A photoelectrochemical study of the anodic oxidation of lead in alkaline solution

The oxidation of lead in sodium tetraborate and in sulphuric acid solution has been studied. The analysis of photocurrent spectra has shown that tetragonal PbO is formed in both electrolytes. Evidence for photoemission from the metal substrate into the anodic oxide film is presented and discussed. The photocurrent behaviour is related to changes in electrode impedance which accompany the interconvertion of PbO and PbO₂.     

Voltammetric study of the oxidation kinetics of 2,4,6,-tribromoaniline in acid medium

The electrochemical oxidation of 2,4,6-tribromoaniline in aqueous sulphuric acid solutions, at different concentrations, and in aqueous 60% perchloric acid solution, at a platinum electrode, has been studied by rotating disc electrode and cyclic voltammetry. An irreversible first oxidation process, controlled by diffusion is observed, in perchloric acid and in sulphuric acid concentration <12.5 M. At higher sulphuric acid concentration, this process is kinetically controlled. The electroactive species is the protonated form Ar−N⁺H₃, which is bielectronically oxidized to [Ar−NH₃]³⁺ cation. Deprotonation of this cation can produce two different species, Ar−N₂₊ H₂ (I) and Ar⁺−N₊H₃ (II). In the latter case (II), the deprotonation occurs from the benzene ring. Hydrolysis of (I) gives 3,5-dibromo-1,2,4-trihydroxybenzene, which is further oxidized to 3,5-dibromo-2-hydroxy-2,5-cyclohexadien-1,4-dione. This charge-transfer equilibrium explains an additional redox pair observed for the first process in cyclic voltammograms. An irreversible second oxidation process corresponding to an initial bielectronic oxidation of (II), in both sulphuric acid at concentration >9.0 M and perchloric acid solutions, is observed. Generally, this second process is kinetically controlled. The rate-determining step of both processes is attributed to the initial charge transfer for the electrode reaction under diffusion control or to an adsorption process, depending on the medium used.     

Organocatalyzed anodic oxidation of aldehydes

A method for the catalytic formation of electroauxiliaries and subsequent anodic oxidation has been developed. The process interfaces N-heterocyclic carbene-based organocatalysis with electro-organic synthesis to achieve direct oxidation of catalytically generated electroactive intermediates. We demonstrate the applicability of this method as a one-pot conversion of aldehydes to esters for a broad range of aldehyde and alcohol substrates. Furthermore, the anodic oxidation reactions are very clean, producing only H(2) gas as a result of cathodic reduction.     

Air oxidation of in situ obtained carbanions in koh-dme-crownsystem

KOH(NaOH)-dimethoxyethane-18-crown-6-O₂ system has been found to be efficient for generation and oxidation of carbanions, obtained from monoarylmethanes.     

Voltammetric study of the oxidation of quercetin and catechin in the presence of cyanide ion

The reaction of electrochemically generated o-benzoquinones from oxidation of quercetin and catechin as Michael acceptors with cyanide ion as nucleophile has been studied using cyclic voltammetry. The reaction mechanism is believed to be EC; including oxidation of catechol moiety of these antioxidants followed by Michael addition of cyanide ion. The observed homogeneous rate constants (k _obs) for reactions were estimated by comparing the experimental voltammetric responses with the digitally simulated results based on the proposed mechanism. The effects of pH and nucleophile concentration on voltammetric behavior and the rate constants of chemical reactions were also described.     

Voltammetric oxidation of vinblastine and related compounds

The indole alkaloids, vinblastine, catharanthine, and vindoline, gave oxidation waves in mixed aqueous/organic media by cyclic, normal-pulse, and differential-pulse voltammetry (d.p.v). Of the systems investigated, d.p.v. at carbon paste anodes in pH 5.6 acetate buffer in ethanol/water (1:1) is recommended for general analytical work; calibration graphs were linear up to about 1 mM, with limits of detection of 4, 20, and 1.5 μM for vinblastine, catharanthine, and vindoline, respectively. Results obtained by cyclic voltammetry as well as by pulse techniques suggest that electron transfer is preceded by deprotonation and followed by additional chemical reactions. Products of the electrode processes form mechanically unstable films on the electrode surface.     

The application of reflectance spectroscopy to a study of the anodic oxidation of cuprous sulphide

A series of discrete copper sulphide phases have been identified during the anodic oxidation of Cu₂S to CuS. The four intermediate phases, viz. Cu_1.95–1.91S, Cu_1.86–1.80S, Cu_1.68–1.65S, and Cu_1.40–1.36S, have been characterized by X-ray diffraction and in situ measurement of reflectance spectra. Their free energies have been calculated from the electrode potentials and compared with other work.     

1,2-Shift in the anodic oxidation of aromatic compounds

The 1,2-shift observed during oxidation of organic substrates can arise by involvement of cation radicals.     

Voltammetric oxidation of anthraquinones in acetonitrile and nitromethane

The voltammetric oxidation of seven anthraquinones to the singly-charged radical cations has been investigated in acetonitrile and nitromethane. The voltammetric waves can be used for the quantitative determination of anthraquinones. Benzoquinones, naphthaquinones and phenanthraquinones are not oxidized in these solvents.     

Efficient N-p-methoxyphenyl amine deprotection through anodic oxidation

[reaction: see text] A new method of deprotection of N-p-methoxyphenylamines using anodic oxidation in acidic medium is presented. The process furnishes a high yield of amine and is compatible with several oxidable functional groups.     

First aromatic ring acetamidation by anodic oxidation

Anodic oxidation of 1-(trifluoromethyl)benzene in dry acetonitrile/Bu₄NBF₄ under constant potential conditions led to 2-(trifluoromethyl) acetanilide in 86% yield. Other experimental conditions, as the use of constant current or the change in the supporting electrolyte were considered.     

Anisotropy in the anodic oxidation of silicon in KOH solution

The electrochemical oxidation and passivation of Si(100) and Si(111) electrodes in KOH solution was studied by potentiodynamic and potential-step measurements. Striking differences were observed between the surfaces. A comparison of the results for n- and p-type electrodes led us to conclude that electrochemical oxidation of silicon in alkaline solution must be triggered by a chemical reaction. The strong influence of temperature on the current-potential and current-time results of (111) surfaces supports the importance of chemical activation. Photocurrent experiments on n-type (111) electrodes show that oxide nucleation is important for growth of the passive layer. A mechanism combining surface chemistry and electrochemistry is proposed to account for the pronounced anisotropy in anodic oxidation.     

The anodic oxidation of gold + palladium alloys

The anodic oxidation of Au+Pd alloys has been studied in solutions of 1 M H₂SO₄ and 0.1 M K₂SO₄ by voltammetric methods. A linear relationship between oxide reduction maximum and bulk alloy composition, often used to determine the surface composition of homogeneous alloys, could be shown to hold only for alloys up to 60 at% gold. At higher gold content the Au oxide peak must be additionally evaluated. With continuous cycling in acid solution the anodic dissolution of Pd, especially from gold-rich places, leads to a rather heterogeneous surface. The O--chemisorption is not governed by a transfer mechanism from Pd to Au surface atoms. The alloys are able to absorb the oxygen species generated in the positive potential region; however, this ability decreases with increase of the gold content.