Redox reactions of transient species formed during pulse radiolysis of 2-pyridine carboxaldehyde and 2-pyridine methanol in aqueous solutions

Rate constants for the reactions of e _aq ^? , H and OH radicals with 2-pyridine carboxaldehyde and 2-pyridine methanol have been determined by pulse radiolysis technique. Reactions of reducing radicals such as acetone ketyl radicals and CO₂ ^? with these compounds were also evaluated at various pHs. The species produced by the reaction of reducing radicals with these solutes was a strong reductant itself. While pyridinyl were produced in the case of 2-pyridine methanol, one-electron reduction of 2-pyridine carboxaldehyde led to the formation of PyCHOH radical. The one-electron reduction potential of PyCHOH radicals was estimated by establishing an equilibrium with MV⁺ radical cations to be ?0.6V vs NHE. OH radical reaction with 2-pyridine carboxaldehyde gave an OH adduct, while in the case of 2-pyridine methanol, OH radicals reacted partly by H-abstraction from the ?CH₂OH group. SO₄ ^? radical reaction with 2-pyridine carboxaldehyde produced a species which was reducing in nature. The rate constants for the reaction of e _aq ^? and OH radicals are compared with similar values obtained in the case of other 2-pyridine derivatives to see if there is any electron-inductive effect.     

Different channels of ·OH radical reaction with tryptophanol in aqueous solutions: A pulse radiolysis study

The pulse radiolysis technique has been employed to study the reaction of ·OH radical with tryptophanol (TPN). Reactions of specific one-electron oxidants like Br₂· - and N₃· and ·H atom were carried out to understand the contribution of different channels of · OH radical reaction with TPN. The studies were carried out in the pH range 3 to 10. One-electron oxidation of TPN (pH 3) produced radical cation absorbing at 570 nm. However, at higher pH, deprotonation of TPN cation radical takes place from N(1) position and indolyl radical absorbing at 520 nm with a p ^K a value of 3.6 is formed. Redox titration with TMPD, ABTS^2- and MV²⁺ was performed to determine the total yield of oxidizing and reducing radicals produced during ·OH reaction.     

Gutmann's Donor Numbers Correctly Assess the Effect of the Solvent on the Kinetics of SNAr Reactions in Ionic Liquids

We report an experimental study on the effect of solvents on the model S_NAr reaction between 1‐chloro‐2,4‐dinitrobenzene and morpholine in a series of pure ionic liquids (IL). A significant catalytic effect is observed with reference to the same reaction run in water, acetonitrile, and other conventional solvents. The series of IL considered include the anions, NTf₂^?, DCN^?, SCN^?, CF₃SO₃^?, PF₆^?, and FAP^? with the series of cations 1‐butyl‐3‐methyl‐imidazolium ([BMIM]⁺), 1‐ethyl‐3‐methyl‐imidazolium ([EMIM]⁺), 1‐butyl‐2,3‐dimethyl‐imidazolium ([BM₂IM]⁺), and 1‐butyl‐1‐methyl‐pyrrolidinium ([BMPyr]⁺). The observed solvent effects can be attributed to an “anion effect”. The anion effect appears related to the anion size (polarizability) and their hydrogen‐bonding (HB) abilities to the substrate. These results have been confirmed by performing a comparison of the rate constants with Gutmann's donicity numbers (DNs). The good correlation between rate constants and DN emphasizes the major role of charge transfer from the anion to the substrate.     

