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.     

Electrochemical behaviour of some neuroleptics: Haloperidol and its derivatives

The electrochemical characteristics of Haloperidol and related compounds, representative neuroleptics of the butyrophenone family, have been investigated as a function of pH and concentration by direct-current, alternating-current and differential-pulse polarography and cyclic voltammetry at a hanging mercury drop electrode. A single cathodic wave representing an irreversible two-electron reduction is obtained, and its half-wave potential differs from that characteristic of aromatic ketone reduction. Adsorption processes disturb the wave behaviour and an adsorption prewave is observed at high concentrations. Quantitative measurements were successful in the concentration range 1 x 10(-4)-1 x 10(-6)M (0.4 mg/l.), the lower concentration representing the detection limit by differential-pulse polarography.     

The electrochemical reduction of some fused ring systems

The electrochemical reduction of some fused ring systems containing four carbonyl moieties has been studied in alcoholic aqueous media. A mechanism has been proposed and clarified in which one carbonyl is reduced to a methylene group via an uptake of four electrons.     

Dynamics and structure of some tricarbonylferrole-iron tricarbonyl derivatives

A series of compounds of the formula Fe₂(CO)_6-x(PR₃)_x(R′C₂R″)₂ (x = 0, R′ and R″ = Ph, R′ and R″ = H, R′ = Ph and R″ = H; x = 1, K = Ph or n-Bu, and R′ and R″ = Ph) were studied by ¹³C NMR to observe their solution properties. The tricarbonylferrole unit was found to be static from ?125 to +95° C, while the π-Fe(CO)₃ group appeared to be fluxional over the same temperature range. Definite assignments of the carbonyl carbon and ferrole ring carbon resonances have been made. A low temperature single crystal X-ray study of Fe₂(CO)₅PPh₃(PhC₂Ph)₂ demonstrated that the phosphine ligand was attached to the ferrole iron contrary to previous belief based on chemical evidence.     

Electrochemical reduction of tricarbonyl compounds: Reduction mechanism of alloxan on a mercury electrode

A study has been carried out on the reduction mechanism of alloxan over a DME corresponding to the first two-electron transfer over the pH range 0–12.Polarographic and voltammetric results show the reduction process to be kinetically controlled. Rate and equilibrium constant data for the prior chemical reaction have been evaluated and several reaction mechanisms for different pH zones have been proposed.     

Electrochemical behavior of ellipticine derivatives: Part I. Oxidation of 9-hydroxy-ellipticine

The electrochemical one-electron oxidation of 9-hydroxy-ellipticinium cations at a platinum electrode has been examined with particular attention to the thermodynamic redox potential and to the dimerization rate of the radical species produced. Both the reactant and the product of the electrochemical reaction are strongly adsorbed at the solution/electrode interface. The initial step of the anodic process is a reversible electron transfer accompanied by a fast deprotonation; E^0′=900?53 pH mV vs. NHE. The resulting neutral radical dimerizes, the rate constant of the surface dimerization being ca. 2.5×10⁹ mol^?1 cm² s^?1.     

Electrochemical reduction of oxygen at a porous flow-through electrode

The electrochemical reduction of oxygen at a porous flow-through electrode is described with emphasis on the effects of concentration, flow speed and surface area. On a packed bed copper electrode in sulfuric acid, it was found that oxygen undergoes a two electron reduction process giving rise to H₂O₂.     

Photo-induced degradation reactions of some alkyl- and aryl-carbonyl derivatives of manganese,molybdenum and tungsten

