Kinetics of solvolysis of the trans-dichlorobis(R-ethylenediamine)cobalt(III) ion (R = N-methyl or N,N′-dimethyl) in urea–water mixtures

First order solvolysis rates of the trans-dichlorobis(R-ethylenediamine)cobalt(III) ion (R=N-Me or N,N-Me₂) complex have been investigated in mixtures formed by adding urea to H₂O, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of the complexes in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the cationic cobalt(III) complexes in the transition state are more stable than the cationic cobalt complexes the initial state in the water+urea mixtures.     

TXRF determination of mercury(II) in water in combination with liquid–liquid microextraction

A highly selective hybrid way of TXRF determination of mercury(II) in drinking water at the level of n(10^–2–10⁰) μg/L is developed. The technique of preconcentration of mercury(II) ions includes directly suspended droplet microextraction with benzene in the form of an iodide molecular complex. The proposed method of determination is characterized by its high degree of sensitivity and reproducibility (c _min = 8 ng/L, s _r = 0.12 (100 ng/L)). The accuracy of the analysis results is confirmed by the introduced–found method.     

Solvent effects on the kinetics of solvolysis of trans-dichlorobis (N-methylethylenediamine)cobalt(III) ion in water–propan-2-ol and water–acetonitrile mixtures

First order solvolysis rates of the trans-dichlorobis (N-methylethylenediamine) cobalt(III) ion have been measured over a wide range of solvent compositions and temperatures in water–propan-2-ol and water–acetonitrile mixtures. The rate of solvolysis is faster in the former mixtures rather than the latter. Plots of log(rate constant) versus the reciprocal of the dielectric constant of the co-solvent, and also versus the Grunwald–Winstein Y-values are non-linear for both co-solvents; this non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. However, extrema in the variation of enthalpy H and entropy S of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of H versus S were obtained and the isokinetic temperature indicates that the reaction is entropy controlled. The application of a free-energy cycle shows that changes in solvent structure affect the pentacoordinated cobalt(III) ion in the transition state more than the hexacoordinated cobalt(III) ion in the initial state. In addition, the stabilizing influence of changes in solvent structure is greater in propan-2-ol–water mixtures than in acetonitrile–water mixtures, and the difference becomes greater as the mole fraction, x2 of the organic co-solvent increases.     

Iron analysis in atmospheric water samples by atomic absorption spectroscopy (AAS) in water–methanol

To distinguish between Fe(II) and Fe(III) species in atmospheric water samples, we have adapted an analytical procedure based on the formation of a specific complex between Fe(II) and ferrozine (FZ) on a chromatographic column. After elution of Fe(III), the Fe(II) complex is recovered with water–methanol (4:1). The possibility of trace iron measurements in this complex medium by graphite-furnace atomic-absorption spectrometry has been investigated. A simplex optimization routine was required to complete the development of the analytical method.     

Critical phenomena in the topochemical reaction of water vapor with solid N,N′-ethylenebis (acetylacetoniminato)copper (II)

The effects of water vapor and temperature on the phase transition between anhydrous N,N-ethylenebis(acetylacetoniminato) copper (II) and the monohydrate have been studied by a piezoelectric microbalance method. The presence of a branch point and critical deceleration of the phase transition dynamics near that point have been observed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 2, pp. 149–153, March–April, 1993.This work was supported by a fundamental research grant from the State Committee on Science and Technology, Ukraine Council of Ministers.     

Palladium(II)-catalyzed Suzuki–Miyaura reactions of arylboronic acid with aryl halide in water in the presence of 4-(benzylthio)-N,N,N-trimethybenzenammonium chloride

This work reported that Suzuki–Miyaura coupling reactions of arylboronic acid with aryl bromide or iodides were mediated by Pd(OAc)₂ and 4-(benzylthio)-N,N,N-trimethylbenzenammonium chloride in the presence of Na₂CO₃ in water under the mild conditions. The corresponding Suzuki–Miyaura coupling products were obtained in good to excellent yields.     

Medium effects on the reactions of coordination complexes: Base hydrolysisof(αβS)(p-hydroxybenzoato)(tetraethylenepentamine) cobalt(III) in iso-propanol–water,tert-butanol–water and dimethyl sulfoxide–water media