Green synthesis of silver nanoparticles using Eucalyptus comadulensis leaves extract and its immobilization on magnetic nanocomposite (GO-Fe3O4/PAA/Ag) as a recoverable catalyst for degradation of organic dyes in water

The magnetically recyclable graphene oxide-Fe₃O₄/polyallylamine (PAA)/Ag nanocatalyst was prepared via a green route using Eucalyptus comadulensis leaves extract as both reducing and stabilizing agent. The catalytic activity of this nanocatalyst was investigated for the reduction reaction of methylene blue and methyl orange in the presence of NaBH₄ in aqueous medium at room temperature. The prepared nanocatalyst was characterized by different methods such as Fourier transformed infrared spectroscopy, X-ray diffraction, scanning electron microscopy–energy dispersive X–ray spectroscopy, thermogravimetric analysis, vibrating sample magnetometer, transmission electron microscopy, and UV–visible spectroscopy. The results show that graphene oxide/PAA/Ag nanocatalyst has good activity and recyclability, and can be reused several times without major loss of activity in the reduction process. The apparent rate constants of the methyl orange (MO) and methylene blue (MB) were calculated to be 0.077 s⁻¹ (3 mg of catalyst) and 0.15 s⁻¹ (2 mg of catalyst), respectively.     

OH radical reactions with ethanolamines: formation of reducing as well as oxidizing radicals

The reaction of OH radicals with ethanolamine, diethanolamine and triethanolamine was studied at pH values below and above the pK_a values of these compounds. The rate constants were found to be lower for the protonated amines than those for their neutral forms. The OH radical reaction led to the formation of both oxidizing as well as reducing species, as observed by their reactions with methyl viologen and ascorbic acid. The oxidizing species formed by OH radical reaction at the amine site was not found to react with the parent molecules and thereby no secondary yield of reducing species was obtained as in the case of glycine (except in the case of triethanolamine at pH 9.2).     

Kinetics and Mechanism Study of Chemical Treatment of Methylene Green by Urea

The kinetics and mechanism studies, for the reduction of methylene green (MG) by urea, in acidic and alkali media, were studied at λ_max=652.8 nm by monitoring the depletion in MG concentration. The reaction was carried out by UV radiation, with variable dye concentration, reducing agent (urea), acid and base under different additive ions that are very common in dye waste water. The reduction followed pseudo first‐order kinetics with respect to different anions, cations, dye, reductant and OH^? ion concentrations. It was found that most of the cations tested showed the inhibitory effect on dye decoloration, due to the formation of insoluble precipitate and followed the order K⁺>Na⁺>Al³⁺>Ca²⁺≈Mg²⁺. Tested anions showed that the dye decoloration was significantly accelerated and followed the order Cl^?>Br^?>I^?>NO^?₃>SO^2?₄. A mechanistic model involving generation of a complex of dye with ions was proposed.     

The elementary stages of electroreduction of alkyl(hetero)arylsulfones in water-organic media

Intermediates (IM) of methyl(2-pyridyl)sulfone (MPS) and tert-butyl(2-pyridyl)sulfone (TBPS) formed upon the transfer of the first electron are studied by methods of laser photoelectron emission (LPE). The capture constants of solvated electrons by the MPS and TBPS molecules were determined. The time-resolved voltammograms of each sulfone measured in water-organic mixtures (20–80% ethanol or 60% DMSO) are found to demonstrate a redox wave with half-wave potentials E _1/2 = −1.34 and −1.37 V for MPS and TBPS, respectively. The dependence of rate constants for the one-electron IM reduction and oxidation on the potential is shown to obey the slow-discharge equation and the absolute magnitudes of rate constants are determined. The characteristic times of homogeneous transformations (decomposition, protonation) of MPS and TBPS radical anions do not exceed 3 × 10⁻⁷ s. The LPE data are compared with the results of preparative electrolysis and the mechanisms of both electrochemical and homogeneous reactions of IM are discussed.     

Kinetics and stereoregulation in the isotactic-specific and living polymerization of methyl methacrylate

Kinetic studies on the isotactic-specific living polymerization of methyl methacrylate with t-C₄H₉MgBr were carried out in toluene at −78°C and the kinetic orders with respect to the monomer and initiator were found to be unity. Propagation rate in the early stage of polymerization and the initiation rate were studied by kinetic measurement and analysis of oligomer distribution. The rate constants for the propagation of the unimer (k₁), dimer (k₂), and trimer anions (k₃) were found to be in the following order; k₁ >> k₂ > k₃. ¹³C NMR spectra of the oligomer anions indicated the coordination of penultimate and antepenultimate ester groups to Mg counterion. Possible mechanism for stereoregulation in the early stage of polymerization was discussed.     

