Formation electrochimique d'alkylantimoine par reduction d'alkyldiiodoantimoine

In acid and basic aqueous-alcoholic media, in a mixture of acetic acid and sodium acetate and in dimethylformamide, alkylantimony diiodides are reducible in a two-electron step with formation of an alkylantimony insoluble in all solvents. In basic media, oxidation into an alkylantimony (V) compound is observed. Electrochemical behaviour and stability of alkylantimony are discussed.     

Etude de la reduction electrochimique d'alkyl-2 ethoxycarbonyl-2 cycloalcanones. Influence des conditions experimentales sur la stereochimie de la reduction

A few 2-alkyl-2-carbethoxy-cycloalkanones are selectively reduced with high yields into 2-alkyl-2- carbethoxy-cycloalkanols, electrochemically on mercury pool in hydroorganic medium. Carbon dioxide is bubbled with nitrogen through the solution to maintain the pH at about 9 to avoid side reactions.Cis and trans esters-alcohols were determinated by gas chromatography. The ratio is depending on several factors: the size of the cycle, the nature of the alkyl substituent, the nature of the cation of the supporting electrolyte, the nature of the cathode material and the temperature of the reaction. This last one is very important. At low temperature (?6°C) we have got preferentially the cis isomer and at high temperature (80°C) we have got almost exclusively the more thermodynamically stable trans isomer.To explain the mechanism we consider the presentation of the molecule on the electrode, the protonating effect of the medium and the thermodynamical equilibrium of the intermediate carbanion.     

Coupure cathodique de liaisons carbone-oxygene—I reduction electrochimique d'alcools insatures application a la reduction tetraelectronique de certaines cetones en carbures

Some unsaturated alcohols are reduced electrochemically on mercury at very negative potential values. For many of them, it has been shown that the carbon-oxygen bond is cleaved with formation of unsaturated hydrocarbons, in a two electron reaction. However, in some cases, there exists for the anion radical another possibility of reaction: protonation to form a saturated alcohol. When the protonation reaction is slow compared to the cleavage reaction, a direct method of electroreduction of ketones to hydrocarbons, through the intermediate alcohol, can be proposed.     

Mecanisme de la reduction electrochimique des carboxy-3 et -4 coumarines et de leurs esters ethyliques

Mechanisms of the electroduction of the title compounds are settled from polarographic and voltammetric studies in protic and aprotic media. Ethyl 4-coumarin carboxylate is reduced according to the sequences H⁺ eeH⁺ in acidic media and eeH⁺H⁺ in basic media; for 4-coumarin carboxylic acid, a DISP 2 mechanism is revealed. These results explain the different behaviour of the two compounds with respect to asymetric synthesis of substituted dihydrocoumarins.

If substituent is in position −3, the reduction leads to hydrodimers. They may result from radical—anion substrate coupling or radical anion dimerization. Reaction of the dianion with the substrate clearly appears in DMF solutions.     

Comportement electrochimique d'oxydes de manganese,en milieu alcalin

The cathodic reduction of Mn₃O₄, Mn₂O₃γ, MnOOH, MnO₂β and MnO₂γ in alkaline medium (8 M NaOH) has been studied by cyclic voltammetry, using a carbon paste electrode. The electrochemical behaviour of MnO₂γ differs from MnO₂β. The reduction of MnO₂γ proceeds by two steps: the first being the reduction of MnO₂γ to Mn₂O₃, the second the reduction of Mn₂O₃ to Mn(OH)₂; while the reduction of MnO₂β gives directly Mn(OH)₂. The oxidation of manganous hydroxide has also been studied.     

Etude de la formation d'un complexe de surface graphite-eau relation avec la reaction d'oxydation

Using a gas flow apparatus under atmospheric pressure with infrared CO and CO₂ detection and an apparatus operating under low pressure with a mass spectrometer for analysis, we have found the following: The chemisorption of water becomes appreciable above 200°C. The water chemisorbed on graphite previously degassed at 1000°C forms a surface complex which on raising the temperature decomposes with simultaneous evolution of CO and H₂, thus showing that hydrogen enters into the composition of the complex. The programmed thermal decomposition of the complex shows two peaks. The first peak corresponds to the sites which disappear with the degassing of the CO and H₂; these sites are formed by the ‘labile’ carbon atoms created by the prior degassing. The second peak corresponds to the sites which are constantly renewed after degassing the CO molecules, and which are therefore formed by graphite atoms normally linked to the graphite lattice. The reaction rate of the oxidation of graphite by water shows two transient rates before attaining a stationary value. Correlations between the CO thermo-desorption and these reaction rates are found which indicate that the formation of a surface complex is a necessary step in graphite-H₂O reaction.     