The base hydrolysis of (S)(p-hydroxybenzoato)-(tetraethylenepentamine)cobalt(III) has been investigated in aqueous–organic solvent media using i-PrOH, t-BuOH and dimethyl sulfoxide (DMSO) as cosolvents at 20.0 T (°C) 40.0 (I=0.02 mol dm ^-3) with 80% (v/v) of cosolvents. Only the base-catalysed path (k_obs=k_OH[OH^-]) is observed. The relative second order rate constant k _OH ^os /k _OH ^ow at I=0 increases nonlinearly with increasing mol fraction (x_O.S.) of the cosolvents, the rate acceleration in alcoholic cosolvents being greater than in DMSO. The destabilization of ^-OH in mixed solvent media alone does not explain the observed rate acceleration. The solvent composition dependence, log k _OH ^os = log k _OH ^ow + a_ix _os ⁱ [i=1,2,k _OH ⁰ denotes k_OH at I=0 in mixed solvent(s) and water (w)] indicates specific solute–solvent interactions. The values of the relative transfer free-energy data [_TG(t.s.) - _TG^o (i.s.)](sw)(25 °C)(G), where t.s. and i.s. denote the transition state and initial state of the substrates respectively, are positive for all substrates at all compositions, indicating a greater destabilizing effect of the mixed solvent on the transition state than on the initial state. The G values also correlate with G^E(G = ax_O.S. + cG^E) for all solvents, supporting the fact that solvent structural effects mediate the rates and energetics of the reaction. However, the solvent effects on the solvation components of H and S are mutually compensating, thus indicating that there is no change in the mechanism.     

Vinylation of trialkylamines with acyl- and cyanoacetylenes via C–N bond cleavage in the presence of water

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Phase equilibria in binary subsystems of urea–biuret–water system

Joint results of the differential scanning calorimetry (DSC) and thermogravimetry (TG) experiments were the basis for the fusion enthalpy and temperature determination of the biuret (NH₂CO)₂NH (synthesis by-product of the urea fertilizer (NH₂)₂CO). Recommended values are Δ_m H = (26.1 ± 0.5) kJ mol⁻¹, T _m = (473.8 ± 0.4) K. The DSC method allowed for the phase diagrams of “water–biuret,” “water–urea,” “urea–biuret” binary systems to be studied; as a result, liquidus and solidus curves were precisely defined. Stoichiometry and decomposition temperature of the biuret hydrate identified, composition of the compound in “urea–biuret” system was suggested.     

Suzuki–Miyaura cross-coupling reactions in water using in situ generated palladium(II)–phosphazane complexes

The phosphazane derivatives(L1–3) were readily obtained by reaction of different ratios of PCl3 and PhNH2. The L1–3 derivatives were found to be efficient ligands in the palladium-catalyzed Suzuki C–C coupling reactions in water. It was determined that with the use of L1–3 /Pd(OAc)2 system as a catalyst, aryl halides undergo Suzuki cross-couplings with arylboronic acids to give the desired products in moderate to excellent yields.     

Dissolution mechanisms of wood cellulose fibres in NaOH–water

Four wood pulps and a microcrystalline cellulose were dissolved in a NaOH 8%–water solution. Insoluble fractions and clear solution fractions were isolated by centrifugation and were observed by optical microscopy and transmission electron microscopy. Molecular weight distribution, carbohydrate composition and cellulose II content were measured. The dissolution of wood cellulose fibres in NaOH 8%–water solutions occurs by successive dismantlement and fragmentation steps governed by the swelling and the shearing of the original structure. The cellulose from insoluble and clear solution fractions is in both case converted in cellulose II and the insoluble fractions contain embedded mannans. Besides, the molecular weight distributions of cellulose from insoluble and clear solution fractions reveal the existence of heterogeneities in dissolution capacity of the cellulose chains, independent to the degree of polymerization, which are related to the chemical environment of the chains in the fibre structure.     

Behavior of double stimuli-responsive copolymer of N-(3-(diethylamino) propyl)-N-methylacrylamide and N,N–diethylacrylamide in aqueous solutions

Double stimuli-responsive copolymer of N-(3-(diethylamino)propyl)-N-methylacrylamide and N,N-diethylacrylamide (molar mass M ≈ 32,000?g?mol^?1) in buffer solutions was studied by the static and dynamic light scattering and turbidimetry. The experiments were performed within pH interval from 7 to 13 and in a wide concentration range. Two types of scattering species were observed in solutions of all concentrations and pH at room temperatures. They were macromolecular unimers and loose aggregates. The increase in concentration and pH led to decrease in phase separation temperatures and width of phase transition.     

A general method for the preparation of N-heterocyclic carbene–silver(I) complexes in water

A new synthetic method leading to N-heterocyclic carbene–silver(I) complexes [(R₂-NHC)₂Ag]⁺ [AgX₂]^? is developed by using benzimidazolium compounds, NaOH (as a base), silver salts and water (as the reaction medium). Single-crystal X-ray structure revealed that compound 1 comprises a linear [Ag-(Et₂-Bimy)₂]⁺ cation and a linear [AgBr₂]^? anion. These two ions are linked through an Ag^I–Ag^I association and staggered at an angle of 90.3°.     

