Anodic film formation on osmium electrodes: Part II. Galvanostatic and potentiostatic measurements

The formation and reduction of anodic films on Os electrodes in 2 M HCl and HClO₄ solutions were studied by anodic and cathodic charging curves. The galvanostatic oxidation of Os in HClO₄ shows the formation of OsO₂ as an intermediate step to OsO₄ that goes in the solution. The cathodic charging curves at Os electrodes previously oxidized at constant potential reveal the anodic film to be made up of a reversibly desorbed oxygen layer and an oxide phase reduced irreversibly. Both layers increase with time under potentiostatic conditions following a logarithmic equation until a constant value is reached. At all times, the content of OsO₂ in the anodic film at high potentials is larger than that of chemisorbed oxygen.In HCl solutions only the reversible reduction of an oxygen layer is observed. The growth of this film also complies with a direct logarithmic law before attaining a limiting coverage. The charge involved in the reduction increases linearly with the potential at a given time of formation. The results are discussed in terms of a Temkin-type isotherm and a place-exchange mechanism.     

Precipitation and characterization of uniform submicrometer lead titanate particles. Part II: Reaction mechanism

The chemical mechanism of the precipitation of lead titanium peroxohydroxide particles in a solution of nitrilotriacetate (NTA) complexes of lead and titanium peroxo-hydroxide is envisioned as the interaction between cations of lead hydroxide and anions of polymeric titanium peroxo-hydroxide.     

Oxygen evolution: the mechanism of formation of porous anodic alumina

Abstract  Aluminium anodization behavior in ammonium sebacate solution (w = 4%) in ethylene glycol, and in several H₃PO₄-containing electrolytes, has been investigated. A new mechanism is proposed for the formation of porous anodic films. The model emphasizes the close relationship between pore generation and oxygen evolution. PO₄ ³⁻ ions incorporated in the anodic films behave as the primary source of avalanche electrons. It is the avalanche electronic current through the barrier film that causes oxygen evolution during anodization. When growth of anodic oxide and oxygen evolution occur simultaneously at the aluminium anode, cavities or pores are formed in the resulting films. Accordingly, the mechanisms of growth of barrier and porous films are not very different in nature. These findings are a decisive new step towards full understanding of the nature of anodic alumina films. Graphical abstract        

Reaction mechanism for formation of condensed thienopyrans and thienothiopyrans

The formation of 2-amino-3-cyanothiophenes has been studied. It has been shown that the intermediates in the synthesis are the Knoevenagel type ,-unsaturated nitriles.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 395–397, March, 1987.     

Anodic oxidations and polarity: exploring the chemistry of olefinic radical cations

The cyclization chemistry of radical cations derived from electron-rich olefins has been examined and the relationship between the polarization of the radical cation and the chemoselectivity of the reaction probed. It was found that more polarized radical cations favor carbon-carbon bond formation while less polarized radical cations favor carbon-heteroatom bond formation. A new approach to the synthesis of quaternary carbons was uncovered and the compatibility of ene diol ethers with anodic olefin coupling reactions examined.     

The reactions of diphenylcarbazide and diphenylcarbazone with cations : Part II. Extraction behaviour and spectra of the reagents

Extraction experiments in the carbon tetrachloride-water system for diphenylcarbazone above pH 7.5 give a value of (2.9 ± 0.3) ·10^-9 for the dissociation constant. At lower pH values the partition coefficients were found to be 7.5 for carbon tetrachloride-water and 39 for toluene-water in 0.1–1 M NaClO₄. The solubility of the carbazone in water is 3.8·IO^-4M in acid perchlorate medium.Spectra of diphenylcarbazone in water, toluene and carbon tetrachloride, of the anion in water and of diphenylformazane in toluene are compared. In the infra-red spectrum the C=O band was present in the carbazone, but not in the sodium salt.     

Reaction of hexamethylditin with dimethyltin dichloride : II. Rate studies and mechanism

Rate data for the concurrent reaction of hexamethylditin with dimethyltin dichloride and with two of the products of this reaction in methanol solution at 30°C have been analysed in terms of individual steps. Line-broadening phenomena in the system have been studied, providing evidence of the intermediacy of dimethyltin and its insertion into tin—chlorine bonds. A general mechanism for this and related reactions is presented.     

Electrogenerated chemiluminescence in mechanistic investigations of electroorganic reactions: Part II. Anodic dehydrogenation of 1,4-dihydropyridines

The anodic oxidation of fifteen 1,4-dihydropyridines in acetonitrile was investigated by electrogenerated chemiluminescence measurements at a rotating platinum disc electrode in the presence of some luminescent compounds D. The emission observed originates from the homogeneous electron transfer between the cation radicals of D and the free pyridiniumyl radicals formed in the dihydropyridine oxidation. It follows from the luminescence-potential curves that, in addition to the pyridiniumyl radicals, the dihydropyrindine cation radicals are also involved in the dehydrogenation process. Therefore, from the different oxidation pathways of dihydropyridines described in the literature an ECE mechanism is preferred in acetonitrile. The substituent effect on the oxidation reaction and on the anodic luminescence is discussed.     