The kinetics study of the reduction of Fast Green dye with cetylpyridinum chloride as cationic surfactant

The kinetics of the reaction of Fast Green dye (FG) with cetylpyridinum chloride was studied in alkaline medium by UV-Visible spectrophotometer. Reduction of Fast Green dye was carried out by varying the fast green dye concentration, cetylpyridinum chloride concentration and concentration of sodium hydroxide. In the present study the reduction of dye was carried out in order to reduce the color content. The interaction of dye was carried out with reducing analyte (cetylpyridinum chloride). The rate of the reaction was determined by varying the above parameters at different temperatures. It was observed that the reduction followed pseudo first-order kinetics with respect to dye, surfactant, OH⁻ ion concentration according to the following reaction pathway. The mechanism for the photo bleaching of the dye has been proposed and well confirmed by the data simulation procedure. The activation parameters of the reaction like entropy of activation (ΔS) and free energy of activation (ΔG) showed the extremely solvated states of transient complex which was less disorderly arranged than the oxidized form of dye, whereas E _a values reflects a high amount of energy required for the reduction of dye with cetylpyridinum chloride.     

The Preparation of New C(α)-Dianions

For more than a decade, Hauser et. al. carried out rather extensive investigations on the preparation and reactions of the multiple anions of β -diketones and related materials. Once the conditions for the preparation of a particular multiple anion seemed established, three reactions were usually attempted with the new multiple anion: alkylations with reagents such as benzyl chloride², Aldol-type condensations with aldehydes or ketones³ such as benzaldehyde and benzophenone, and Claisen-type condensations with esters such as methyl benzoate⁴. Other electrophilic reagents, such as nitriles⁵ and isocyanates⁶, were also condensed with certain of these multiple anions in selective situations; however, the alkylations, acylations, and carbonyl additions received the greatest amount of attention⁷. In some cases only one of the three types of reactions (Aldol) worked well⁸; however this did not mean that conditions could not be found for the other two reaction types. In addition, the synthetic potential of these multiple anion reactions was quite extensive (Scheme 1).     

Reactivity of O2 − radical anions on hydrated ZrO2 surface

The concentration of O₂ ^? radical anions generated on the surface of hydrated ZrO₂ in an H₂O₂ solution was found to depend on H₂O₂ concentration. It was shown that this method can be used for detecting H₂O₂ in solutions at concentration as low as 0.01 wt%. The radical anions were found to react with organic molecules, even at room temperature. The decomposition kinetics of O₂ ^? radical anions was double-exponential with two reaction rate constants. The existence of two distinct rate constants suggests that two types of O₂ ^? radical anions with similar spectroscopic properties but different reactivity are present on the surface of hydrated ZrO₂. It is highly likely that different arrangements of hydroxyl groups near the radical anions account for the presence of the two types of O₂ ^? with different reactivity. The rate constants obtained in the presence of the organic compounds studied were found to conform with the expected order of reactivity: toluene > benzene ? hexane.     

Admittance measurements of kinetic and adsorption parameters of anodic mercury complex formation with macrobicyclic (222) ligand in propylenecarbonate and dimethylformamide

Admittance measurements were applied to investigation of the charge-transfer rate and mechanism of anodic complex formation between mercury and macrobicyclic ligand (222) as well as to the cathodic reduction of Hg²⁺-(222) complex formed in the bulk. From measurements in PC and DMF using adsorbable and non-adsorbable base electrolyte anions it was shown that the reactant adsorption effects are observable only if adsorption of ClO₄^? and (222) takes place at the same time. Corresponding charge-transfer rates were evaluated and potential dependence of the adsorption capacity for two ligand concentrations was given. At the half-wave potential apparent rate constants k_1/2 listed below were found (data from Fig. 9).     