Molecular thermodynamics of salt effect in vapor–liquid equilibrium of ethanol–water systems

Scaled particle theory was used to derive a general expression for the salt effect parameter, K, of isobaric vapor–liquid equilibrium for ethanol–water-1-1 type electrolytic systems, which appears in the Furter equation. This expression was essentially a sum of two terms: 1, the hard sphere interaction term calculated by Masterton–Lee's equation, 2, the soft sphere interaction term calculated by Y. Hu's molecular thermodynamical model, in which the diameters of nacked ions were replaced by that of solvated ions, the solvation coefficients (i.e., in the radio of the latter to the former) were taken to be adjustable parameters, their magnitude implies the ionic solvation rules. A correlation equation for the local dielectrical constant around central ions with liquid concentration was obtained by mapping out experimental points. The calculated salt effect parameters of 9 ethanol–water-1–1 type electrolytic systems were in good agreement with the literature values within the wide range of liquid concentration.     

Molecular organization and aggregation of mucin at air–water interface in the presence of chromium(III) complexes

The interfacial organization of mucin (glycoprotein) in the presence of chromium(III) complexes has been assessed from the surface pressure–molecular area (π–A) isotherms in Langmuir films at air–water interface and the surface energy of their LB films through contact angle measurements. At pH 7.0, the electrostatic interaction of [Cr(en)₃]Cl₃ with mucin was found to bring about changes in the average surface area from 3.26 to 1.47 nm²; suggesting the possible formation of large aggregates of mucin. Adsorption experiments using surface potential measurements reveal that [Cr(en)₃]Cl₃ binds at a much faster rate to the available binding sites in mucin when compared to [Cr(salen)(H₂O)₂](ClO₄) which binds coordinatively to mucin.     

A novel N–O ligand for palladium-catalyzed Mizoroki–Heck reaction in neat water

1-(Morpholin-1-yl-pyridin-2-yl-methyl)-2-naphthol as a novel efficient N–O ligand has been developed for palladium-catalyzed Mizoroki–Heck reaction in neat water without the protection of an inert atmosphere. The reactions proceed smoothly and give the desired products in moderate to excellent yields. The catalyst system is reusable.     

On the kinetics of pozzolanic reaction in metakaolin–lime–water system

The kinetics of pozzolanic reaction metakaolin–lime is studied in the present work. Metakaolin is prepared by calcination of enriched kaolin (deposit “Senovo”, Bulgaria) at temperature of 830 ± 10 °C in a labscale muffle oven. The reaction is performed in intensively stirred water suspension at different temperatures in the range 20–100 °C. The kinetics is analyzed by comparing the experimental data with theoretical curves, derived according to appropriate kinetic and diffusion models taking into account the grain size distribution of metakaolin. The macroscopic mechanism and activation energy of the reaction are determined. It is found, that the activation energy decreases gradually from 71 to 45 kJ/mol[Ca(OH)₂] with the increase of the reaction degree from 0.2 up to 0.6, respectively, which is a characteristic for transition regime reactions.     

Bulk properties and hydration numbers of components of water–salt,water–urea,and water–urea–salt systems

With an increase in the concentration of additives, the hydration numbers of compounds decrease. Thus, in a saturated 54.6% solution, urea loses approximately 3/4 of the initial amount of water, forming an aquacomplex of the composition (NH₂)₂CO?H₂O. In a supersaturated 44% solution, the sodium chloride aquacomplex is dehydrated by 2/3, and in a supersaturated 67% solution, sodium sulfate is dehydrated by 5/6. The density of these solutions is 1.354÷1.360 g/cm³ (44% NaCl) and 1.800÷1.849 g/cm³ (67% Na₂SO₄). In a saturated urea solution, NaNO₃, NaCl, and Na₂SO₄ complexes lose 53÷55% of hydration water. It is shown that the interactions in the binary water–urea system somewhat increase the hydration number of the salts (structural hydration). The hydration water density, a structurally important characteristic, increases in the series of solutions of urea, NaNO₃, NaCl, and Na₂SO₄. In the same series of additives, the excess volume of binary water–urea and water–salt systems becomes more negative.     

A new hybrid (experimental–theoretical) quantitative method for detection of relative anomer concentrations in water

A novel and inexpensive hybrid (combined experimental and theoretical) approach was used to quantitatively identify anomer proportions of d-glucose, d-galactose and d-mannose in water. The study involves three parts: recording of experimental FT-IR spectra of monosaccharides in water, calculation of vibrational frequencies of all stable anomers of monosaccharides and regression analysis of theoretical and experimental intensities. No expensive experimental processes and high-level calculations were needed during the study. The results were compared with the data from pure experimental and molecular dynamic studies. It has been concluded that in water while d-glucose and d-mannose have two stable anomers, α-pyranose and β-pyranose, d-galactose has four stable anomers, α-pyranose, β-pyranose, α-furanose and β-furanose. Comparison of detected results with the literature data showed that the developed method is working for the quantitative detection of anomer proportions of monosaccharides in water.