Study of the electrochemical reduction of dioxygen in acetonitrile in the presence of weak acids

The electrochemical reduction of dioxygen has been studied in acetonitrile at glassy-carbon electrodes. The initial step is the reversible one-electron reduction to form superoxide. In the presence of hydrogen-bond donors (water, methanol, 2-propanol), the superoxide forms a complex with the donor resulting in a positive shift in the potential that can be analyzed to obtain formation constants for these complexes. Stronger acids result in protonation of the superoxide followed by reduction to produce HO2-. In the absence of hydrogen-bond donors, the reduction of superoxide occurs at very negative potentials, and this second reduction peak is very much drawn-out along the potential axis, indicating a small value of the transfer coefficient, alpha. The addition of hydrogen-bond donors, HA, brings about a positive shift in this peak, without a noticeable change in shape. The reaction occurring at the second peak is a concerted proton and electron transfer (CPET) in which the electron is transferred to superoxide and a proton is transferred from HA to the superoxide, forming HO2- and A- in a concerted process. An estimation of the standard potential for this reaction shows that the second reduction always occurs at a high driving force, which explains the small value of alpha that is observed. Consistent with a CPET, a kinetic isotope effect, HA versus DA, was detected for the three hydrogen-bond donors. The increasing positive shift of the second peak with increasing water concentration has been interpreted as being a consequence of the change in the formal potential, as water is both a reactant in the process and a participant through the hydrogen-bond stabilization of the anions.     

The mechanism of electrochemical reduction of tricarbonylchromium-tropylium tetrafluoroborate in acetonitrile

The mechanism of electrochemical reduction of tricarbonylchromium-tropylium tetrafluoborate has been investigated in anhydrous MeCN together with that of the free tropylium tetrafluoborate. The bi-(cycloheptatrienyl), free and bis complexed with Cr(CO)₃ is the reduction product of the macroscale electrolysis of the free and complexed cations respectively. However, the mechanism of formation of the two dimers is different: the free dimer is formed through a radical-cation coupling mechanism; in constrast, the complexed dimer is produced by the coupling of the tricarbonylchromiumcycloheptatrienyl anion with the starting cation.     

The electrochemical reduction mechanism of trivalent chromium in the presence of formic acid

The electrochemical reduction process of trivalent Cr in the presence of formic acid is studied. The compositions of Cr complexes in electrolytes and products on cathode are investigated using ultraviolet–visible absorption spectroscopy (UV–Vis) and X-ray photoelectron spectroscopy (XPS), respectively. The geometric structures of the original and transition state ions during the electroreduction process are optimized, using density functional theory with General Gradient Approximation/Perdew-Wang 91 (GGA/PW91) calculation. The trivalent Cr primarily exists in the form of [Cr(H₂O)₆]³⁺ in the solution. [Cr(H₂O)₆]³⁺ exhibits regular-octahedron structure which is unfavorable for center Cr³⁺ to contact cathodic electrons. In the presence of formic acid, formate ion promotes the formation of the reactive intermediate, [Cr(H₂O)₄CHOO]²⁺, which possesses irregular-octahedron structure with Cr³⁺ ion as a vertex. In this case, Cr³⁺ ion can contact the cathode and then obtain electrons easily.     

Application of electrochemical impedance spectroscopy for characterisation of the reduction of benzophenone in acetonitrile solutions

In this study, the reduction of benzophenone (Bzph) Ph₂C=O (Ph: phenyl group) on glassy-carbon electrode was studied in acetonitrile by means of cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Bzph undergoes two one-electron reductions. The first reduction leads to the formation of radical anion [Ph₂·–O]^? and appears to be reversible and diffusion controlled. The second reduction results in the generation of benzhydrol dianion [Ph₂C–O]^2? and seems to be irreversible. A third quasi-reversible wave observable at more anodic potentials can be ascribed to benzhydrol free radical [Ph₂CH–O] and benzydrol anion [Ph₂CH–O]^? redox couple. The EIS spectra demonstrate that the first reversible reduction of Bzph is characterised by the lowest charge transfer resistance while the resistance for the irreversible reduction is significantly greater. The electrochemical behaviour of Bzph on film consisting of multi-walled carbon nanotubes seems to be different. Thus, the findings reveal slower electrode kinetics which can be associated with electrode passivation.     

Keto-enol tautomerism in the electrochemical reduction of parabanic acid

Convolution potential sweep voltammetry has been applied to the electrochemical reduction of parabanic acid in the 10–100 V/s scan rate range. The overall reduction corresponds to a quasireversible two-electron, two-proton transfer to give an enediol intermediate compound. This compound undergoes an enol-keto transformation in a following homogeneous chemical reaction. This chemical reaction appears to be acid-base catalyzed in the 2 M-4×10⁻³ M H⁺ concentration range. The catalytic and electrochemical constants were determined.     

Effects of proton donors on the electrochemical reduction of benzo[c]cinnoline in acetonitrile

The polarographic behaviour of benzo[c]cinnoline in acetonitrile in the presence of various proton donors (water, phenol, benzoic acid and perchloric acid) is reported. In aprotic medium benzo[c]cinnoline is reduced in two one-electron waves, followed by one two-electron wave. The most probable final reduction product is 2,2′-diaminobiphenyl. The addition of proton donors causes a shift and/or appearance of new polarographic waves, which can be related to the reduction of various protonated species.     