Stereochimie de l'electroreduction de bromocyclopropanes—III. Diastereoselectivite de la reduction electrochimique de gem-dibromocyclopropanes

Cleavage of a single carbon bromine bond occurs in the two electrons reduction of 2,2 disubstituted 1,1 dibromo cyclopropanes.Analysis of the diastereoisomeric mixture of monobromo compounds obtained at the term of electrolysis at a mercury cathode in a hydroethanolic media allows to determine which of the two bromine atoms is preferentially cleaved.

When R² = Ph, R¹ = H or CH₃ only a low stereoselectivity is generally observed. Nevertheless, yields up to 84% of the cis isomer are achieved when R¹ = H.Best results are obtained when R² is a carboxy group (CO₂H, CO₂Me or CO₂Et) especially when the reduction is carried out in the presence of quaternary ammonium cations. The cis isomer is always preferentially formed; yields up to 92% when R² = CO₂H and R¹ = Ph, and up to 99% when R² = CO₂Et and R¹ = CH₃ can be achieved.Influence of electrolyses conditions (working potential, nature of the supporting electrolyte) on the stereochemical course of the reduction is investigated.     

Mecanisme d'echange electrochimique du systeme Ni/Ni (II) dans l'eutectique LiCl—KCl fondu

We suggest, for the exchange reaction Ni/Ni (II) in the fused eutectic LiCl—KCl the following reaction scheme: Ni(I)_ad + e ? Ni the rate determining transfer step 2Ni(I)_ad ? Ni + Ni(II) fast disproportionation step The experimental curve showing the variation of

with the charge transfer overpotential η, agrees with the theoretical curve corresponding to the proposed scheme. The comparison of the slope of the theoretical curve, lni₀ = f(ln[Ni(II)]) with the experimental one is in good agreement with our assumption.     

Oxydation electrochimique d'amino-3 pyrazolines

The electrochemical oxidation of 3-amino pyrazoline in acetonitrile leads to the corresponding dimers and pyrazoles.     

Sur l'electrosorption d'ethylxanthate au cours de la polarisation anodique d'electrodes de platine et de galene

The main results concerning the galena flottation by xanthates which are relevant from an electrochemical point of view are presented. The usual kinetic theories were unable to explain the results obtained in previous electrochemical studies on xanthate oxidation at various electrodes. A modified treatment is reported. It reconciles data indicating the same phenomenology as that taking place when a monolayer is chemisorbed as well as quantitative results suggesting the presence of a thick layer. The result previously obtained with platinum and galena electrodes can thus be quantitatively interpreted. It is concluded that the complexity of the chronoamperometric data derives more from the adsorbed layer structure than from the reaction scheme. The latter is composed of the xanthate physisorption followed by chemisorption. The adsorbed phase with tens of monolayers is assumed to be analogous to a lamellar phase of a liquid crystal. It is electrically conducting and its thickness is not dependent on the substrate. However. the nature of the latter controls the oxidation rate and the possible occurrence of passivation. The electrochemical results and those obtained by other technics on dispersed systems are well can be interpreted by the proposed model.     

Oxydation electrochimique de l'ethylene en solution aqueuse a des electrodes d'or

A systematic study has been made of the influence on the nature and quantity of products formed by electro-oxidation with gold electrodes of aqueous solutions of ethylene, the pH, the potential applied to the electrode, the temperature and the rate of electrolysis.Details have been given, whatever the experimental conditions, of the formation of dosable quantities of acetaldehyde, glyoxal, glycolaldehyde, formaldehyde, acetic acid, oxalic acid and carbonic acid gas.Beginning with the distribution of these various products in correlation with the various parameters, a possible mechanism has been suggested for the electro-oxidation of ethylene with gold electrodes, with the formation on the electrode of a species of activated or protonized acetaldehyde.     

Comportement electrochimique d'une electrode de graphite en milieu tetrachloroaluminate de lithium—carbonate de propylene

The electrochemical behaviour of the system LiAl/Propylene-carbonate—LiAlCl₄/graphite has been investigated.Such a system can store energy and give it back at a potential of 4V. At the charging state, the AlCl^?₄ ion is oxidized into chlorine which saturates the solution.Graphite is not oxidized before the evolution of chlorine in propylene carbonate solution: no intercalation compound was evidenced.     