Quantum mechanical investigation of effects in formation of carbonyl and thiocarbonyl cations. Part I. Fragmentation reactions of 1,2-hemithiodione radical cations

In order to study decomposition reactions of ionic oxygen and sulphur-containing compounds, such as hemithiodione radical cations, a quantum chemical investigation of the formation of formyl, thioformyl, acyl and thioacyl cations and radicals was performed. Calculations were carried out mainly at the 6–31G^* level involving complete geometry optimizations. In the ionization of aldehydes and thioaldehydes, no important energy differences were found between the oxygen and sulphur analogues studied. A stepwise generation of formyl and thioformyl cations from formaldehyde and thioformaldehyde, by hydrogen atom abstraction followed by expulsion of unpaired electrons from the resulting radicals, showed the radicalization of formaldehyde to be only 12.6 kJ mol^?1 more favoured than that of thioformaldehyde. The electron expulsion from formyl radical was 23.8 kJ mol^?1 more favoured than that from thioformyl radical. Substitution of hydrogens of formyl and thioformyl groups by methyls lowered the total formation energies of carbonyl and thiocarbonyl cations 119.2 and 96.2 kJ mol^?1. The formation energy difference between acyl and thioacyl cations was also very small.     

Chemistry of 1,2,4-trioxanes relevant to their mechanism of action. Part 1: Reaction with Fe(II) salts

Reactions of 6-(1′-phenylvinyl)-1,2,4-trioxanes 2-5, with FeCl₂·4H₂O, FeBr₂, and a combination of hemin (bovine) and reduced glutathione (GSH) under various conditions have been studied.     

Ellipsometric study of dissolution of anodic WO3 films in aqueous solutions. 2. Reaction mechanism

The present work demonstrates that the dissolution of anodic WO3 can be described in terms of general mechanisms already used to account for the physicochemical processes involved in the dissolution of oxides of other metals. The chemical dissolution of anodic WO3 films is characterized by in situ ellipsometry, which has proven to be a suitable technique for studying the processes taking place.(1) The capability of ellipsometry for distinguishing between thickness decrease and hydration and generation of porosity/roughness helps to analyze the thickness range within which reliable kinetic information can be obtained. The dissolution rate law found for anodic WO3 does not depend on the oxide thickness. The dissolution of anodic WO3 films includes hydration and roughening, besides thickness decrease, and is kinetically controlled by the OH- concentration at the oxide/electrolyte interface. Apart from OH-, the specific nature of electrolyte anions seems to play no kinetic role, except in promoting porosity/roughness.     

2,2′-dihalo-bis(perfluorocycloalkenes): Part II. Reaction with pyridine

Pyridine reacts with 2,2′-dichloro(perfluorobicycloalken-l-yl) under hydrolytic conditions to give the pyridinium betaine of a 1,3-diene. Hydrolysis of the allylic fluorines occured exclusively in the cyclobutenyl ring bearing the oxygen anion. The intermediate ylide, if indeed formed at all, was too unstable to isolate and identify.     

Formation of membranes by means of immersion precipitation : Part II. the mechanism of formation of membranes prepared from the system cellulose acetate-acetone-water

Using equations and boundary conditions derived in Part I¹, calculations have been performed on the ternary diffusion processes that occur in a cellulose acetate (CA) -acetone casting solution immersed into a water bath. The necessary concentration-dependent thermodynamic and hydrodynamic parameters have been derived from experimental data on the three limiting binary mixtures. Calculations show that immersion of the polymer solution into the coagulation bath results in an instantaneous increase of the polymer concentration at the surface of the solution. For a CA-acetone casting solution the thickness of this concentrated surface layer will increase until the onset of liquid-liquid demixing by means of nucleation and growth of the diluted phase that fixes the asymmetric polymer distribution in the film. The moment of onset of the demixing process depends on the thickness of the film. However, addition of a certain minimum amount of water to the casting solution results in an instantaneous onset of liquid-liquid demixing upon immersion, yielding a membrane with a very thin skin layer. The model calculations have been confirmed by light transmission measurements performed on immersed casting solutions.     

Radiochemical study of gas-phase reactions of diethylstannyl cations Et2SnT+ with oxygen-containing compounds: II. Reaction of diethylstannyl cations with butanol

Gas-phase reaction of nucleogenic diethylstannyl cations Et₂SnT⁺ with butan-1-ol has been studied by the radiochemcal method, and probable reaction mechanisms have been proposed. During the process diethylstannyl cations undergo isomerization into tertiary Me₂EtSn⁺ cations and rearrangement with elimination of ethane. Thermochemical parameters of reactions of diethyl-substituted cations derived from Group 14 elements (Et₂TM⁺; M = C, Si, Ge, Sn) with alcohols have been analyzed by the M06L aug-cc-pVDZ quantum chemical method.     

Benzisoxazolium cations—II : Reaction with anthranilic acid: Intramolecular reaction of an N-aroyl-N-alkylsalicylamide

Reaction of 2-ethylbenzisoxazolium fluoborate (1) with anthranilic acid gives O-aroylsalicylamide (5a) and the quinazolone (6a), whose structure is established by an unambiguous synthesis of its methyl ether (6b). The O-aroyl amide (5a), formed via the ketoketenimine (Scheme 1) from the salt 1, undergoes O→N migration to the imide (8) which through an intramolecular reaction followed by dehydration is converted to the quinazolone 6a (Scheme 2). Independently the O-aroyl amide (5a) could be transformed to the quinazolone (6a) under basic conditions. The formation of quinazolone (6a) suggests the labile N-alkyl-N-aroyl isomer (8), which is expected to be in equilibrium with the O-aroyl isomer (5a), is captured. (Scheme 2).