Voltammetric investigation of the anodic dimerization of p-halogenoanilines in DMF: Reactivity of their electrogenerated cation radicals

The electrodimerization mechanisms of p-chloro- and p-bromoaniline were studied in unbuffered DMF medium. By combined application of conventional and fast voltammetry (100 kV s^?1 range), the primary radical cation intermediates, formed by the one electron oxidation of each p-halogenoaniline were characterized. The overall reaction path involves a dimerization via a N–C bond formation and de-halogenation at the para position. A detailed mechanistic investigation demonstrates that this proceeds through a fast reversible deprotonation of the primary radical cation followed by the subsequent N–C bond formation between the resulting radical and its parent radical cation which is the rate-determining step of the sequence (e-p-RRC-p kinetic sequence). The effect of a relatively strong, but weakly nucleophilic base, 2,6-lutidine, has been also investigated and confirmed the involvement of the fast deprotonation pre-equilibria. The fast voltammetric experiments were simulated and the apparent rate constants for the overall deprotonation/dimerization sequence obtained on the basis of peak potential shift analysis were thus confirmed.     

Studies of the behavior of 5-hydroxyindole-3-acetamide at a solid electrode

The electrochemical oxidation of 5-hydroxyindole-3-acetamide has been studied in phosphate buffers in the pH range 1.9–9.9 at a pyrolytic graphite electrode on the basis of voltammetric, spectral changes and product characterization. The oxidation of 5-hydroxyindole-3-acetamide occurred in a single well-defined peak. The initial electrode reaction has been proposed to involve a 1e^?, 1H⁺ oxidation step at the –OH group to give the corresponding free radical, which resonates, at different positions in the ring. On the basis of these results, a probable oxidation route of 5-hydroxyindole-3-acetamide to various O–O and C–C linked hydroxylated dimers, which may be toxic in nature, has been proposed. It has been concluded that during the oxidation of 5-hydroxyindole-3-acetamide, hydrolysis also occurs to give corresponding acetic acid.     

Monodispersed redox submicrometer particles created by polyaniline-coated polystyrene latex

A spherical electroactive particle, 65 nm in diameter, was synthesized by coating a sulfonated polystyrene latex particle with the electric conducting polymer (polyaniline). It was suspended in acidic solution, and hence its electrochemical properties were obtained by the usual electrochemical techniques. The following behavior is predicted: when the particle comes into contact with the electrode by diffusion, the polyaniline layer takes the same potential as the electrode owing to the electric conduction; consequently, the whole polyaniline layer reacts at the electrode simultaneously as if the number of electrons transferred was very large. The anodic voltammetric peak, due to the oxidation of the radical cation to the dication, was controlled by diffusion of the particle. The diffusion coefficient obtained by the limiting current was half the value evaluated from the Stokes–Einstein relation. The steady-state voltammogram at a microelectrode and a rotating disk electrode showed a ?0.2 V potential shift from the surface waves of the electrochemically synthesized polyaniline film, probably due to stabilization of the radical cation in the colloidal form. The number of the charge relevant to the electrode reaction was n=4.2×10⁵, determined not only by the bulk electrolysis, but also by the analysis of chronoamperometric curves. The overall process is diffusion of the particle composed of n redox sites, followed by n parallel sluggish charge-transfer reactions.     

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

The electrochemical oxidation mechanism of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) is studied in a 0.1 M N(C₄H₉)₄ClO₄ + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential bulk electrolysis at platinum electrodes. The primary radical cation formed by the one electron oxidation of DPMB undergoes a deprotonation process, which is the rate-determining step, followed by a radical–radical coupling. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s^?1 and 40 Hz, respectively. Diffusion coefficients of DPMB at different temperatures were calculated from the quasi-reversible convoluted cyclic voltammograms. DigiSim® and COOL software were used to fit the quasi-reversible cyclic and square wave voltammetric responses, respectively. Formal potentials, formal rate constants and positive transfer coefficients at different temperatures were evaluated from the fitting of cyclic voltammograms. The experimental activation parameters were also determined. The effects of the analytical concentration of the reagent and the temperature, as well as the addition of trifluoracetic acid and a strong base such as lutidine on the electrochemical responses are discussed. A general reaction mechanism as well as probable structures for dimeric products are proposed.Besides, the presence of an acid–base equilibrium in DPMB solutions is also studied by employing UV–Vis spectroscopic measurements at different trifluoracetic acid concentrations. An apparent value of (1.5 ± 0.2) × 10³ M^?1 was estimated for the DPMB basic constant at 20.0 °C     

