Electrochemical reduction of substituted pyrimidines in acetonitrile

Polarographic studies of several substituted pyrimidines were reinforced by the results from cyclic voltammetry, controlled-potential electrolysis, and spectrophotometric examination of electrolyzed solutions, as well as by the examination of model compounds. Pyrimidines substituted with non-reducible groups (amino, methyl) are reduced in a single, one-electron (1e), diffusion-controlled process, very similar to that for pyrimidine itself. Pyrimidine-4-carboxylic acid exhibits three reduction waves: a very drawn-out acid-reduction wave with unusual properties and at more negative potential, an adsorption prewave and a wave corresponding to the 1e reduction of the pyrimidine moiety, 2-Chloro-and 2-bromopyrimidine each exhibit two polarographic waves; the first, corresponding to irreversible scission of the carbon-halogen bond, has electrochemical properties quite different from those normally expected; the second is due to reduction of the electro-generated pyrimidine. Results are compared with those for the reduction of bromobutane, bromobenene, and 2-bromopyridine.     

Electrochemical reduction of ytterbium in perchloric media

The electrochemical behavior of Yb³⁺ in perchloric media was studied by cyclic voltammetry and current reversal chronopotentiometry at several temperatures. The results show that the reversible electrochemical reduction of Yb³⁺ is followed by homogeneous reactions. The experimentally determined diffusion coefficients of ytterbic ion are 0.41×10^?5, 0.48×10^?5 and 0.53×10^?5 cm² s^?1 at 4.2, 9.2 and 14.7°C, respectively, in 0.5 M NaClO₄ solutions. From these data a value of 16 kJ mol^?1 (3830 cal mol^?1) was obtained for the activation energy for diffusion of ytterbic ion. From the activation energy the diffusion coefficient of ytterbic ion at 25.0°C was estimated. The value of 0.67×10^?5 cm² s^?1 was obtained. In all the experiments the initial pH was maintained at 4.1.     

Electrochemical behavior of phthaloyl peroxide in aqueous media

The electrochemical behavior of phthaloyl peroxide C₈H₄O₄ on an Au disk electrode in a 0.05 M aqueous solution of Na₂SO₄ was studied by cyclic voltammetry (CVA). It demonstrated a high activity in cathodic reduction with the formation of an irreversible peak on the CVA curve at E =–0.81 V. Additionally, during the anodic oxidation of C₈H₄O₄, the surface of the Au electrode became passivated by compounds which prevented its oxidation during the registration of repeated cycles. Apparently, these compounds are surface complexes of phthaloyl peroxide with a gold cation.     

Electrochemical reduction of actinide ions in aqueous solution

Electrochemical reduction of heavy elements from aqueous solution to amalgams was studied by radiopolarography and radiocoulometry. Mechanism of actinide reduction on a mercury pool is discussed through simulation techniques. Special emphasis is placed on redox reactions and potentials, kinetics of the process and effect of acetate and citrate ions as complexing agents. Three groups of actinides have been found. The first group represents actinium and from uranium to berkelium. Reduction occurs in the experimental conditions via an irreversible 3–0 process. The second group consists of the elements from fermium to nobelium, which are reduced in non-complexing solutions, or with acetate ions, similarly as barium and radium, via a reversible 2–0 reaction. Finally, californium and einsteinium behave as intermediate elements. It is noticeable that such groups are also observed in the actinide series by studying the structure of the trivalent aqua ions. On the basis of the above mentioned investigations of actinides and lanthanides several examples of electrochemical application are presented. Californium has been separated from preceding transuranium and lanthanide elements (except europium) by electrochemical reduction to amalgams in acetic solution. Separation factors from 25–90 are achieved with appropriate cathodic potentials. Similarly, this element could be separated from several heavier actinides with citric media. The electrochemical preparation of mixed uranium-nickel and uranium-tin amalgams from aqueous acetate solutions is investigated. The dependence of redox potentials of mixed amalgams on different atomic ratio UNi and USn in amalgams is measured. The large shift of redox potentials of mixed amalgams to the positive direction is detected when the atomic ratio UNi or USn in amalgams reaches 15. The thermal distillation of mercury from mixed amalgams with different UNi and USn atomic ratios was carried out and the products were identified by chemical analysis and X-ray diffraction. The intermetallics UNi₅ and USn₃ were prepared from mixed amalgams with the atomic ratios UNi=15 and USn=13. The uranium and neptunium amalgams are prepared by electrolysis of aqueous acetate solutions and are processes into metals or nitrides U₂N₃, NpN by thermal distillation of mercury in vacuum or in nitrogen atmosphere.     

