Aqueous Organic Solvent Media Research Articles

Kinetics Solvent Effect and Mechanism of Hydrolysis of Ethyl Caprylate Ester in Aqueous Organic Solvent Media

The specific rate constant of ethyl caprylate in alkali catalised hydrolysis in water-acetone mixture covering range of 30 to 70% (v/v) of acetone has been determined at temperature 20 to 400c. The rate of reaction decreases with increase in percentage of Acetone from 30 to 70% (v/v). The observed Activation energy decreases progressively with increase in acetone content of the medium. The effect of molar concentration of water and Dielectric constant on the reaction kinetic has also been studied. The thermodynamic parameters (DG*, DH* and DS*) has been determined which showed strong dependency on solvent composition.

Cationic all-halogen bonding rotaxanes for halide anion recognition.

A family of cationic halogen bonding [2]rotaxanes have been synthesised via an active-metal template synthetic strategy. 1H NMR spectroscopic anion titration investigations reveal these interlocked host systems recognize halides selectively over oxoanions in aqueous-organic solvent media. Furthermore, systematically modulating the rigidity and size of the rotaxanes' anion binding cavities via metal complexation, as well as by varying the number of halogen bond-donor groups in the axle component, was found to dramatically influence halide anion selectivity.

Stabilization of horseradish peroxidase in aqueous-organic media by immobilization onto cellulose using a cellulose-binding-domain

A fused protein consisting of a cellulose-binding domain (CBD) and horseradish peroxidase (HRP) was bound to cellulose beads and evaluated in aqueous-organic solvent systems. The CBD–HRP fusion protein containing two different functionalities, a catalytic domain and a binding domain, preserved both capabilities in this non-conventional environment. A six-fold increase in the half-life of the enzyme in buffer resulted from immobilization onto cellulose via CBD. The immobilized enzyme was also more stable than the native enzyme in increasing concentrations of acetone (0–92%). There was a general decrease in activity as the solvent concentration in the mixture increased (in all solvent types: THF, acetone, acetonitrile and ethanol). However, the immobilized enzyme was at all times more active than the soluble enzyme forms. The thermostability of the enzyme in buffer, at 40–60 °C, was also improved by immobilization. The soluble CBD–HRP fusion protein exhibited greater stability (both to organic solvents and temperature), but lower activity, in comparison with the native HRP. This work demonstrates for the first time the use of a cellulose-bound CBD-enzyme as a catalyst in aqueous-organic solvent media.

The effect of solvent on the kinetics of the oxidation of benzaldehydes by quinolinium chlorochromate in aqueous organic solvent media

The kinetics of the oxidation of benzaldehyde and para-substituted benzaldehydes by quinolinium chlorochromate in water-dimethylformamide mixtures has been studied under pseudo-first-order conditions at 25?0.2?C. The operation of non-specific and specific solvent-solute interactions was explored by correlating the rate data with solvent parameters through a correlation analysis technique. Both electron-releasing and electron-withdrawing substitutents enhance the rate of oxidation and the Hammett plot shows a break in the reactivity order indicating the applicability of a dual mechanism.

Solvent and chelation effects on the photoreduction of cobalt(III)-amine complexes in aqueous-organic solvent media

The photoreduction of trans-[Co(NH3)4Cl2]+, trans-[Co(en)2Cl2]+, [Co(dien)Cl3], [Co(trien)Cl2]+, and [Co(tetren)Cl]2+, ions has been studied using a low pressure Hg vapour lamp as light source (254 nm) in aqueous–organic solvents [0–30% (v/v) MeOH or 1,4-dioxane]. Quantum yields for CoII production by redox decomposition have been determined in all the cases, and increase considerably with the increase in concentration of MeOH or 1,4-dioxane in the binary solvent mixtures under investigation. A plot of log(quantum yield) versus the Grunwald–Winstein Parameter, Y, which is a measure of solvent ionizing power, shows that a different blend of general and specific solvent interacts with the solute. This kind of specific solvent interaction on the reactant/excited state has been analysed using multiple regression: viz. Krygowski–Fawcett and Kamlet–Taft equations. Reasons for the difference in reactivity with chelation are also discussed.

