Effect of pH on the electrochemical reduction of some heterocyclic quinones

The reduction of 1,10-phenanthroline-5,6-quinone(I), 5,8-quinolinequinone(II) and 6,7-dichloro-5,8-quinolinequinone(III) was investigated using cyclic voltammetry and coulometry at mercury electrodes and 50% dimethylsulfoxide+water solvent. Each compound is reduced to the corresponding hydroquinone in a diffusion-controlled, reversible two-electron process. The pH-dependence of the reversible potential indicated that the quinone forms were unprotonated, but the hydroquinones could be protonated at the heterocyclic nitrogen atom with pK_a = 5.3 for I and 3.5 for III. Careful analysis of the cyclic voltammetric peak shape revealed that the difference between the standard potentials for the introduction of successive electrons, E₂⁰ ? E₁⁰, was 70 ±20, >100 and 80 ± 20 mV for I–III. Investigation of the pH-dependence of E₁⁰ and E₂⁰ showed that the pK_a of the semiquinone of I was about 8.     

Evaluation of unimolecular rate constants for some outer-sphere electrochemical reactions

Unimolecular rate constants, k_et (s^?1), and transfer coefficients, α_cet, for the outer-sphere electroreduction of several Co(III) ammine and ethylenediamine complexes have been evaluated by electrostatically adsorbing the reactants at silver electrodes coated with chloride or bromide monolayers. Sufficiently strong diffuse-layer adsorption is thereby produced so to enable k_cet and α_cet to be determined by means of linear sweep voltammetry, employing sufficiently fast sweep rates (10–100 V s^?1) and dilute reactant concentrations (≤ 50 μM) so that the bulk solution reactant contributes negligibly to the observed faradaic transients. Comparison with corresponding rate constants and transfer coefficients for the solution reactants enables the influence of precursor-state stability upon the latter rate parameters to be assessed.     

On the electrochemical reduction mechanism of indolinone nitroxide radicals in DMF

A study of the electroreduction of 1,2-dihydro-2-phenyl-2-alkyl (or phenyl)-3 H-indol-3-one-1-oxyls (nitroxide radicals) in dimethylformamide is presented. The techniques of ac and dc voltammetry, controlled potential coulometry, ESR and UV spectrometry were used. In DMF-H₂O nitroxide radicals are reduced in two steps: the first is the reversible monoelectronic formation of the corresponding N-hydroxides, while the second (which involves a chemical reaction) is the irreversible production of 2,3-dihydro-2-phenyl-2-alkyl (or phenyl)-3-hydroxy indoles. A multistep scheme is suggested, based upon the electrochemical and spectrometric data.     

Electrochemical reduction of some rhodanine derivatives at the DME

The electroreduction of two series of rhodanine derivatives has been investigated at the DME. The first wave was assigned to the saturation of the azomethine linkage and the reduction of the ethylenic double bond in series I and II, respectively. On the other hand, the second wave in both series was assigned to the reduction of the C-S group and partly to the catalytic hydrogen evolution catalyzed by these compounds.     

The influence of disproportionation of the anion radical on the electrochemical reduction of 1,9-dimethyldibenzo[b,f]pentalene

The electrochemical reduction of 1,9-dimethyldibenzo[b,f]pentalene has been investigated in DMF by both cyclic voltammetry (CV) and dc polarography. The influence of the depolarizer concentration and electrode material (Pt and Hg) was studied. It was found that the first electron transfer is a reversible one under most conditions studied, whereas the second electron transfer was irreversible on Pt and quasi-reversible on the HMD. An overall EDISP.EC mechanism was suggested and discussed. It turned out that the first reduction process was accompanied by a disproportionation of the anion-radical to its parent hydrocarbon and dianion. The experimental waves were analysed on the basis of various theoretical procedures.     

Stereochemistry of reactions in the 1,2-dimethylsilacyclopentane ring systems. I. Stereospecific transformations

Preparations, separations of geometric isomers, and structural assignments based on nmr and on chemical evidence are described for a number of 1-substituted 1,2-dimethylsilacyclopentanes. A number of stereospecific reactions have been observed, and the stereochemistry is in all cases the same as that observed for acyclic silanes. A discussion of the role of ring strain in determining stereochemical outcome and reaction rates is presented.     

