There is a good agreement between the observed and calculated rate constants at varying conditions of experiments. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 789–796, 1999     

Kinetic, mechanistic and spectral investigation of ruthenium (III)-catalysed oxidation of atenolol by alkaline permanganate (stopped-flow technique)

Kinetics of ruthenium (III) catalyzed oxidation of atenolol by permanganate in alkaline medium at constant ionic strength of 0.30 mol dm³ has been studied spectrophotometrically using a rapid kinetic accessory. Reaction between permanganate and atenolol in alkaline medium exhibits 1 : 8 stoichiometry (atenolol : KMnO₄). The reaction shows first-order dependence on [permanganate] and [ruthenium (III)] and apparently less than unit order on both atenolol and alkali concentrations. Reaction rate decreases with increase in ionic strength and increases with decreasing dielectric constant of the medium. Initial addition of reaction products does not affect the rate significantly. A mechanism involving the formation of a complex between catalyst and substrate has been proposed. The active species of ruthenium (III) is understood as [Ru(H₂O)₅OH]₂₊. The reaction constants involved in the different steps of mechanism are calculated. Activation parameters with respect to the slow step of the mechanism are computed and discussed and thermodynamic quantities are also calculated.     

Effect of Anionic Surfactant on the Oxidation of DL-Aspartic Acid by N-Bromophthalimide: A Kinetic Study

The kinetics of oxidation of DL-Aspartic acid (Asp) by N-bromophthalimide (NBP) was studied in the presence of sodium dodecyl sulfate (SDS) in acidic medium at 308 K. The rate of reaction was found to have first-order dependence on [NBP], fractional order dependence on [Asp] and inverse fractional order dependence on [H⁺]. The addition of reduced product of the oxidant, that is, [Phthalimide] has decreased the rate of reaction. The rate of reaction increased with increase in inorganic salts concentration, whereas a change in [Cl^?], ionic strength of the medium and [Hg(OAc)₂] had no effect on the oxidation velocity. The rate of reaction decreased with a decrease in dielectric constant of the medium. COOH-CH₂-CN was identified as the main oxidation product of the reactions. The various activation parameters have been computed. A suitable reaction mechanism consistent with the experimental findings has been proposed. The micelle-binding constant has been calculated.     

Thallation of fluorenes: A kinetic and mechanistic study

The kinetics of thallation of fluorene with thallium triacetate (TTA) in HOAc-H₂SO₄ solutions led to the rate expression The rate of thallation is found to increase with increasing acid concentration, and a sixth-order dependence on [H₂SO₄] is observed. Decrease in solvent polarity increases the rate of thallation. The effect of substituents is in accord with the electrophilic substitution at an aromatic system. Thallation occurs at 4-position. A mechanism similar to aromatic bromination is proposed for the thallation of fluorene.     

Enantioselective hydrogenation of alkenes with iridium-PHOX catalysts: a kinetic study of anion effects

In the asymmetric hydrogenation of unfunctionalized olefins with cationic iridium-PHOX catalysts, the reaction kinetics and, as a consequence, catalyst activity and productivity depend heavily on the counterion. A strong decrease in the reaction rate is observed in the series [Al[OC(CF3)3]4]- >BArF- >[B(C6F5)4]- >PF6- >BF4- >CF3SO3-. With the first two anions, high rates, turnover frequencies (TOF >5000 h(-1) at 4 degrees C), and turnover numbers (TONs) of 2000-5000 are routinely achieved. The hexafluorophosphate salt reacts with lower rates, although they are still respectable; however, this salt suffers from deactivation during the reaction and extreme water-sensitivity, especially at low catalyst loading. Triflate and tetrafluoroborate almost completely inhibit the catalyst. In contrast to the hexafluorophosphate salt, catalysts with [Al[OC(CF3)3]4]-, BArF-, and [B(C6F5)4]- as counterions do not lose activity during the reaction and remain active, even after all the substrate has been consumed. In addition they are much less sensitive to moisture and, in general, rigorous exclusion of water and oxygen is not necessary. A first-order rate dependence on the hydrogen pressure was determined for the BArF- and the PF6- salts. At low catalyst loading, the rate dependence on catalyst concentration was also first order. The rate dependence on the alkene concentration was strikingly different for the two salts. While the reaction rate observed for the BArF- salt slightly decreased with increasing alkene concentration (rate order -0.2), a rate order of approximately 1 was determined for the corresponding hexafluorophosphate at low alkene concentrations.     