Amperometric determination of optically active 1-phenylethanol using chiral nitroxyl radical-modified polypyrrole films prepared by electrochemical polymerization

Polymeric chiral nitroxyl radical catalysts were prepared on an electrode surface by electrochemical polymerization of (6S, 7R, 10R)-2,2,7-trimethyl-10-isopropyl-1-azaspiro[5.5]undecane-1-yloxyl precursors containing a pyrrole side chain. This modified electrode gave a reversible electron transfer for the nitroxyl radical/oxoammonium ion redox couple in cyclic voltammetry at +0.72 V vs. Ag|AgCl. The modified electrode was used for electrocatalytic oxidation of (R)-(+)- and (S)-(?)-1-phenylethanol as an optically active sec-alcohol. Cyclic voltammetric studies showed that the catalytic current for the oxidation of (S)-(?)-1-phenylethanol is greatly enhanced as compared with a small enhancement in the oxidation current for the (R)-isomer. The (S)-isomer can be detected selectively in a mixture of (R)-(+)- and (S)-(?)-1-phenylethanol, even in the presence of an excess amount of (R)-isomer.     

Electrochemical reduction of phenyl-substituted cyclopentadienes: a case of an ‘indirect father–son’ self-protonation process

The electrochemical reduction of 1,2,3,4-tetraphenyl-1,3-cyclopentadiene and 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene has been investigated in DMF. At low temperatures (≤−30°C) and under anhydrous conditions cyclic voltammetry experiments indicate that both substrates are reducible in two successive, chemically reversible, one-electron steps, affording the corresponding radical anions and dianions. More complex voltammetric behaviour is detected at higher temperatures, when the radical anions are protonated by the substrate itself, giving rise, via the so-called ‘father–son’ self-protonation process, to further reducible compounds generating additional basic intermediates. In exhaustive electrolyses, 1/3 of the starting substrate is converted into dihydroreduction products and 2/3 into its conjugate bases. On the other hand, the quantitative formation of the dihydroreduction products is observed when exhaustive electrolyses are carried out in the presence of phenol, which acts as the proton donor instead of the substrate. Under voltammetric conditions, however, evidence of deprotonation of the substrate by the phenoxide anions thus generated is obtained, indicating the occurrence of an ‘indirect father–son’ self-protonation process, which takes place since phenol is kinetically more acidic, but thermodynamically less acidic, than the substrate itself. The mechanisms of decay of the radical anions are proposed on the basis of the comparison of experimental and simulated voltammetric data; this approach allowed also the determination of the characteristic kinetic rate constants, some of which are strongly dependent on the structure of the substrate, and of the importance of the homoconjugation reaction involving phenol and phenoxide anion. The stereochemistry of the dihydroreduction process and the voltammetric behaviour of the corresponding products are considered also, even in relation to the voltammetric behaviour of the starting substrates.     

Cyclic voltammetric studies of alumina supported monometallic Pt and bimetallic PtSn catalysts using carbon paste electrodes

The present work relates to the cyclic voltammetric studies of alumina supported monometallic Pt and bimetallic PtSn catalysts using solid carbon paste electrodes. The cyclic voltammetric data of the solid catalysts, which were reproducible, were in agreement with those of electro-deposited Pt and PtSn electrode systems. The reversibility of the H-adsorption–desorption system in representative 0.3 Pt–Al₂O₃ monometallic catalyst was confirmed by the difference between the anodic and cathodic peak potentials. The unit ratio of the cathodic to anodic peak currents observed, indicated the equality of the transfer coefficient in the kinetics for the respective processes. The O₂-adsorption–desorption reaction was found to be irreversible as expected. The lower area of the curve for H-adsorption–desorption in the case of bimetallic catalysts gives an indication for the formation of alloys. The use of a carbon paste electrode in studying the cyclic voltammetric behavior of heterogenous catalysts yielded qualitative information about the activity of the catalysts. Good correlations were found between cyclic voltammetric parameters and activity results in both monometallic and bimetallic catalysts.     