Homogeneous Catalysis of the Water Gas Shift Reaction by Bridged Dinuclear Pyrazolate Rhodium Complexes. FT-IR, 1H and 13C NMR In Situ Studies

Homogeneous catalysts for the water gas shift reaction prepared from Rh2(μ-Pz)2(COD)2 (Pz = pyrazolate ion and COD = 1,5-cyclooctadiene) in aqueous organic solvent media (pyridine, 4-picoline or 2-ethoxyethanol) and Rh2(μ-Pz)2(CO)2(TPPMS)2 (TPPMS = meta-sulfonatophenyl-diphenyldiphosphine) in acidic aqueous media under mild conditions are described. In situ FT-IR, 1H and 13C NMR spectroscopic studies of the Rh2(μ-Pz)2(COD)2 catalytic system reveal the presence of hydrido-Rh complexes with linear and bridging carbonyls. These complexes also catalyze the reduction of nitrobenzene to aniline.

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 (αβS)(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 (kobs=kOH[OH-]) is observed. The relative second order rate constant kOHos/kOHow at I=0 increases nonlinearly with increasing mol fraction (xO.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 kOHos = log kOHow + ∑aixosi[i=1,2,kOH0 denotes kOH at I=0 in mixed solvent(s) and water (w)] indicates specific solute–solvent interactions. The values of the relative transfer free-energy data [ΔTG(t.s.) - ΔTGo (i.s.)](s←w)(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 GE(G′ = axO.S. + cGE) 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 ΔH‡ and ΔS‡ are mutually compensating, thus indicating that there is no change in the mechanism.

Polarity Index: The Guiding Solvent Parameter for Enzyme Stability in Aqueous-Organic Cosolvent Mixtures

Enzyme catalysis in aqueous–organic cosolvent mixtures has wide applications. However, inadequate attention has been paid to the issue of stability of enzymes in such media. The results with polyphenol oxidase, peroxidase, acid phosphatase, and trypsin show that solvents with polarity indexes of 5.8 and above are “good” solvents. These solvents when used as cosolvents in aqueous–organic solvent media do not denature the enzymes irreversibly. Enzyme(s) exposed to these solvents retain most of their activity even after 48 h of exposure, whereas solvents with polarity indexes of <5.1 denature the enzyme completely within 0–4 h in most of the cases studied. It appears that at higher concentrations (50% and above) cosolvents effectively compete with the water layer around the enzyme. Fluorescence spectroscopy shows that, although the presence of all the organic cosolvents cause conformational changes in the enzyme molecule at a concentration of 50% (v/v), these changes were completely reversible (when the concentration of organic solvent is diluted with aqueous buffer) in case of solvents having polarity indexes of 5.8 and above. In cases of the solvents having polarity indexes of 5.0 and below, the exposure at 50% concentration changed the conformation of the enzymes irreversibly. Thus, a simple parameter, viz. polarity index, may help in medium engineering of enzyme catalysis in nonaqueous surroundings.

Catalysis of the water gas shift reaction by polymer-immobilized rhodium complexes

Catalysts for the water gas shift prepared from RhCl 3·3H 2O immobilized on several porous aminated polymers in neutral aqueous and aqueous organic solvent media are described. Those using poly(4-vinylpyridine) as the support proved to be the most active among those surveyed, and the effects of varying different experimental parameters were investigated. The turnover frequencies for the production of H 2 displayed a nonlinear dependence on the partial pressure of CO in the 0–1.5 atm range but were independent of the polymer loading in the range 0.5 to 1.5 × 10 −4 mol RHCl 3H 2O/ g of polymer. An activation energy of 18.4 ± 0.9 kcal/mol was obtained from a linear Arrhenius plot based on H 2 production over the temperature range 80–110°C. The activity of the poly(4-vinylpyridine)-anchored catalyst was found to be insensitive to pH in the 3–5 zone but to fall off sharply outside this range. A catalysis mechanism consistent with the quantitative behavior is proposed. This species also catalyzes the Reppe-type transformations (hydrogenation and hydroformylation) of alkenes as well as the reduction of nitrobenzene to aniline.