Influence of copper(II) catalyst on the oxidation of l-histidine by platinum(IV) in alkaline medium: a kinetic and mechanistic study

The kinetics of oxidation of l-histidine (His) by platinum(IV) in the absence and presence of copper(II) catalyst was studied using spectrophotometry in alkaline medium at a constant ionic strength of 0.1 mol dm^?3 and at 25 °C. In both cases, the reactions exhibit a 1:1 stoichiometry ([His]:[Pt^IV]). The rate of the uncatalyzed reaction is dependent on the first power of each of the concentrations of oxidant, substrate and alkali. The catalyzed path shows a first-order dependence on both [Pt^IV] and [Cu^II], but the order with respect to both [His] and [OH^?] is less than unity. The rate constants increase with increasing ionic strength and dielectric constant of the medium. The catalyzed reaction has been shown to proceed via formation of a copper(II)^_histidine intermediate complex, which reacts with the oxidant by an inner-sphere mechanism leading to decomposition of the complex in the rate-determining step. Platinum(IV) is reduced to platinum(II) by the substrate in a one-step two-electron transfer process. This is followed by other fast steps, giving rise to the oxidation products which were identified as 2-imidazole acetaldehyde, ammonia and carbon dioxide. A tentative reaction mechanism is suggested, and the associated rate laws are deduced. The activation parameters with respect to the slow step of the mechanism are reported and discussed.     

Kinetic study of ruthenium(VI) -- catalyzed oxidation of 2-methoxy- ethanol by aqueous alkaline hexacyanoferrate(III)

The kinetics of ruthenium(VI) catalyzed oxidation of 2-methoxyethanol by hexacyanoferrate(III) ion in an aqueous alkaline medium at constant ionic strength shows zero order dependence on hexacyanoferrate(III) and first order dependence on Ru(VI). Dependence of substrate concentration shows a Michaelis – Menten type behaviour. The rate increases with the decrease in alkali concentration. A reaction mechanism involves the formation of an intermediate complex between the substrate and ruthenium(VI). This complex decomposes slowly, producing ruthenium(IV), which is reoxidized by hexacyanoferrate(III) in subsequent steps. The theoretical rate law obtained is in complete agreement with the experimental observations.     

Anomalous decrease in lamellar spacing by shear flow in a nonionic surfactant/water system

We study the effects of shear flow on the structure of a lamellar phase in a C16E7 [hepta(oxyethylene glycol)-n-hexadecyl ether]/water system (40-55 wt % of C16E7) at 70 degrees C using small-angle neutron scattering in the range of shear rate of 10(-3)-30 s(-1). At the shear rate 0.1-1 s(-1), the repeat distance (d) is decreased significantly (down to about 40% of d at rest in the most significant case) and discontinuously with increasing shear rate. With the further increase in the shear rate, d increases through a sharp minimum (referred to as d*). Such a shear rate dependence of d is obtained for all the principal orientations of lamellae. As the concentration of C16E7 decreases from 55 to 40 wt %, d increases from 6.5 to 8.5 nm at rest whereas d* remains almost constant (approximately equal to 5 nm). Moreover, d* is found to be almost equal to the thickness of bilayers obtained from the line shape analysis of small-angle X-ray scattering at rest. The results strongly suggest that the water layer is excluded by shear flow and that the lamellar phase segregates into surfactant-rich and water-rich regions, although these regions do not reach macroscopic size.     

Studies of redox polymerization. I. Aqueous polymerization of acrylamide by an ascorbic acid–peroxydisulfate system

The polymerization of acrylamide initiated by an ascorbic acid–peroxydisulfate redox system was studied in aqueous solution at 35 ± 0.2°C in the presence of air. The concentrations studied were [monomer] = (2.0–15.0) × 10^?2 mole/liter; [peroxydisulfate] = (1.5–10.0) × 10^?3 mole/liter; and [ascorbic acid] = (2.84–28.4) × 10^?4 mole/liter; temperatures were between 25–50°C. Within these ranges the initial rate showed a half-order dependence on peroxydisulfate, a first-order dependence on an initial monomer concentration, and a first-order dependence on a low concentration of ascorbic acid [(2.84–8.54) × 10^?4 mole/liter]. At higher concentrations of ascorbic acid the rate remained constant in the concentration range (8.54–22.72) × 10^?4 mole/liter, then varied as an inverse halfpower at still higher concentrations of ascorbic acid [(22.72–28.4) × 10^?4 mole/liter]. The initial rate increased with an increase in polymerization temperature. The overall energy of activation was 12.203 kcal/mole in a temperature range of 25–50°C. Water-miscible organic solvents depressed the initial rate and the limiting conversion. The viscometric average molecular weight increased with an increase in temperature and initial monomer concentration but decreased with increasing concentration of peroxydisulfate and an additive, dimethyl formamide (DMF).     