Probing TCNQ‐mediated Metal Reduction Reactions at Liquid‐liquid Interface with SECM

Metal reduction at the interface between two immiscible electrolyte solutions (ITIES) has been studied with scanning electrochemical microscopy (SECM). Metal cations in the aqueous phase are reduced by 7,7,8,8‐tetracyanoquinodimethane anion (TCNQ^?) residing in the oil phase, methyl isobutyl ketone (MIBK). TCNQ^? is formed at the SECM tip by reducing TCNQ, which results in a positive feedback loop between the tip and the ITIES when an electron is donated to a metal cation. The effect of the Galvani potential difference on the rate of the interfacial electron transfer was investigated, establishing the potential difference either by an additional substrate electrode in the aqueous phase or by an a common ion in both phases. It is shown that the Galvani potential difference as a driving force does enable TCNQ^? mediated Cu²⁺ reduction. Finite element method (FEM) simulations were run to provide information on the reaction kinetics and stoichiometry.     

Application of the pulse radiolysis technique to reactions of organic radical ions

The nanosecond pulse radiolysis technique has been applied to study the rate constants for charge transfer and substitution reactions of radical ions. Electron transfer from biphenyl anion to styrene derivatives shows a correlation with the reduction potential of the acceptors expected from the Marcus theory. The positive charge transfer from biphenyl cation to the same acceptors shows a much larger rate constant, suggesting a considerable shift of the free energy relationship to the positive side of G^o. The substitution reaction of fluorenone anion with organic halides shows an S_N2 character, while that of diethyl fumarate shows electron transfer nature. The dimerization of radical anions has been proven for benzophenone and fluorenone, when their lifetime of the parent anions are prolonged by countercations.     

Theoretical and Experimental Investigation of Functionalized Cyanopyridines Yield an Anolyte with an Extremely Low Reduction Potential for Nonaqueous Redox Flow Batteries

Cyanopyridines and cyanophenylpyridines were investigated as anolytes for nonaqueous redox flow batteries (RFBs). The three isomers of cyanopyridine are reduced at potentials of −2.2 V or lower vs. ferrocene^+/0 (Fc^+/0), but the 3-CNPy⋅⁻ radical anion forms a sigma-dimer that is re-oxidized at E≈−1.1 V, which would lead to poor voltaic efficiency in a RFB. Bulk electrochemical charge-discharge cycling of the cyanopyridines in acetonitrile and 0.50 M [NBu₄][PF₆] shows that 2-CNPy and 4-CNPy lose capacity quickly under these conditions, due to irreversible chemical reaction/decomposition of the radical anions. Density-functional theory (DFT) calculations indicated that adding a phenyl group to the cyanopyridines would, for some isomers, limit dimerization and improve the stability of the radical anions, while shifting their E_1/2 only about +0.10 V relative to the parent cyanopyridines. Among the cyanophenylpyridines, 3-CN-6-PhPy and 3-CN-4-PhPy are the most promising as anolytes. They exhibit reversible reductions at E_1/2=−2.19 and −2.22 V vs. ferrocene^+/0, respectively, and retain about half of their capacity after 30 bulk charge-discharge cycles. An improved version of 3-CN-6-PhPy with three methyl groups (3-cyano-4-methyl-6-(3,5-dimethylphenyl)pyridine) has an extremely low reduction potential of −2.50 V vs. Fc^+/0 (the lowest reported for a nonaqueous RFB anolyte) and loses only 0.21 % of capacity per cycle during charge-discharge cycling in acetonitrile.     

Myoglobin Reconstituted with Ni Tetradehydrocorrin as a Methane‐Generating Model of Methyl‐coenzyme M Reductase

Myoglobin reconstituted with Ni tetradehydrocorrin was investigated as a model of F430‐containing methyl‐coenzyme M reductase, which catalyzes anaerobic methane generation. The Ni^II tetradehydrocorrin complex has a Ni^II/Ni^I redox potential of ?0.34 V vs. SHE and EPR spectroscopy indicates the formation of a Ni^I species upon reduction by dithionite. This redox potential is approximately 0.31 V more positive than that of F430. The Ni^I tetradehydrocorrin moiety is bound to the apo‐form of myoglobin to yield the reconstituted protein. Methane gas is generated in the reaction of the model with methyl iodide in the presence of the reconstituted protein under reductive conditions, whereas the Ni^I complex itself does not produce methane gas. This is the first example of a protein‐based functional model of F430‐containing methyl‐coenzyme M reductase.     