Photo-induced degradation of CH₃Mn(CO)₅ in pentane solution results in the formation of Mn₂(CO)₁₀, methane and carbon monoxide. Both CH₃D and CH₄ are formed when CH₃Mn(CO)₅ is photolyzed in C₆D₆. Photolysis of C₆H₅CH₂Mn(CO)₅ in pentane solution produces Mn₂(CO)₁₀, toluene and bibenzyl. Analogous photodegradation of C₆H₅Mn(CO)₅ in pentane solution yields Mn₂(CO)₁₀, benzene and carbon monoxide, but not biphenyl. The thermally unstable complex C₂H₅Mn(CO)₅ was studied by photolyzing it in solution at ?40°C. GC analysis indicates that both ethylene and ethane are formed, and that the mole ratio of these products is dependent on the initial concentration of C₂H₅Mn(CO)₅. These results are consistent with a β-hydrogen elimination mechanism for this reaction. Photolysis of CpMo(CO)₃CH₂C₆H₅ in pentane solution produces [CpMo(CO)₃]₂ and toluene, whereas photolysis of CpW(CO)₃CH₂C₆H₅ affords [CpW(CO)₃]₂, CpW(CO)₂(η³-benzyl), toluene, and a small amount (2%) of bibenzyl. When CpM(CO)₂(η₃-benzyl) (M = Mo, W) complexes are subjected to photolysis under similar conditions, the only identifiable product is toluene. CpW(CO)₃C₆H₅ degrades photochemically in pentane solution to form [CpW(CO)₃)₂ and benzene, together with a small amount (6%) of biphenyl.     

Electrochemical behavior of decyl mercuric halides at mercury cathodes in dimethylformamide

Polarograms for decyl mercuric halides in dimethylformamide containing tetraalkylammonium perchlorates exhibit two waves. When large-scale electrolyses of decyl mercuric halides are performed at potentials corresponding to the first polarographic wave, the couldometric n value is unity and didecylmercury is obtained in quantitative yield; electrolyses carried out at potentials on the plateau of the second polarographic wave afford only decane and the n value is essentially 2. Double-potential-step chronocoulometry and staircase voltammetry indicate that, at potentials corresponding to the first polarographic wave, the decyl mercuric halide (which is itself adsorbed onto mercury to the extent of less than a monolayer) undergoes reversible one-electron reduction to adsorbed decyl mercury radicals and to adsorbed decyl mercury radical “polymer”; the adsorbed radicals have a lifetime of approximately 10^?3s and disproportionate into didecylmercury and elemental mercury. In the presence of electrolytically released halide ion, the adsorbed radicals are reoxidized to the decyl mercuric halide; alternately, the adsorbed species are reoxidized to decyl mercury cations at a potential approximately 600 mV more positive than that required for reoxidation to the decyl mercuric halide. At potentials corresponding to the second polarographic wave, reduction of decyl mercuric halides is an irreversible process producing decyl carbanions which are protonated by traces of water in the solvent to give decane.     

The anodic oxidation of fluorene and some of its derivatives: Conditions for the formation of a new conducting polymer

Quinones undergo a one-step two-electron reduction in molten acetamide at the mercury and platinum electrodes. The reversibility of reduction increases in the order benzoquinone < napthaquinone = phenanthrquinone < anthraquinone.     

Enthalpies of mixing of some aqueous electrolytes at 373.15 K

The enthalpies of mixing of a variety of aqueous electrolytes at an ionic strength of 1.000 mol·kg^?1 have been measured at 373.15 K. The mixtures studied were LiBr + KBr, NaBr + KBr, KBr + (C₄H₉)₄NBr, NaCl + Na₂SO₄, NaCl + MgCl₂, MgSO₄ + Na₂SO₄, MgCl₂ + MgSO₄, NaCl + MgSO₄, and MgCl₂ + Na₂SO₄. In contrast to most previous enthalpies of mixing of alkali halides, the mixtures with multivalent ions studied here often have large temperature dependences. The results for the mixtures involving MgSO₄ are consistent with a higher degree of ion pairing as the temperature increases. The present measurements, together with Gibbs energies of mixing at 298.15 K, allow the calculation of Gibbs energies of mixing at the higher temperature. This appears to be the most accurate and convenient method available for determining Gibbs energies of mixing at high temperatures.     

Electrochemical reduction of α-unsaturated carbonyl compounds: Part VI. Effect of electrogenerated bases in very weakly protic solvents

Unsaturated carbonyl compounds were reduced electrochemically in carefully dried aprotic solvents. In the absence of protonating impurities, the i—V curves could be modified due to the de-activation of a part of the substrate by the action of the electrogenerated bases. In DMF solutions, the electrogenerated base which is able to deprotonate the ketone is mainly the dianion and discrepancies occur with, classical polarograms, of this class of reactants as observed at the level of the second step. For more conjugated ketones, a very cathodic step is assumed to correspond to the reduction of the conjugated anion of the ketone but the instability of this anion (inducing polymerisation reactions) prevents the acquisition of careful analytical evidence.     