Oxydation electrochimique d'une serie d'indoles aryl et alkyl substitues dans l'acetonitrile

The electrochemical oxidation of various substituted indoles at a platinum electrode in nonbuffered acetonitrile shows two different types of behaviour, depending on the substituents: (i) 2,3-diphenyl, 2,3-diphenyl-4-methyl, 2,3-diphneyl-5-methyl and 2,3-dimethyl indoles lead to a dimer oxidizable at more positive potentials than the starting material. These dimers have been isolated and characterized. Oxidation of 2,3-diphenyl indole leads to a tetramer, (ii) 2,3-diphenyl-7-methyl and 1,2-diphenyl indoles lead to dimers oxidizable at more negative potentials than the starting material.     

Etude du mecanisme et de la cinetique de la reduction des ions uranyles dans des solutions d'acide phosphorique

The electrochemical reduction of uranyl ions in 0,1-9 M phosphoric acid has been investigated by polarography, cyclic voltammetry, chronopotentiometriy and potentiostatic coulometry.In concentrated phosphoric acid solutions (H₃PO₄ < 1 M), an adsorption pre-wave appears in addition to the main wave of diffusion. The electrochemical reaction involves a single electron. Kinetic usually corresponds to a quasi-reversible system, but it evolves according to the variation of U(VI) and H₃PO₄ concentrations. So, when the concentration of U(VI) increases and/or that of H₃PO₄ reduces, the system becomes reversible.     

Influence de l'adsorption du depolarisant ou d'un produit de la reaction electrochimique sur les courants polarographiques—XVIII. Etude de la dimerisation de radicaux mercuroorganiques par voltammetrie a variation lineaire de tension

Compounds with the general formula ArHgX, adsorbed on a mercury electrode are reduced to free radicals, which dimerize on the surface of the electrode. A study by linear potential sweep voltammetry shows that the dimerization involves two free radicals and establishes the validity of the theory elaborated previously for that type of reaction.     

Mise en oeuvre des methodes d'impusions galvanostatiques rapides: determination de la resistance de transfert de charge et de la capacite differentielle d'une electrode

Reading within a very short time the voltage response to galvanostatic pulses requires the use of a microelectrode having a suitable geometry and an area not exceeding a few mm². The comparison of different methods (single, double pulse, charge step-decay) applied to the H⁺/H system and to some Me^z+/Me systems show that the double pulse method is the only one to yield the determination of the R_t and C_d characteristic parameters 60–80 ns after the beginning of polarization.     

Etude d'un modele de reaction electrochimique d'insertion—I. Resolution pour une commande dynamique a petit signal

The small signal ac impedance of an electrochemical reaction of ion and electron insertion in a host lattice has been studied. The diffusion equations have been solved for plane, cylindrical and spherical electrodes. It is shown how quantitative informations about real systems could be obtained by analysis of the impedance data by the Nyquist complex plane and Bode plane methods.     

Mecanisme de la passivation electrochimique du platine en milieu chlorures fondus premiere partie:: Milieu deoxygene

The mechanism of platinum electrochemical passivation in molten LiCl-KCl at 450°C has been examined by the chronopotentiometry.This passivation due to precipitation of a double platinum and potassium chloride (K₂PtCl₆) allows the verification of two kinds of relationships according to the temperature and current density range. At low temperature (<450°C) and current density (<250 mA/cm²), a faradaic relation (Iτ = Cte) is observed, while at higher temperature (450°C) and current density (250 mA/cm²), a relation I√τ = Cte is established.The activation energies required for the transport process of platinum ions are determined both in the eutectic and in the K₂PtCl₆ crystals. By the comparison of the activation energy necessary for the formation of a passivating layer, it may be concluded that the mechanism of thickening in the solid K₂PtCl₆ layer at high temperature is by means of a diffusion process.On the basis of these results, a theory is proposed. It explains the two kinds of law relevant to experimental conditions. It takes into account the initial conditions and it is based on the vacancy theories characteristic of oxidation.     

Etude electrochimique de l'oxydation simultanee de la galene (PbS) et de l'ethylxanthate—condition de formation du xanthate de plomb en surface de la galene

The electrochemical oxidation of galena (PbS) has been investigated in the pH range 9–10, by controlled potential electrolysis using a special bed electrode. In the absence of xanthate, the oxidation of PbS leads to the formation of elemental sulphur, thiosulphate and lead species (Pb(OH)₂). Simultaneous oxidation of galena and xanthate has also been studied and the products characterized by their uv-visible spectrum. The oxidation produces lead xanthate and dixanthogen. It has been established that lead xanthate comes from an ion exchange reaction between excess xanthate in solution and lead species resulting from the lead sulphide oxidation.