Single‐Electron Transfer in σ‐Complexation Reactions of 2,6‐Dimethoxy‐3,5‐dinitropyridine with Para‐X‐phenoxide Anions in Aqueous Solution

Second‐order rate constants have been measured spectrophotometrically for reactions of 2,6‐dimethoxy‐3,5‐dinitropyridine 1 with 4‐X‐substituted phenoxide anions (X = OMe, Me, H, Cl, and CN) 2a–e in aqueous solution at various temperatures. The effect of phenoxide substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to nonlinear correlations of Δ^≠H and Δ^≠S with Hammett's substituent constants (σ). Each Hammett plots exhibits two intersecting straight lines for the reactions of 1 with the phenoxide anions 2a–e , whereas the Yukawa–Tsuno plots for the same reactions are linear. The large negative ρ values (?4.03 to ?3.80) obtained for the reactions of 1 with the phenoxide anions possessing an electron‐donating group supports the proposal that the reactions proceed through a single‐electron transfer mechanism.     

Electrode kinetics of the metal complexes with aminopolycarboxylic acids: Part II. Cadmium-ethylenediamine-N,N,N′,N′-tetraacetate (EDTA) complexes

The d.c. polarographic current-potential curves of Cd(II)-EDTA complexes were examined in the pH range 0.5–10.0, to elucidate the mechanism of their electrode processes and to determine the relevant electrochemical kinetic parameters. It was shown that the first wave observed below pH 3 at ?0.58 to ?0.65 V vs. SCE is the reversible reduction wave of Cd(II) aquo-ion with kinetically-controlled limiting current, and the second wave observed above pH 1.5 at ?0.75 to ?1.21 V vs. SCE corresponds to the simultaneous irreversible reduction of four complex species, CdH₃L⁺, CdH₂L, CdHL^? and CdL^2?, where CdH_pL^(p?2)+ and L^4? denote the protonated complex species with p protons and the unprotonated EDTA ion, respectively. Analysis of the dependence of limiting current on the hydrogen ion concentration led to the conclusion that the preceding reaction determining the behaviour of limiting current is CdH₃L⁺?Cd²⁺+H₃L^? with k₃^d=6.3×10² s^?1 and k₃^f=3.3×10⁶ s^?1M^?1, where k₃^d and k₃^f are the dissociation and formation rate constants, respectively. On the other hand, from analysis of the dependence of half-wave potentials of the second wave on the hydrogen ion concentration, the kinetic parameters of the four complex species were evaluated, and are given in Table 1. Further, it was shown that the cathodic rate constants of these four charge transfer processes at some reference potential together with those of Cd(II)-HEDTA complexes fulfil the linear free energy relationship.     

One-electron redox reactions of trifluoperazine in aqueous solutions

Absolute rate constants have been measured for the reactions of the primary and specific one-electron oxidant radicals with the protonated form of trifluoperazine (TFP). The primary radicals, e^- _aq and OH·, react with TFP at diffusion controlled rates. The transients thus produced have been characterized. Halogenated aliphatic peroxyl radicals oxidize TFP with rate constants between 10⁷ and 10⁸ dm³ mol^-1 s^-1, depending on the structure of the peroxyl radical. The reactivity of peroxyl radicals has been found to vary with Taft's inductive parameter. Oxidation of TFP at acidic pH has been studied using stopped-flow technique. The reaction between TFP radical cation and ascorbic acid has also been examined using pulse radiolysis technique. The results indicate that TFP radical cation is repaired by ascorbate. One-electron reduction potential of TFP · + /TFP at pH 3.5 has been calculated to be 0.964 V vs. NHE.