Solvent effect on the kinetics of Beckmann rearrangement

The rate constants for the Beckmann rearrangement of cyclohexanone oxime p-toluenesulfonate in 11 solvents are satisfactorily described by a three-parameter linear correlation including polarizability, electrophilicity, and molar volume of the solvent. The first two factors favor the reaction, whereas increase of the solvent molar volume makes the reaction slower, presumably due to steric hindrances to solvation.     

Solvent effect on the kinetics of carbamoylation of alcohols

Solvent effect on the rate of the reaction of phenyl isocyanate with alcohols may be described by multiparameter linear equations. Correlation analysis of the effects of various factors showed that specific solvation inhibits the reaction and that increased solvent polarity favors the process.     

Potentiometric microdetermination of nitro- and bromophenols by oxidation with lodate and periodate

Continuing the work started to study the redox reactions between iodate or periodate and monohydric phenols potentiometrically, standard solutions of oxidants are used for microdetermination of nitro- and bromophenols, by iodide reduction of unreacted iodate or periodate and back titrating excess iodide with Hg(II) using silver amalgam and SCE combination. The procedure is also evaluated in comparison with those mentioned in literature, indicating the reliability and high accuracy of the given procedure. The possible schemes for these redox reactions are suggested and compared with those mentioned in literature. These schemes also explain the formation of dimers as oxidation products at higher concentration of both reactants.     

Physicochemical characterization of ion-exchange membranes in water-methanol mixtures

A complete physicochemical characterization of two ion-exchange membranes—CM2 and Nafion^®117—used in electrodialysis and in direct methanol fuel cells (DMFC) has been carried out. For each membrane, in different methanol-water mixtures—0%, 20%, 40%, 60%, 80% and 100%—and at different temperatures (25.0; 40.0 et 55.0 °C), we have measured the variations of the geometrical dimensions, the proton electrical conductivity, the swelling rate and the amount of methanol in the membrane. The FTIR analysis of Nafion^®117 was performed at different methanol contents of the external solution.The results show that the CM2 membrane presents the best geometrical stability, and the lowest conductivity at any methanol content. At high methanol contents, Nafion^®117 is 10 times more conductive than the CM2 membrane. It was found that the methanol is absorbed more by Nafion^®117, and its effect is more noticeable on the microstructure of this membrane, under standard conditions. The high methanol permeability of these membranes, particularly of the Nafion^®117, induces bad cell efficiencies and lifetimes.     

The conductance of tetraethylammonium salts in water-methanol mixtures

Conductance measurements are reported for tetraethylammonium halide and perchlorate salts in water-methanol mixtures over the complete composition range at 25° C. Limiting conductances and association constants are evaluated together with the distance parameters. The maxima in the limiting ionic Walden products are considered in some detail. Ionic association is generally weak, and the lack of reliable correlation betweenK _A and the solvent dielectric properties supports the assumption of specific ion-solvent interaction.     

Solvent effect on metal oxidation rates in aprotic media

A correlation between the rates of oxidation of metals with organometallic chlorides in aprotic media and the physicochemical properties of the solvents was established. An equation fitting the experimental data for ten reaction series was suggested. The kinetic parameters of oxidation of cadmium with diphenyl-bismuth chloride in aprotic media, calculated with the correlation equations, coincide with the experimental data.     

Preparation and characterization of polypyrrole-silica colloidal nanocomposites in water-methanol mixtures

The effect of methanol cosolvent on the synthesis of polypyrrole-silica colloidal nanocomposites using ultrafine silica sols in combination with both FeCl3 and APS oxidants has been investigated. Two protocols were evaluated: the addition of methanol to an aqueous silica sol and the addition of water to a methanolic silica sol. The latter protocol proved to be more robust, since it allowed colloidally stable dispersions to be prepared at higher methanol content (up to 50 vol% using the APS oxidant). This allowed greater control over the particle size of the nanocomposite particles. In general, the spectroscopic data, the particle size range, silica contents and electrical conductivities of these nanocomposites were similar to those reported earlier for purely aqueous formulations. Polypyrrole contents ranged from 49 to 71% by mass and particle diameters varied from around 160 to 360 nm. In terms of colloid stability, the APS oxidant was preferred for nanocomposite syntheses in the presence of methanol. However, the FeCl3 oxidant generally gave higher conductivities and narrower size distributions under comparable conditions. HF etching experiments combined with transmission electron microscopy studies indicated that, to a first approximation, these nanocomposite particles had core-shell morphologies, with a hydrophobic polypyrrole core and a hydrophilic silica shell that compose approximately one monolayer of silica sol particles. Finally, aqueous electrophoresis measurements suggested that the polypyrrole-silica nanocomposites were silica-rich and that the methanolic silica sol was more hydrophobic (lower surface charge density) than the aqueous silica sol.     

