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.     

Influence de la nature de l'electrolyte sur la reactivite electrochimique du chromate de baryum incorpore dans une electrode a pate de carbone

The electrochemical behaviour of baryum chromate is investigated by means of a carbon paste electrode with conducting binder. Current potential curves recorded, when electrolyte and binder are 1 M aqueous solutions of strong mineral acids, evidence only the reduction of chromium (VI) ions resulting from the more or less complete dissolution of the sample at the beginning of the experiment. The behaviour is different in the acids CH₃COOH and HCOOH. The curves then exhibit two peaks corresponding to the electrochemical transformations of chromium: the first is due, as in strong mineral acids, to the reduction of chromium (VI) ions, coming from the dissolution of the finest particles at the beginning of the experiment, into Cr³⁺ ions; the second one is characteristic of the progressive electrodissolution of the solid, via a chemical-electrochemical mechanism, during the potential sweep; its shape and position are greatly influenced by the morphology of the sample.     

Synthese asymetrique par voie electrochimique—V. Influence du potentiel et des agents protonants sur le rendement optique

A general scheme is proposed for the mechanism of asymmetric electrosynthesis, performed in the presence of an adsorbed inductor, in the case of reductions involving an intermediate carbanion. The interactions between substrates and inductors, and specially the preprotonation of the substrate by the adsorbed inductor, allow a preferential presentation of the substrate at the electrode surface. When the intermediate carbanion is conjugated, it may undergo an inversion of its initial configuration in the electrical field of the electrode, before the fixation of a proton. Then, the configuration of the final product and the optical yield depend on the competition between the influence of the working potential and the ability of the solution, on the electrode side, to act as a proton donor.     

Reduction electrochimique des cetones insaturees—VII. Influence de l'acidite du milieu sur le comportement cathodique des cetosulfones

The ketosulfones are known to be reduced electrochemically, in weakly acidic medium with scission of the CS bond. Nevertheless, in strongly acidic media, the electroactive entity being the protonated form of the carbonyl group, the reaction may lead to pinacols. In the case of γ-ketosulfones, the electroreduction in very weakly protic media is followed by the loss of proton of the acidic substrate due to the effect of electrogenerated bases. This chemical cleavage of the CS bond yields to α-ethylenic ketones reducible at the electrolysis potentiel. On the other hand, for the long chained ketosulfones the reduction may occur through a partial intramolecular electron transfer.     

Influence de l'adsorption chimique du soufre sur le comportement electrochimique du cuivre dans une solution d'acide nitrique

By the chemisorption of sulphur on copper on obtains electrodes with properties different from those of clean copper annealed in pure hydrogen. Electrochemical measurements show that the chemisorbed sulphur slows down the dissolution of the metal in nitric acid solution.

The protective role of sulphur can be verified in two ways: by making a cell either with one electrode of clean copper and the other copper covered with chemisorbed sulphur, or by using a specimen partially covered with sulphur.

Zusammenfassung

Durch die Chemisorption von Schwefel auf Kupfer erhält man Elektroden mit anderen Eigenschaften als denen von reinem Kupfer, das in reinem Wasserstoff erhitzt wurde. Elektrochemische Messungen zeigen, dass der chemisorbierte Schwefel die Auflösung des Metalls in Salpetersäure verlangsamt.

Die Schutzwirkung des Schwefels kann auf zweierlei Art verifiziert werden: indem man eine Zelle mit einer Elektrode aus reinem Kupfer und der anderen aus Kupfer überzogen mit chemisorbiertem Schwefel herstellt oder eine Probe verwendet, die teilweise mit Schwefel bedeckt ist.     

Etude, par voie electrochimique, de l''influence du fer (II) sur la reduction de cetones en solution dans le N, N-dimethylformamide (DMF)

The electrochemical reduction, in DMF, of a series of aromatic ketones has been studied in the presence of iron salts. With these ketones, the first reduction peak of the ketone is always preceeded by another peak which appears at a less negative potential. This peak is attributed to a fast chemical reaction between Fe(II) and the anion radical of the ketone, which shifts the electrochemical peak potential. With fluorenone, several species are formed corresponding to 1/1, 3/4, 1/2, 1/4 iron/ketone ratios. For the other ketones, the 1/1 species is mainly formed.     