A mechanism for the hydrazinolysis of benzoic acid in the presence of ion-exchange catalyst

A mechanism for the hydrazinolysis of benzoic acid via cyclic transition states is proposed on the basis of kinetic and IR spectroscopic studies.     

Deuteriation of benzoic acid in the presence of homogeneous potassium tetrachloroplatinate(II) catalyst

The kinetic behavior of deuteriation of benzoic acid with D₂O acidified with HCl in the presence of homogeneous K₂PtCl₄ catalyst has been investigated in the 100–130°C temperature interval. The quasiunimolecular H/D rate constants at 100 and 130°C corresponding to an exchange process in ortho positions of the substituted benzene ring hydrogens were determined by¹H NMR integration signal. These same constants for meta and para positions have been deduced by analysis of the composite¹H NMR signal, and the Arrhenius activation energies for these exchange reactions were estimated.     

Esterification of allylic alcohols with benzoic acid in the presence of diethyl azodicarboxylate and triphenylphosphine

Stereoselectivity of esterification reaction of some allylic alcohols, mainly unsaturated pyranosides, with benzoic acid in the presence of diethyl azodicarboxylate and triphenyl phosphine was investigated. The reaction was found to proceed with inversion, without allylic rearrangement.     

On the chemical and electrochemical one-electron reduction of peroxynitrous acid

Peroxynitrous acid was reduced by cathodic linear sweep voltammetry at a gold electrode and by iodide at pH 3.2 and 5.6. The cathodic reduction wave was identified by measuring its decay in time, which was the same as observed by optical spectroscopy. The iodide oxidation was followed by optical measurement of the triiodide formation. Both reductions show one-electron stoichiometry, with the product n(alpha)alpha = 0.23 +/- 0.04 from the electrochemical experiments, in which alpha is the transfer coefficient and n(alpha) the number of electrons transferred, and an diiodine yield of ca. 0.5 equiv per equivalent of peroxynitrous acid. The voltammetric reduction was irreversible up to scan rates of 80 V s(-1). Both reductions were pH independent in the range studied. The voltammetric reduction is most likely an irreversible elemental reaction followed by a chemical decay that cannot be observed directly. Because of the pH independence, we conclude that both reductions have a common short-lived intermediate, namely [HOONO]*-. We estimate the electrode potential of the likely ONOOH/ONOOH*- couple to be larger than 1 V. The commonly used electrode potential E degrees (ONOOH, H+/NO2*, H2O) does not describe the chemistry of peroxynitrous acid.     

The electrochemical reduction of CP2TiCl2 in the presence of trimethylphosphine

The electrochemistry of the complexes Cp₂TiCl_x(PMe₃)_2-x in anhydrous THF has been studied. The most stable complexes at the three oxidation states of titanium are Cp₂TiCl₂, Cp₂TiCl(PMe₃) and Cp₂Ti(PMe₃)₂, and each of these species is readily formed by electrolysis. It has also been demonstrated that oxidation/reduction of these species is followed by facile and rapid ligand exchange to form the preferred species in the new oxidation state provided a stoichiometric concentration of the required ligand is present. The consequences of this redox and ligand exchange chemistry for the synthetic reactions catalyzed by lower oxidation states of Ti are discussed. Finally, the voltammetry of a titanocycle is reported, and it is shown that the corresponding Ti^III metallocycle is stable.     

Electrochemical reduction of substituted pyrimidines in acetonitrile

Polarographic studies of several substituted pyrimidines were reinforced by the results from cyclic voltammetry, controlled-potential electrolysis, and spectrophotometric examination of electrolyzed solutions, as well as by the examination of model compounds. Pyrimidines substituted with non-reducible groups (amino, methyl) are reduced in a single, one-electron (1e), diffusion-controlled process, very similar to that for pyrimidine itself. Pyrimidine-4-carboxylic acid exhibits three reduction waves: a very drawn-out acid-reduction wave with unusual properties and at more negative potential, an adsorption prewave and a wave corresponding to the 1e reduction of the pyrimidine moiety, 2-Chloro-and 2-bromopyrimidine each exhibit two polarographic waves; the first, corresponding to irreversible scission of the carbon-halogen bond, has electrochemical properties quite different from those normally expected; the second is due to reduction of the electro-generated pyrimidine. Results are compared with those for the reduction of bromobutane, bromobenene, and 2-bromopyridine.     

Anomalous behavior in the two-step reduction of quinones in acetonitrile

The electrochemical reduction of eight quinones, 9,10-anthraquinone (1), duroquinone (2), 2,6-di-tert-butyl-1,4-benzoquinone (3), 2,6-dimethoxy-1,4-benzoquinone (4), 9,10-phenanthrenequinone (5), tetrachloro-1,2-benzoquinone (6), tetrabromo-1,2-benzoquinone (7) and 3,5-di-tert-butyl-1,2-benzoquinone (8), have been studied in acetonitrile. In every case it was found that cyclic voltammograms differed in significant ways from those expected for simple stepwise reduction of the quinone to its radical anion and dianion. The various types of deviations for the eight quinones have been cataloged and some speculation is offered concerning their origins.