Adsorption and oxidation of aniline and anisidine by chromium ferrocyanide

The interaction of aniline and p-anisidine with chromium ferrocyanide has been studied. Maximum uptake of both anilines was observed around pH 7. The adsorption data obtained at neutral pH were found to follow Langmuir adsorption. Anisidine was a better adsorbate because of its higher basicity. In alkaline medium (pH>8) both aniline and anisidine reacted with chromium ferrocyanide to give colored products. Analysis of the products by GC-MS showed benzoquinone and azobenzene as the reaction products of aniline while p-anisidine afforded a dimer. IR analysis of the amine-chromium ferrocyanide adduct suggests that the outer metal ion of chromium ferrocyanide and amino group of amines are responsible for the interaction. A possible reaction mechanism for the product formation in alkaline medium has been proposed. The present study suggests that metal ferrocyanides might have played an important role in the stabilization of organic molecules through their surface activity in the prebiotic condensation reactions.     

Solvent effect on transformation kinetics of ozone and toluene complex

Medium effect on the decomposition rate of C₆H₅CH₃.O₃ in solvents: n-C₆H₁₄, CHCl₃, CH₂Cl₂, (CH₃)₂CO, CH₃OH and CH₃CN has been studied. Consumption rate of this complex increases in polar solvents.

---

C₆H₅CH₃.O₃ n-C₆H₁₄, CHCl₃, CH₂Cl₂, (CH₃)₂CO, CH₃OH, CH₃CN. .

     

Enthalpies of solution of urea in water-methanol mixtures at 298.15 K

The standard enthalpies of solution of urea in water-methanol mixtures were determined over the entire range of solvent compositions at 298.15 K. A comparison of the results obtained with the published data on mixtures of water with ethanol and n-propanol revealed a differentiating effect of the alcohol concentration on the enthalpy of solution of urea. Only for water-methanol solutions, an increase in the alkanol content in the mixture (0.6 mole fractions < x ₂ < 1.0 mole fractions) caused an increase in the degree of solvation of urea.     

Time-resolved emission of flavin adenine dinucleotide in water and water-methanol mixtures

Time-resolved fluorescence decay of flavin adenine dinucleotide (FAD) was studied at room temperature in water and water-methanol mixtures by a fluorescence upconversion technique. The observations were focused on the most initial decay phase (200 ps), before the residual fluorescence assumes a single exponential decay, typical for an extended conformation of the fluorophore. Within the first few picoseconds, where most of the electron transfer coupled quenching takes place, the emission decay curves could be fitted by a stretched exponent, compatible with the inhomogeneous distance dependent electron transfer model. This implies that the population of the excited FAD molecules exhibits a large number of non-identical states, each with its own separation between the donor (adenine) and acceptor (isoalloxazine) moieties, having its own rate of electron transfer. To evaluate the distribution of the separation between the donor-acceptor pair, we carried out molecular dynamics simulations of closed conformation of the FAD in water and water-methanol mixtures, sampling the structure at 10 fs intervals. The analysis of the dynamics reveals that within the 4 ps time frame, where most of the nonexponential fluorescence relaxation takes place, the relative motion of the donor-acceptor pair is consistent with a one-dimensional Brownian motion, where the diffusion coefficient and the shape of the confining potential well are solvent dependent. The presence of methanol enhances the diffusion constant and widens the width of the potential well. On the basis of these parameters, the relaxation dynamics was accurately reconstructed as an electron transfer reaction in an inhomogeneous system where the reactants are diffusing within the time frame of the observation.     

Temperature effect on the kinetics of persulfate oxidation of p-chloroaniline

The kinetics of heat-facilitated persulfate oxidation of p-chloroaniline（PCA） in aqueous solutions was measured at five different temperature conditions and at four different oxidant concentrations.The PCA degradation was found to follow a pseudo-first-order decay model when the persulfate was excessive.The pseudo-first-order rate constants of PCA degradation by persulfate（50 mmol/ L） at pH 7.0 are 0.12×10^-4,0.28×10^-4,0.43×10^-4,0.83×10^-4,1.32×10^-4 s^-1 at 10,20,30,40 and 50℃,respectively. Under the above reaction conditions,the reaction has activation energy of 49.97 kJ/mol.The observed rate was found to be function of temperature and oxidant concentration.Raising temperature and increasing persulfate concentration can significantly accelerate the PCA degradation.     