Cuticular “cell membrane complex‘ modification of wool fabrics by sulphite treatment in aqueous organic solvent media

Cuticle modification on knitted wool fabrics treated with sodium sulphite in an isopropanol water medium has been studied. Isolation of the cuticle and its components, as well as amino acid composition determinations after acid and enzymatic hydrolysis from untreated and treated wool, indicate only slight modification. However, from the amino acid composition of the lipoproteic material dissolved during the treatment and from transmission electron microscopic observations, a modification of the ft‐layer in the cuticle‐cuticle ‘cell membrane complex’ has been clearly observed.

Shrinkage Modifications of Wool Fabrics by Sulphite Treatments in Aqueous Organic Solvent Media: Effect of the Organic Solvent on the Internal Lipids

If a certain solubilization of lipoproteic material takes place during treatments of knitted wool fabrics with sodium sulphite in an isopropanol/water medium, and if these treatments prevent wool shrinkage at some particular conditions of solvent con centration and temperature, then a relationship exists between the dissolved lipoproteic material and the shrinkproofing effect. The lipids of the fiber play an important role in obtaining this effect.

Kinetics and mechanism of formation and dissociation of copper(II) and nickel(II) complexes of the Schiff base bis(acetylacetone)ethylenediimine in aqueous-organic solvent media

In NH4NO3+NH4OH buffered 10% (v/v) dioxan-water media (pH 7.0–8.5), thePseudo-first-order rate constant for the formation of the title complexes M(baen),i.e. ML, conforms to the equation 1/kobs=1/k+1/(kKo.s · TL), where TL stands for the total ligand concentration in the solution, Ko.s is the equilibrium constant for the formation of an intermediate outer sphere complex and k is the rate constant for the formation of the complex ML from the intermediate. Under the experimental conditions the free ligand (pKa>14) exists virtually exclusively in the undissociated form (baenH2 or LH2) which is present mostly as a keto-amine in the internally hydrogen-bonded state. Although the observed formation-rate ratio kCu/kNi is of the order of 105, as expected for systems having “normal” behaviour, the individual rate constants are very low (at 25°C, kCu=50 s−1 and kNi=4.7×10−4s−1) due to the highly negative ΔS≠ values (−84.2±3.3 JK−1M−1 for CuL and −105.8±4.1 JK−1M−1 for NiL); the much slower rate of formation of the nickel(II) complex is due to higher ΔH≠ value (41.2±1.0 kJM−1 for CuL and 78.2±1.2 kJM−1 for NiL) and more negative ΔS≠ value compared to that of CuL. The Ko.s values are much higher than expected for simple outer-sphere association between [M(H2O)6] and LH2 and may be due to hydrogen bonding interaction. In acid media ([H+], 0.01–0.04 M) these complexes M(baen) dissociate very rapidly into the [M(H2O)6]2+ species and baenH2, followed by a much slower hydrolytic cleavage of the ligand into its components,viz. acetylacetone and ethylenediamine (protonated). For the dissociation of the complexes kobs=k1[H+]+k2[H+]2. The reactions have been studied in 10% (v/v) dioxan-water media and also ethanolwater media of varying ethanol content (10–25% v/v) and the results are in conformity with a solvent-assisted dissociativeinterchange mechanism involving the protonated complexes.

BASE HYDROLYSIS OF ISO-THIOCYANATOPENTAAMMINE COMPLEXES OF COBALT(III), CHROMIUM(III) AND RHODIUM(III) IN AQUEOUS-ORGANIC MIXED SOLVENTS

Abstract In the base hydrolysis of the title complexes in aqueous-organic solvent media (water-ethanol and water-acetone) to the corresponding hydroxo-complexes the observed rate law is in accord with the expression: The observed K values (3.5 to 26 M−1 at 25°) and the nature of their solvent dependence suggest the formation of an ion-pair in a pre-equilibrium involving the substrate complex and OH− ion, and not of a conjugate base. Rate constants (k) for the transformation of the ion-pair to the product increase with increased proportions of the organic component in the solvent. The nature of the dependence of k on different solvent parameters indicates significant associative character with OH− ion acting as the attacking nucleophile. Activation enthalpy, ΔH≠, decreases with ethanol concentration; this along with the observed large positive slopes of the log k vs 1/D plots rule out the possibility of H2O acting as the attacking nucleophile. ΔH≠ and ΔS≠ corresponding to k in different solvent composition...