Dennis H. Evans

Electrochemical oxidation of 2,4,6-tri-tert-butylphenol

Abstract The electrochemical oxidation of 2,4,6-tri- tert -butylphenol as well as the phenoxide and phenoxy radical derived from it has been investigated in acetonitrile and ethanol+water. The ease of oxidation decreases in the order phenoxide, phenoxy radical, phenol with the separation between potentials for phenoxide oxidation and phenoxy radical oxidation being 1.2 V in acetonitrile. The phenoxide is oxidized to the stable phenoxy radical in a highly reversible reaction in acetonitrile and alkaline ethanol+water. Oxidation of the radical produces a phenoxonium ion which is attacked by water giving 2,4,6-tri- tert -butyl-4-hydroxy-2,5-cyclohexadienone. This two electron product is also formed upon oxidation of the phenol in either solvent. However, in acidic media the hydroxydienone dealkylates give 2,6-di- tert -butylhydroquinone which is oxidized to the final product 2,6-di- tert -butyl-1,4-benzoquinone. The dealkylation is quite rapid in anhydrous acetonitrile but the rate is depressed by the addition of water. A novel double potential step experiment was used to characterize the oxidation of the phenoxy radical. A step to a potential where the phenoxide is oxidized to the phenoxy radical is followed by a step to a potential where the phenoxy radical is oxidized. The current during the second step is unusually small because protons produced by the oxidation of the radical deactivate the phenoxide. The current-time curve was found to agree with that predicted by digital simulation.

Use of microelectrodes for the study of a fast chemical step in an electrode reaction

Abstract Cyclic voltammetry at a platinum microdisk electrode with a 5 μm radius has been employed to determine the rate constant for the conversion of the B form of 1,1′-dimethylbianthrone to the A form. Scan rates from 200 to 5000 V/s were employed and the rate constant was found to be 500 s−1 at 361 K in dimethylformamide containing 0.60 M tetraethylammonium perchlorate. The measurements were carried out without a potentiostat and using a two-electrode configuration with a platinum counterelectrode as pseudo-reference. It was experimentally determined that solution iR drop and counterelectrode polarization were negligible up to scan rates of 8000 V/s.

Cyclic voltammetric study of tert-nitrobutane reduction in acetonitrile at mercury and platinum electrodes: Observation of a potential dependent electrochemical transfer coefficient and the influence of the electrolyte cation on the rate constant

Abstract The reduction of tert-nitrobutane in acetonitrile has been studied by cyclic voltammetry. By the fit of data to theoretical voltammograms obtained from digital simulations, the electrochemical transfer coefficient, α , was found to be potential dependent at mercury and platinum. This result was confirmed by potential step experiments. The potential dependence of α is discussed according to Marcus theory of electron transfer and compared to the measurements of others. In addition, the electrochemical rate constant, k s , was found to depend on the size of the electrolyte cation. The rate constant decreased as the size of the tetraalkylammonium ion in the electrolyte increased. It is speculated that this effect may be partly a double layer effect resulting from differing planes of closest approach for different electrolyte cations.

Reversible voltammetric response for a molecule containing four non-equivalent redox sites with application to cytochrome c3 of Desulfovibrio vulgaris, strain Miyazaki

Abstract The cyclic voltammetric and differential pulse polarographic behavior of cytochrome c 3 of Desulfovibrio vulgaris , strain Miyazaki, has been evaluated in terms of a model employing four reversible redox centers. Both types of experiments can be fit by digital simulations using the four standard potentials: E 1 0 =−0.467, E 2 0 =−0.519, E 3 0 =−0.539 and E 4 0 =−0.580 V vs. SCE. The results are interpreted to mean that the four redox centers are chemically different and only weakly interacting. The relationships between the observed macroscopic standard potentials and the microscopic standard potentials for reduction of individual sites are discussed.

Rate constants for the electrode reactions of some quinones in aprotic media at platinum, gold and mercury electrodes

Apparent values of heterogeneous electron transfer rate constants for reduction of 1,4-benzoquinone in DMF at platinum, gold and mercury electrodes have been measured by cyclic voltammetry and found to be much larger than reported earlier. Results were also obtained for benzoquinone in acetonitrile and for the reduction of 1,4-naphthoquinone, 9,10-anthraquinone and 3,5-di-tert-butyl-1,2-benzoquinone in DMF at mercury. The apparent rate constants for all the quinones with the various metals and solvents employed fall in the range of 0.1–1.3 cm s−1 which is only slightly lower than the values typically found for simple electron transfer reactions in nonaqueous solvents.

Heterogeneous electron transfer kinetics for a variety of organic electrode reactions at the mercury-acetonitrile interface using either tetraethylammonium perchlorate or tetraheptylammonium perchlorate electrolyte

Abstract The electrode reactions of 26 compounds have been studied by cyclic voltammetry at mercury electrodes using acetonitrile as solvent and either 0.10 M tetraethylammonium perchlorate (TEAP) or 0.10 M tetra-n-heptylammonium perchlorate (THpAP) as supporting electrolyte. The standard potentials of the compounds ranged from −0.07 to −2.90 V vs. the Ag/AgNO 3 reference electrode. In several of the redox couples studied, the charge on the ionic partner was localized on a few atoms which is expected to cause the outer reorganization energy to be high, giving small electron transfer rate constants. However, is some instances it does not appear that the degree of charge localization is great enough to account fully for the small rate constants. It is argued that the inner reorganization energy should not be ignored in such cases. The rate constants observed for THpAP were always smaller than those for TEAP when the values were small enough to measure in at least one electrolyte. This general result was explained by a suppression of the probability of electron transfer by an adsorbed film containing mainly tetra-n-heptylammonium ions. The interpretation was supported by measurement of the activation parameters for nitroethane in the two electrolytes. The activation enthalpies were almost identical but the preexponential factor for THpAP was only 1/44 that for TEAP.