Adsorption kinetics and equilibrium modeling of cesium on copper ferrocyanide

The copper ferrocyanide (CuFC) prepared in this study was characterized using X-ray diffraction and scanning election microscopy. The distribution of particle sizes of the CuFC suspension was determined. The adsorption kinetics data were evaluated for an intraparticle diffusion model, a pseudo-first order model and a pseudo-second order model at temperatures of 288, 298 and 308 K, respectively. It was found that the adsorption process of Cs⁺ on CuFC was best described by a pseudo-second order kinetic model, with a correlation coefficient (R ²) equal to 1.000, and the adsorption rate constant increased with increasing temperature. This result indicated that chemisorptions took place during the adsorption process. The adsorption equilibrium data fit well to the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. The mean adsorption energy (E) between 11 and 13 kJ/mol at different temperatures indicated that ion exchange was the main mechanism during the adsorption process. Thermodynamic parameters were also evaluated during the adsorption. The values of the standard Gibbs free energy change (ΔG ^o) and standard enthalpy change (ΔH ^o) suggested that the adsorption was a spontaneous and endothermic process. The distribution coefficient (K _d) was more than 2.94 × 10⁶ mL/g when the pH of solution was between 2.6 and 10.9, and the initial Cs⁺ concentration was 100 μg/L. The existence of K⁺ and Na⁺ did not affect the adsorption of Cs⁺ on CuFC when the concentration of K⁺ and Na⁺ in the solution was below 20 and 1,000 mg/L, respectively.     

Dynamics of ionic and hydrophobic solutes in water-methanol mixtures of varying composition

We have carried out a series of molecular-dynamics simulations of water-methanol mixtures containing either an ionic or a neutral atomic solute to investigate the effects of composition of the mixture on the diffusion of these solutes. Altogether, we have considered 17 different systems of varying composition ranging from pure water to pure methanol. The diffusion coefficients of ionic solutes are found to show nonideal behavior with variation of composition of the solvent mixture. The extent of nonideality of the solute diffusion is found to be similar to the nonideality that is observed for the diffusion and orientational relaxation of water and methanol molecules in these mixtures and is attributed to the enhanced stability of the hydrogen bonds and formation of interspecies complexes in the mixtures. The neutral solute shows characteristics of hydrophobic solvation and its diffusion decreases monotonically with increase of methanol concentration. The present simulation results are compared with those of experiments wherever available.     

Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

Kinetics of oxidation of bis(ethylenediamine)mercaptoacetatocobalt (III) ion by periodate in water-t-butanol and water-methanol mixtures at different perchloric acid concentrations

Summary The kinetics of oxidation of the bis(ethylenediamine)mercaptoacetatocobalt (III) ion by periodate were investigated as a function of perchloric acid concentration, ionic strength and temperature. Furthermore, the effect of a nonaqueous component of the solvent on the rate constant and activation parameters H and S in the water-t-butanol and water-methanol mixtures was estimated. The solvent effect is discussed from the viewpoint of the change in solvation of initial and transition states on going from water to a mixed solvent.     

Solvent effect on rate and mechanism of oxidation of chromium(III) by periodate

Summary The kinetics of chromium(III) oxidation by periodate were studied in various EtOH–H₂O solvent mixtures covering the 0.0 to 58.0 wt% EtOH range, at five different temperatures in the 15–35°C range. The rate of reaction increases with increasing EtOH content. Thermodynamic activation parameters have been calculated and an appropriate mechanism is suggested.     

Insights into ion specificity in water-methanol mixtures via the reentrant behavior of polymer

In the present work, we have for the first time systematically investigated the ion specific reentrant behavior of poly(N-isopropylacryamide) (PNIPAM) in water-methanol mixtures. Turbidity measurements demonstrate that SCN(-) and ClO(4)(-) depress the reentrant transition, whereas other anions enhance the transition. As the anion changes from chaotropic to kosmotropic, the minimum critical phase transition temperature (T(min)) decreases and the corresponding volume fraction of methanol (X(M)) shifts to a larger value. Our results demonstrate that anion specificity is due to the anionic structure making/breaking effect on water/methanol complexes. Cations are found to have a lesser but still significant effect on the reentrant transition, and as T(min) decreases the corresponding X(M) also shifts to larger values as with the anions. Our studies show that cation specificity is induced by specific interactions between cations and PNIPAM chains. Furthermore, both anion and cation specificities are amplified as X(M) is increased due to the formation of additional water/methanol complexes. Calorimetry measurements demonstrate that the ion specificity is dominated by changes in entropy.