Synthesis of highly substituted pyrroles using ultrasound in aqueous media

Abstract

A two-step protocol for the synthesis of highly substituted pyrroles in aqueous media and without catalyst is described. The first step is the dimerization of a 1,3-dicarbonyl compound by ceric ammonium nitrate/ultrasound to produce a tetracarbonyl derivative. This derivative is then combined with an amine in the absence of any catalysts to obtain the pyrrole via a Paal–Knorr reaction. This route is an improvement when compared with classical methodologies toward Green Chemistry objectives.

Asymmetric reduction of prochiral inclusion complex in aqueous media

Abstract

In the formation of an inclusion compound, the interaction of both ionic and hydrogen bonds were shown to be involved in the asymmetric reduction of a keto acid in the presence of 6-deoxy-6-amino-β-cyclodextrin with NaBH₄ in an aqueous buffer solution. Furthermore, multiple interactions were elucidated and the enatioface differentiating reduction was induced in the concerted interactions. Asymmetric reduction of a keto acid in the presence of newly prepared alkylamino-substituted β-cyclodextrin (CD) gave a higher optical yield. There should be a steric effect by a substituent group on the CD to form the conformation favoured by the multi-interaction between host and guest A mechanistic scheme for the enatioface differentiating attack of a reducing reagent on the included substrate is suggested.     

Electrochemical formation and reduction of silver oxides in alkaline media

The anodic oxidation of silver electrodes in NaOH solution and the reduction of the silver oxides formed were studied by potential step chronoamperometry. Oxidation of Ag to Ag₂O is a diffusion-controlled reaction, the diffusion control being established in the solid phase. Oxidation of Ag₂O to AgO proceeds via a nucleation and growth-controlled process. The amount of AgO decreased with increasing step height. The current—time curves for this reaction have been analysed with the Kolmogoroff—Avrami equation. Reduction of AgO to Ag₂O occurs initially on the outside of the electrode, and the rate of the reaction is limited by diffusion of ions across the thickening layer of Ag₂O. Reduction of Ag₂O to Ag proceeds via a nucleation and growth reaction.     

Electrochemical derivatization of carbon surface by reduction of in situ generated diazonium cations

The derivatization of a glassy carbon electrode surface was achieved by electrochemical reduction of several in situ generated diazonium cations. The diazonium cations were synthesized in the electrochemical cell by reaction of the corresponding amines with NaNO2 in aqueous HCl. The versatility of the method was demonstrated by using six diazonium cations. This deposition method, which involves simple reagents and does not require the isolation and purification of the diazonium salt, enabled the grafting of covalently bounded layers which exhibited properties very similar to those of layers obtained by the classical derivatization method involving isolated diazonium salt dissolved in acetonitrile or aqueous acid solution. Cyclic voltammetry and electrochemical impedance spectroscopy carried out in aqueous solutions containing electroactive redox probe molecules such as Fe(CN)6(3-/4-) and Ru(NH3)6(3+) confirmed the barrier properties of the deposited layers. The chemical composition of the grafted layers was determined by X-ray photoelectron spectroscopy and surface coverage in the range 3 x 10(-10) to 6 x 10(-10) mol cm(-2) was estimated for films grown in our experimental conditions.     

pH effects on the reduction of bilirubin in aqueous media

The electrochemical reduction of bilirubin at the mercury electrode was studied in aqueous media embracing an extended pH range; a variety of electrochemical techniques was used. Reduction in neutral solution results in three waves—the first a reversible adsorption prewave, followed by a pH dependent, reversible, two electron wave and finally by a one electron irreversible wave. In basic solution, only two waves are discernable: the reversible adsorption prewave, followed by a three electron wave resulting from the coalescence of the second and third waves seen in neutral solution. Familiarity with the detailed electrochemistry of bilirubin reduction provides the prerequisite basis for developing a sensitive analytical method for its determination.     

Computational electrochemistry: aqueous two-electron reduction potentials for substituted quinones

The electrode potentials of some quinone derivatives in aqueous solution have been calculated. The calculations are carried out at the Hartree–Fock and B3LYP levels with the inclusion of entropic and thermochemical corrections to yield free energies of redox reactions. The Polarisable Continuum Model (PCM) is used to describe the solvent. The average error of calculation of electrode potentials is less than 0.03 V and is decreased compared to the average error of methods presented previously. The role of relaxation energies and frequency calculations in improving the results has been investigated.     

Synthesis of substituted furans by platinum-catalyzed cyclization of propargylic oxiranes in aqueous media

The reaction of propargylic oxiranes with platinum catalyst in aqueous media is described. Furans having a variety of substituents were conveniently synthesized with high efficiency.     