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 la composition et de la temperature sur les reactions electrochimiques et le domaine de stabilite electrochimique des borates de sodium fondus

The oxidation and reduction reactions, limits of the electrochemical stability range, were determined by cyclic voltammetry in molten sodium borates 3 Na₂OB₂O₃ (I), Na₂O2B₂O₃ (II) and Na₂O4 B₂O₃ (III) between 750°C and 1000°C.These reactions are: on gold electrode the oxidation of O^2? ions to oxygen and the reduction of borate ions to elemental amorphous boron; on platinum electrode the oxidation and passivation of the metal and the reduction of borate ions to form platinum borides Pt₃B and Pt₂B.The electrochemical stability ranges are respectively in solvent I 0.67 V (750°C), 0.97 V (850°C), 0.92 V (1000°C); in solvent II 1.94 V (750°C), 1.92 V (850°C), 1.85 V (1000°C); in solvent III 1.85 V (850°C), 1.75 V (1000°C).     

Influence de la concentration des agents tensio-actifs sur la vitesse terminale d'ascension des gouttes

The rising of droplets in water contaminated with two surfactants, sodium salt dodecylhydrogenosulphate (L.S.S.) and Teepol, in the range 200 < N_Rep < 1000 was studied. The dispersed phases used were cyclohexane, benzene, toluene, and n-heptane. Experiences were conducted at a constant temperature of 30°C. It was shown that the presence of the two agents decreases the final rising rate of the droplets with an increasing surfactant concentration and droplet diameter. For the same interfacial tension values, this phenomenon is enhanced in the case of Teepol. An equation is proposed for the computation of the critical droplet diameter as well as the droplet rising rate. These equations demonstrate the effect of the chemical nature of the surface agent and reproduce the experimental results with variations less than 10%.     

Influence du mode d'excitation sur la thermoluminescence du silicate tricalcique

Thermoluminescence is very convenient to study excited centers in solids and specially structural defects (crystallographical and chemical). However, it is necessary to use several reproducible excitation processes. In this work, some excitation processes of tricalcium silicate: plasma, γ and U.V. rays are studied. A new thermoluminescence apparatus which allows use of small quantities of samples (20 mgr) is described.     

Reduction electrochimique du nitrobenzene,en milieu micellaire aqueux,a divers pH. Influence de la nature des tensio-actifs

The electrochemical reduction of nitrobenzene has been studied in the presence of some anionic, cationic and non-ionic surfactants using a glassy carbon electrode or a dropping mercury electrode (polarography). The results are depending upon the electrode nature, the surfactant polarity and the pH of the solutions.On glassy carbon electrode and with cationic surfactants, the reduction is always a one step process for the totality of the studied pH scale (2–12). With the anionic and non-ionic surfactants, the potential is shifted towards the more negative values for a one step process or for a two step process, according to the pH value.For the polarographical studies, the electrochemical reduction of nitrobenzene is generally retarded by the presence of monomer surfactant molecules adsorbed on the mercury electrode, principally in acid media. In micellar solutions, the CTAB decreases the protonation rate of the nitrobenzene radical anion and facilitates the reduction by a dianion way. The anionic and non-ionic surfactants increase the radical anion stability and slow down the rate of the radical anion reduction.     

Synthese asymetrique par voie electrochimique—III. stereochimie de la reduction du phenylglyoxylate de (−)menthyle

Electrochemical reduction, at a mercury cathode, of (-)menthyl phenylglyoxylate, leads to a mixture of the two epimers of (-)menthyl mandelate, the respective proportions of which depend on numerous factors. The R(-) mandelate is nearly always preferentially obtained and the experimental data are in good agreement with the following hypothesis: in very acidic media, protonization of the intermediate carbanion occurs principally at the side of the molecule near by the electrode surface, before inversion of the initial configuration of the carbanion; in neutral or basic media, protonization takes place essentially in the solution. The way of presentation of the substrate at the electrode, the working potential and the components of the electrical double layer, act also as determining factors.     