Mechanism of oxidations in partially aqueous media: Kinetics of oxidation of thiosemicarbazide and thiosemicarbazone by chloramine-T and dichloramine-T in aqueous methanol

Kinetics of oxidation of thiosemicarbazide (TSC) and its hydrazone (Benzaldehyde thiosemicarbazone) by chloramine-T (CAT) and dichloramine-T (DCT) in aqueous methanol medium in the presence of perchloric acid has been studied. Oxidation of TSC by both the oxidants showed first order dependences in [oxidant], fractional order in [TSC] and nearly inverse first order in [H⁺]. The conversion of TSC into its benzaldehyde hydrazone changed the rate dependence in [CAT] from first to second order, while the dependence in [DCT] remained unchanged. The dependence in [TSC] changes from fractional order to zero order in both CAT and DCT oxidations. The rate followed inverse fractional order kinetics in [H⁺] in both the cases. Increase in ionic strength of the medium slightly decreased the rate, while the decrease in dielectric constant of the medium increased the rates of oxidations for both the oxidants. But the addition of reaction products, p-toluenesulphonamide and chloride had no effect on the rate. Oxidation of TSC with both the oxidants has been shown to follow Michaelis-Menten type mechanism. In hydrazone oxidations oxidants have been shown to disproportionate in slow steps to HOCl, which in turn attacks the substrate in fast steps to give the final products. [TSC] was varied at different temperatures and the constants of rate limiting steps were calculated at each temperature. Using the latter constants the activation parameters have been computed from the Arrhenius plots. The rate constants have been predicted from the rate law for the variation of [H⁺] at constant [TSC] and [oxidant]. The predicted values are in reasonable agreement with the experimental rate constants, providing additional support to the suggested mechanisms.     

二羟基二过碘酸合镍(IV)氧化氨基丙酸的动力学

在碱性介质中研究了二羟基二过碘酸合镍（Ⅳ）配离子（DDN）氧化α－氨基丙酸的动力学，结果表明，反应对DDN为一级，对α－氨基丙酸为正分数级，准一级速率常数，k_(obs)，随［OH~－］增加而增加，随［IO~-_4］增加而减小，无盐效应也未检出自由基1/k_(obs)对［IO~-_4］有直线关系，表明二羟基一过碘酸合镍（Ⅳ）（DMN）是氧化剂的活性物种，据此很出了一个包括DDN与DMN存在前期平衡和内层一步双电子转移的反应机理．导出的速率方程园满地解释了全部实验现象，并且计算出速控步聚的速率常数，前期平衡常数和活化参数．     

Kinetics and mechanism of oxidation of L-Alanine by <Emphasis Type="Italic">N</Emphasis>-bromophthalimide in the presence of sodium dodecyl sulfate

The kinetics of oxidation of L-Alanine (Ala) by N-bromophthalimide (NBP) was studied in the presence of an anionic surfactant, sodium dodecyl sulfate, in acidic medium at 308 K. The rate of reaction was found to have first-order dependence on [NBP], fractional order dependence on [Ala] and inverse fractional order dependence on [H⁺]. The addition of reduced product of the oxidant [Phthalimide] has decreased the rate of reaction. The rate of reaction increased with increase in inorganic salts concentration i.e., [Cl⁻] and [Br⁻], whereas a change in ionic strength of the medium and [Hg(OAc)₂] had no effect on oxidation velocity. The rate of reaction decreased with a decrease in dielectric constant of the medium. CH₃CN was identified as the main oxidation product of the reaction. The various activation parameters have been computed and suitable mechanism consistent with the experimental findings has also been proposed. The micelle-binding constant has been calculated. Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 3, pp. 386–396. The article is published in the original.     

二过碘酸合铜(III)氧化乙醇胺的动力学及机理

近年来，三价铜的过碘酸配合物作为氧化剂在有机物的定量测定中得到广泛的应用问．Movius风和Murthy门分列报导了二过碘酸合铜（Ill）氧化一元醇的反应动力学，但二者所得结果有一定的差别·由于C“m）处于最高氧化态，反应体系又比较复杂·目前人们对它的认识并不太清楚·本文就CU（）氧化乙醇胺（则进行了动力学及机理的研究·1实验部分所用仪器及测定人。s的实验手续同前文＊．2结果与讨论2．1准一级速车常教k。bs的求得在D川。三IO可＊u（m）IO条件下，以w人一＊。）对时间土的图为直线，表明反应对［Cb（Ill）］为一级·准一级速…