Electrochemical reductive breakdown of solid-phase chlorinated rubber in aprotic and aqueous media

A study has been made of the electrochemical reductive breakdown (ECRB) of solid-phase chlorinated butyl rubber (CBR). It has been established that CBR does not accept electrons under conditions of fast sweep of electrode potential; however, with prolonged cathodic polarization of the substrate at the polymer/cathode/solution three-phase boundary, it undergoes reduction and subsequent chemical conversion, including intermolecular cross-linking and breakdown of the macromolecules. The rate of ECRB of the CBR depends on the potential and materials of the cathode, and also on the nature of the solvent. In a aprotic medium, the process of CBR breakdown leads to the formation of long polyconjugated structures that color its surface black in the vicinity of the three-phase boundary. In an aqueous medium, no such color is observed, but the products of ECRB are found to contain conjugated double bonds with a shorter conjugation chain than in the case of the aprotic medium. It has been established that he starting reaction in ECRB of CBR is reduction of the C-Cl bond and subsequent elimination of Cl^–.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 6, pp. 710–715, November–December, 1989.     

Electrochemical behaviors and determination of melamine in neutral and acid aqueous media

The electrochemical behaviors of melamine (MEL) were studied at paraffin-impregnated graphite electrode in PBS (pH 7.0) and 0.5 M H₂SO₄. Various methods including UV–vis thin-layer spectroelectrochemistry, infrared spectra (IR) and electrochemicatry have been performed to investigate the characteristics. In 0.1 M PBS (pH 7.0), MEL loses two electrons to form a dication, which couples head-to-head with a neutral molecule of MEL to form a dimer accompanying the production of azocompound, the dimer plays a role of a monomer in the following polymerization. In 0.5 M H₂SO₄, unstable MEL mostly hydrolyzes to form ammeline, ammelide, s-triazine-2,4,6-trion, and tricyanic acid, respectively; The hydrolysis could be accelerated by electrochemical method; Meanwhile, MEL associates tricyanic acid to give a plane molecule cake by hydrogen bonding. The spectra responses of MEL at 205 and 234 nm are linearly increasing in a same concentration range of 1.0 × 10⁻⁷–1.0 × 10⁻⁵ M in 0.5 M H₂SO₄ (determination limit, 1 × 10⁻⁸ and 3 × 10⁻⁸ (3σ)). The proposed method was successfully applied to the determination of MEL in real sample.

     

Electrochemical behaviors and determination of melamine in neutral and acid aqueous media

The electrochemical behaviors of melamine (MEL) were studied at paraffin-impregnated graphite electrode in PBS (pH?7.0) and 0.5?M H₂SO₄. Various methods including UV?Cvis thin-layer spectroelectrochemistry, infrared spectra (IR) and electrochemicatry have been performed to investigate the characteristics. In 0.1?M PBS (pH?7.0), MEL loses two electrons to form a dication, which couples head-to-head with a neutral molecule of MEL to form a dimer accompanying the production of azocompound, the dimer plays a role of a monomer in the following polymerization. In 0.5?M H₂SO₄, unstable MEL mostly hydrolyzes to form ammeline, ammelide, s-triazine-2,4,6-trion, and tricyanic acid, respectively; The hydrolysis could be accelerated by electrochemical method; Meanwhile, MEL associates tricyanic acid to give a plane molecule cake by hydrogen bonding. The spectra responses of MEL at 205 and 234?nm are linearly increasing in a same concentration range of 1.0?×?10^?7?C1.0?×?10^?5?M in 0.5?M H₂SO₄ (determination limit, 1?×?10^?8 and 3?×?10^?8 (3??)). The proposed method was successfully applied to the determination of MEL in real sample.     

Electrochemical oxygen reduction on nitrogen doped graphene sheets in acid media

Nitrogen doped graphene were prepared via exfoliated graphite oxide. This graphene exhibited significantly high oxygen reduction activity. High electric conductivity, high surface area, large amount of edge sites and pyridinic N site in rGS (reduced graphene sheets) contribute to the high ORR (oxygen reduction reaction) activity. The rGS showed a potential to replace expensive Pt for oxygen reduction reaction in PEMFC.     

Electrochemical behaviour of oxygen reduction on polymer carbon electrodes in alkaline media

The preparation of polymer carbon electrocatalysts by the controlled pyrolysis of polyfurfuryl alcohol polymer is described. The potentiostatic method was used to study the electrochemical behaviour of the oxygen reduction reaction on the prepared catalyst electrodes in potassium hydroxide electrolyte. A pure polymer carbon electrode and a cobalt chloride doped polymer carbon electrode were shown to be active in oxygen reduction, but the electrode containing cobalt chloride seemed the most active. The main reaction product at the pure polymer electrode is hydrogen peroxide, involving two electrons, whereas at a poly(CoCl₂) electrode the reduction process reaches partly its ultimate state, and involves at most three electrons.