Interpretation des courbes de polarisation cathodique du cuivre dans les solutions chlorurees acides desaerees: etude simultanee de la corrosion du cuivre et de la reduction des ions H+

During cathodic polarization of copper in deaerated acidic chloride solutions, two simultaneous processes occur: the dissolution of the metal and the reduction of the H⁺ ions. One can explain the steady state curves lg I = f(E) by means of the theory of adsorbed species. Each interpretation differs according to the nature of the adsorbed species (Cu H⁺_ads for the first one, Cu H_ads for the second one). They are made up of three consecutive reactions involving a rate determining step (R.D.S.) and another step during which the desorption of the adsorbed species brings out the two simultaneous experimental phenomena.     

Influence du millieu de trempe sur la cinetique d'hydratation du silicate tricalcique

Two samples of tricalcium silicate, one air quenched, the other by means of liquid nitrogen, show different structure defects detected by thermoluminescence. Their hydration kinetics, followed by isothermal microcalorimetry and thermogravimetric analysis, point out that the fastest quenching (liquid nitrogen) induces the highest hydration rate and demonstrate once again the part played by structure defects on the reactivity.     

Reduction electrochimique de la phenothiazine et du fluorobenzene a basse temperature

A well characterized polarographic wave is observed at low temperature (?30°C) for phenothiazine $\text{1}$ (E_{$\text{1}\text{2}$} = ?2.85 V) and fluorobenzene (E_{$\text{1}\text{2}$} = ?2.97 V) in dimethylformamide. The electrochemical reduction of $\text{1}$ leads probably to an NN dimer with consumption of 0.5 F.     

Dissolution electrochimique des films anodiques du titane dans l''acide sulfurique

Electrochemical dissolution of anodic oxide films on Ti in H₂SO₄ is studied by the use of an optical—electrolytic cell adapted for in situ ellipsometric and electrochemical measurements. Films were grown on electropolished Ti surfaces by anodic oxidation in 0.5 mol dm⁻³ H₂SO₄ for duration of 30 s, in the potential range from 0 to 100 V. Electrochemical dissolution of these films was performed in the potential range between −0.4 and −0.9 V sce. The potential at which electrochemical dissolution is fastest and homogeneous is determined. In addition it is shown that for potentials more cathodic than −0.8 V, TiH₂ is formed on the electrode surface, if polarization lasts sufficiently long. THe commencement of formation and dissolution of anodic films was monitored by potentiostatic j-E curves and variations of ellipsometric azimuths P and A. by potentiostatic and ellipsometric measurements it was confirmed that complete reduction of anodic films is possible. A possibility is indicated for calculating the refractive index of anodic film during its dissolution by observing the time of successive diminution of film thickness and using the variations of ellipsometric azimuths P and A.     

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.     

Reduction indirecte du graphite par des radicaux-anions formes par voie electrochimique

Graphite may behave as an oxidant towards electrochemically generated anions radicals formed at a potential more cathodic than its own standard potential. When this material is placed in the vicinity of the cathodic surface where are formed those anions radicals, a reoxidation process takes place. This process appears to be similar to a redox catalysis in heterogeneous phase.     

Evolution microstructurale des composites du systeme ciment alumineux-granulat calcaire. II. Influence sur la porosite et les resistances mecaniques

The analysis of the evolution of mechanical strengths of different composite materials based on aluminous cement has allowed the authors to point out two phenomena:

• 1.1. a noticeable improvement in strength by the use of calcareous aggregates or fillers due to the formation of a transitional layer rich in hydrated monocarboaluminate which grows and develops at the aggregate (or fillers) - cement matrix interface,
• 2.2. a direct connection between mechanical strengths and a porosity of the material, named “critical porosity”, which corresponds, in the present case, to pores with diameter higher than 10,000 angstroms.