Voltammetry in the presence of ultrasound: surface and solution processes in the sonovoltammetric reduction of nitrobenzene at glassy carbon and gold electrodes

The application of power ultrasound in electrochemistry may promote or usefully modify electrode reactions. In the work reported here the four-electron reduction of nitrobenzene in alkaline (pH 13) aqueous media was studied as a model system. The electrochemical reduction is known to follow a complex mechanism [E. Laviron, A. Vallat and R. Meunier-Prest, J. Electroanal. Chem., 379 (1994) 427], involving protonations as well as a dehydration step; both surface and solution pathways for this reduction may be observed [C. Nishihara and H. Shindo, J. Electroanal. Chem., 221 (1987) 245], depending on the nature and state of the electrode. Both under silent and ultrasonic conditions nitrobenzene is reduced on glassy carbon electrodes in a chemically reversible one-electron process followed by an irreversible three-electron reduction. At sufficiently negative potentials the reduction process remains overall four-electron, with phenylhydroxylamine as the major product even at the high current densities obtained with intense ultrasound. Glassy carbon electrodes are shown to be suitable for kinetic studies, although damage, as endorsed by an increase in roughness and capacitance and probably initiated by mechanical damage at very short electrode–horn distances, was detected by a.c. impedance, voltammetric and atomic force microscopy (AFM) methods. On gold electrodes a more complicated mechanism due to a surface reaction pathway arises. A comparison of sonovoltammetric and rotating disk voltammetric results gives evidence for the homogeneous pathway being dominant under applied ultrasound conditions. The transition between surface and solution pathways is mass flux as well as concentration dependent, and an estimate for the rate of the surface catalyzed reaction for a concentration of 1.44 mM nitrobenzene (k = 4±2 × 10^?2 cms^?1), attributed to protonation of the nitrobenzene radical anion, is obtained from combined rotating disk and sonovoltammetric data. Damage to the gold surface as monitored by various techniques is small, but manifests itself by an interesting decrease in double layer capacitance.     

Dicyanomethylene derivatives of squaric acid: electrochemical behaviour and ESR investigation

The electrochemical behaviour of mono- and bisdicyanomethylene-substituted squaric acid dianion was investigated in dimethylformamide and acetonitrile on a Pt electrode. In cyclic voltammetry the oxidation of these dianions involves two successive, chemically reversible, one-electron processes, affording the corresponding radical anions and neutral species. The redox potentials are strongly dependent upon the number of dicyanomethylene substituents. Whereas the neutral species are stable only in the time scale of the voltammetric experiments, the radical anions can be accumulated by macroscale electrolysis. The ESR spectra of these radicals are reported. The monosubstituted dianion is protonated by electrochemical oxidation of hydrogen, whereas the disubstituted ones are not. Compounds obtained from the disubstituted dianions and the methyl viologen dication are also described.     

Electrocatalytic activity of a cobalt hexacyanoferrate modified glassy carbon electrode toward ascorbic acid oxidation

A cobalt hexacyanoferrate (CoHCF) modified glassy carbon (CoHCF/GC) electrode was prepared electrochemically. The voltammetric responses of CoHCF are stable and the electrochemical behaviour is related to the concentrations of supporting electrolyte and counterions. The CoHCF/GC electrode shows electrocatalytic activity toward the oxidation of ascorbic acid in phosphate buffer solution. The electrocatalytic rate constant of the CoHCF/GC electrode for the oxidation of ascorbic acid is determined using rotating disk electrode measurements.     