Polarographic studies on the electrode kinetics of the nickel(II) complexes with glycine and some of its derivatives: I. glycine complexes

D.C. polarograms of the Ni(II)-glycine complexes were measured with varying pH values at constant total glycine concentration (0.05 M) and conversely with varying total glycine concentration at constant pH values (3.5, 4.5, 7.5 and 9.5). The current—voltage curves obtained were analysed to determine the limiting currents, the transfer coefficients, and the half-wave potentials, by using the non-linear least squares method. From the dependence of the half-wave potentials on the free glycine concentration, the mechanism of the electrode processes was elucidated. It was shown that the first wave corresponded to the reduction of the Ni²⁺ aquo-complex, the second to the NiG⁺ complex, the third to the NiG₂ complex and the fourth to the NiG^?₃ complex. The kinetic parameters for the four charge transfer processes were determined.The second and third waves were kinetic in character and the rate constants of the dissociation and association reactions between NiG⁺ and NiG₂ and between NiG₂ and NiG^?₃ were determined by analysing the dependence of the kinetic limiting currents on the free glycine concentration.     

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 effect of pH on some outer-sphere electrode reactions at carbon electrodes

The apparent heterogeneous rate constant, k°_app, for three couples known to undergo outer-sphere electron transfer reactions has been investigated at carbon, gold, and platinum electrodes as a function of solution pH. The rates are found to be pH dependent at two different types of glassy carbon and at carbon paste but not at metallic electrodes. The rate is found to decrease with increasing pH for Fe(CN)^{$\text{3?}\text{4?}$}₆ and IrCl^{$\text{2?}\text{3?}$}₆, and the opposite effect is observed for the Ru(NH₃)^{$\text{3+}\text{2+}$}₆ couple. For the three compounds examined, the magnitude of the pH dependent rate changes can be associated with a change in the magnitude of the potential of the reaction layer induced by a change in surface charge of ～ 2 × 10^?7 C cm^?2. The pH dependence of the observed rate constant at glassy carbon can be attenuated by heat treatment, chemical reduction, or esterification of the surface. This behaviour is consistent with the theory that the pH dependence of k°_app arises from the presence of functional groups on the carbon surface.     

Thermal decomposition kinetics of p-toluenesulphonamide and some of its n-halogeno and n-metal derivatives

The TG, DTG and DTA traces of p-toluene sulphonamide, chloramine-T, dichloramine-T, dibromamine-T and the Ag and Hg derivatives of p-toluene sulphonamide are given and interpreted. The kinetic parameters (energies and entropies of activation and pre-exponential factors) for the main decomposition stages have been calculated.     

Polarographic reduction of oxygen dissolved in molten acetamide at 85°C

The reduction of oxygen in molten acetamide has been studied polarographically. The two waves observed are irreversible. The first wave has been analysed by the Koutecký procedure and the kinetic parameters calculated. The ECEC mechanism proposed for oxygen reduction from aqueous media holds good in this medium.     

Reduction of dicarbonyl compounds on a DME: Part VI. A comparative study of the electrochemical behaviour of aromatic dicarbonyl compounds

Polarographic and voltammetric studies have been carried out on the reduction of benzyl and acetylbenzoyl in a basic medium.The effect of pH, reactant concentration, drop time and ethanol content on the polarographic and kinetic parameters is shown. The occurrence of adsorption phenomena is inferred from C - E curves. The degree of adsorption decreases as the ethanol content increases. Tafel slopes and reaction orders have been obtained at potentials corresponding to the foot of the wave. On the basis of these experimental data, reduction mechanisms are proposed in the zone where Tafel's law is obeyed.The results obtained here and those corresponding to the acidic medium, together with those concerning phenylglyoxal and phenylglyoxylic acid reported in the literature, are thoroughly discussed. A parallelism in the electrochemical behaviour of these compounds is made evident.