The synthesis of some bidentate bismuth-containing ligands

The synthesis and properties including mass spectra of (o-diphenylphosphinophenyl)diphenylbismuthine, o-C₆H₄(PPh₂)(BiPh₂) and (o-diphenylarsinophenyl)-diphenylbismuthine, o-C₆H₄(AsPh₂)(BiPh₂) are described. The failure of attempts to prepare α, ω-bis(disphenylbismuthino)alkanes is discussed.     

The lithium aluminium hydride reduction of cyclohexadienylmanganese complexes

Reduction of cyclohexadienyltricarbonylmanganese or its ring-substituted derivatives with lithium aluminium hydride leads to dihydro-derivatives. Preparative and NMR spectral information is presented and interpreted on the basis of the CH-bridged cyclohexenyl structure recently established for these products by Lamanna and Brookhart [3].     

Kinetics and mechanism of the electrochemical reduction of 2,2′-bipyridine: Part III. Accurate kinetics and thermodynamics of the base catalyzed consecutive reactions

The isomerization reactions of the dihydrobipyridine Y, formed primarily from 2,2′ bipyridine by electrolysis at constant potential, were investigated in detail. The gross rate constants of the scheme Y→A agB agC were optimized from measured concentration vs. time curves at different temperatures with the hydroxyl ion concentration as a parameter. Between Y, A, B, C base catalyzed steps were found but also first-order parallel reactions independent of the pH. The reducible intermediate B is not a single substance but exists in two forms: B and BH⁺ that are connected by a fast equilibrium. A detailed reaction scheme was erected and the 9 individual rate constants and their dependence on the temperature as well as the equilibrium constants and the reaction enthalpies and entropies were determined.     

The control of potential in continuous flow systems

The control of the potential of the working electrode in closed loop flowing systems is discussed. Various relative positions of the reference electrode are considered. No satisfactory arrangement of a three electrode configuration is possible for the general case; only when all but one of the current paths are blocked is a three electrode system satisfactory. Generally, however, a five electrode arrangement is adequate for all purposes. Electrical circuits for practical operation are given. With suitably designed tube electrodes reactions with very high exchange currents can be studied free from any uncertainty due to ohmic resistances. An application to the control of parasitic current corrosion is noted.     

Thermal and X-ray diffraction investigations of some lanthanum(III) and neodymium(III) oxide-alkali persulfate binary systems

Different molar ratios of La₂O₃ or Nd₂O₃:Na₂/K₂S₂O₈ have been prepared, and the results of their TG and DTA investigations, under an atmosphere of static air, are reported. The effects of either La₂O₃ or Nd₂O₃ on the thermal decomposition of the persulfates from ambient to 1050°C, using a derivatograph, have been studied. It has been found that La₂O₃ lowers the initial decomposition temperatures of these alkali persulfates through catalytic activity. Nd₂O₃ shows little or no catalytic effect and therefore it acts as an insulator. Intermediate and final products are identified by X-ray diffraction analysis. The stoichiometric molar ratios of the solid state reactions are 1:3::R₂O₃:M₂S₂O₈. (R = La or Nd. M = Na or K), which give double salts of formulae: NaLa(So₄)₂, KLa(SO₄)₂, NaNd(SO₄)₂, and KNd(SO₄)₂. No sulfates or oxysulfates of lanthanum or neodymium have been identified.     

The deaquation reactions of some metal salt hydrates at elevated pressures

The deaquation reactions of BaCl₂·2H₂O, BaBr₂·2H₂O and CoCl₂·6H₂0 were studied by the thermal analysis techniques of thermogravimetry, differential thermal analysis (DTA), and electrical conductivity in the pressure range from one to 170 atm. In general, the effect of pressure on the TG curves increased the T_i and T_f values and also the reaction interval, (T_t—T_i). The DTA curves exhibited splittings into multiple peaks as a result of the increased pressure. These splittings were interpreted as due to the evolution of a liquid water phase followed b     

The thermal stability of some pulping materials

The thermal stabilities of three annual plants (reed, kenaf and Glycyrrihiza glablra L.) are studied. Various types of pulps were prepared from reed stalks by employing the soda, kraft and soda-anthraquinone processes. Their thermal stabilities are reported.It is shown that the thermal stability measured thermogravimetrically depends, to some extent, on the amount of lignin remaining in the raw materials, i.e. the kappa number.The activation energy of the decomposition process and the decomposition temperatures of the pulps are also reported.