A study of aldehyde oxidation at glassy carbon, mercury, copper, silver, gold and nickel anodes

Abstract The rate, potential and mechanism of the anodic oxidation of aliphatic aldehydes have been found to be highly dependent on solution conditions and electrode material. Aldehyde oxidations in neutral acetonitrile on glassy carbon occur at very positive potentials (ca. +3 V vs. SCE) and the peak potentials correlate with the ionization potentials of the aldehydes. In aqueous base, aldehyde oxidation is assisted by reversible addition of hydroxide to the carbonyl group to form electroactive gem-diolate (II). Oxidations of aldehydes in aqueous base on Hg, Ni, Ag and Au all yield the corresponding carboxylate via two-electron oxidation plus aldol and Cannizzaro byproducts and the oxidations occur at potentials far negative of the unassisted oxidation in neutral acetonitrile. On Ni, Cu and possibly Hg the oxidation involves the formation of a metal oxide which acts as a chemical oxidizing agent. On Ag and Au the oxidations take place on a surface which is not covered by a phase oxide. A mechanism involving a direct electrochemical process with oxidation of gem-diolate adsorbed on an oxide-free metal surface is proposed. A pulsed electrolysis technique was utilized to circumvent deactivation of Ag and Au electrodes during electrolysis and preparation of an “aurized” gold surface with a much slower deactivation rate is described.

Effect of metal ions on the electrochemical reduction of benzil in non-aqueous solvents

The reduction of benzil has been studied in N,N-dimethylformamide (DMF), dimethyl-sulfoxide (DMSO) and acetonitrile (AN) in the presence of group IA and IIA metal ions. In DMF, the first voltammetric peak for the reversible, one-electron reduction of benzil obtained with tetra-n-butylammonium perchlorate as electrolyte, was observed to shift toward positive potentials as potassium, sodium or lithium perchlorate was added due to rapid, reversible ion pair formation between the metal cation and benzil radical anion. The ion pair formation constants were 2, 7.8 and 42 M−1 for K+, Na+, and Li+ respectively. n nIn the absence of metal ions, benzil radical anion is reduced in an irreversible, one-electron step which occurs about 1 V negative of the first peak. When K+, Na+, or Li+ (<30 mM LiClO4) was added, two new peaks appeared between the original reduction peaks for benzil. The more positive of these disappeared at rapid scan rates and was attributed to the reduction of an ion triplet containing two cations per radical anion. The more negative peak was assigned to the reduction of the ion pair. Values of K2 k−21/2 (where K2 is the ion triplet formation constant and k−2 is the rate constant for ion triplet dissociation) are 380, 300 and 4100 M−1 s−1/2 for K+, Na+, and Li+ respectively. n nIn the presence of barium, strontium, calcium and magnesium perchlorates, benzil was reduced in a single, two-electron process. For the first three metals, an anodic peak was observed in the cyclic voltammograms. Double potential step chronoamperometry showed that both the reduction and return oxidation were two-electron, diffusion controlled processes. The shapes of the voltammograms and their dependence on scan rate were best explained by a reaction scheme involving reversible reduction to the radical anion, rapid ion pair formation, and quasi-reversible reduction of the ion pair with the standard potential for ion pair reduction being slightly positive of that for the reduction of benzil to the radical anion. The reduction in the presence of magnesium perchlorate was a completely irreversible, two-electron process at all scan rates employed. n nThe effect of metal ions on the reduction in DMSO was very similar to that observed in DMF but in AN the ion pairing interactions were much stronger. In the presence of sodium perchlorate in AN, benzil was reduced in two closely spaced, reversible, one-electron steps, first to the sodium—benzil radical anion ion pair, then to a species containing two sodium ions per benzil dianion.

Infrared studies of quinone radical anions and dianions generated by flow-cell electrolysis

Infrared spectra of the anion radicals and dianions of a series of 1,4-benzoquinones, 1,4-naphthoquinones and 9,10-anthraquinones have been obtained. The characteristic carbonyl stretching frequency decreases an average of 155 cm−1 upon going from quinone to radical anion for six benzoquinones and an additional decrease of about 125–150 cm−1 was observed upon going from radical anion to dianion for naphthoquinones and simple anthraquinones. Dependence of the carbonyl frequency upon substituents for radical anions of the benzoquinones parallels that observed for the parent quinones. A special flow cell for electrolytic generation of the radical anions and dianions was constructed and evaluated. Radical anions could be detected at concentrations as low as 1 mM.

Reversible dimerization of phenoxyl radicals formed by anodic oxidation of phenolates. A quantitative study by cyclic voltammetry

Abstract The anodic oxidation of 2,6-di-t-butyl-4-methylphenolate (IId), as well as the 4-ethyl (IIc) and 4-n-butyl (IIe) derivatives, has been studied by cyclic voltammetry in acetonitrile with a platinum disk electrode. The initially formed phenoxyl radicals (III) dimerize reversibly to form the para-quinol ethers and the reaction has been characterized quantitatively by cyclic voltammetry. The values of kf1/2K−3/4 are 0.20, 0.23 and 0.6 s−1/2 (mol/l)1/4 for IIIc-e respectively where kf is the dimerization rate constant and K is the dimarization equilibrium constant. Values of K have been estimated from peak potential measurements.