Electrocatalytic oxidation of methanol at a nickel hydroxide/glassy carbon modified electrode in alkaline medium

The modified nickel hydroxide/glassy carbon electrode (MNGC) shows a stable voltammetric response even when it is stored under dry conditions. The modified electrode catalyses methanol oxidation in alkaline medium via Ni³⁺ species (mainly NiOOH). The mechanism of methanol oxidation changes from diffusion control at low methanol concentration to a catalytic reaction at higher methanol concentration. The effects of both scan rate and methanol concentration on the anodic peak height as well as current decay measurements demonstrate that methanol oxidation starts as NiOOH is formed on the electrode surface.     

Calculation of cyclic voltammetric responses for the catalytic reduction of substrates via intramolecular electron transfer within a catalyst-substrate complex formed on electrode surfaces

Cyclic voltammetric responses are calculated for the case where the reduction of a substrate proceeds only via intramolecular electron transfer to the substrate molecule when it is bound to reduced catalyst molecules present in a single monolayer on the electrode surface. The dependences of the voltammetric responses on the equilibrium constant for the formation of the catalyst-substrate complex, the rate of its formation and the rate of intramolecular electron transfer are discussed. Procedures are described for estimating the rate constant for the intramolecular electron transfer reaction and for discriminating between mechanistic possibilities.     

Cyclic voltammetric study of the disproportionation reaction of the nitro radical anion from 4-nitroimidazole in protic media

We have chosen 4-nitroimidazole (4-NIm) as a prototype of nitroimidazolic compounds in order to carry out an exhaustive cyclic voltammetric study in protic media, 0.1 M Britton–Robinson buffer+0.3 M KCl with ethanol as a co-solvent. The one electron reduction of 4-NIm in protic media at alkaline pH produces a stable nitro radical anion on the time scale of the cyclic voltammetric technique. The nitro radical anion decays according to a coupled chemical reaction and we have focused the study to follow this reaction using cyclic voltammetric methodology. The one electron reduction of the 4-NIm to form the nitro radical anion and the subsequent reaction of the radical obey an EC₂ mechanism that follows very well the cyclic voltammetry theory for the disproportionation reactions previously described by Olmstead and Nicholson (Anal. Chem. 41 (1969) 862). We have obtained the disproportionation rate constant k₂ which is strongly dependent on both pH and ethanol content according to the following regression equations: log k₂=?0.932pH+12.771 and log(10^-3k₂)=?1.998log[% EtOH]+3.873. The results obtained from this study in protic media differ substantially from previous studies in aprotic media wherein the nitro radical anion was not stabilized.     

Electrochemistry of some novel hole transport materials

Several charge transfer agents have been investigated using cyclic voltammetry. We report multistep mechanisms involving both chemical and electrochemical steps, for the three compounds in the study. The overall mechanism consists of two quasi-reversible electron transfer events, each followed by a chemical step, an ECEC mechanism. The first cyclic voltammetric wave for each compound was studied separately. After the second quasi-reversible electron transfer, a third oxidation wave is observed under conditions of high concentration and slower scan rates. This is attributed to direct oxidation of the product of the second chemical step, which is identified as oligomer formation. This hypothesis is consistent with the physically observed formation of a film on the surface of the electrode, presumably from electrooxidative oligomerization of the compound.     

Electrochemical oxidation in different media of bis(p-phenylene crown ether) as a symmetrical molecule bearing two identical redox centers

The oxidation of the long chain polyether p-disubstituted benzene derivative (BO5O5) and the related macrocyclic paracyclophane (BBO5O5) have been investigated by cyclic voltammetry at different concentrations (10⁻³–3×10⁻² M) and potential scan rates (0.05–250 V s⁻¹). At about 10⁻³ M in CH₃CN or CH₂Cl₂ containing a perchlorate salt as the supporting electrolyte, BO5O5 was oxidized reversibly into moderately stable radical cations. The formal potential E°′ corresponding to this first oxidation step was found to be shifted towards more positive values when the cation was changed from Bu₄N⁺ to Na⁺, and this variation may be ascribed to the coordination of the alkali cation by the polyether chains. At higher concentrations of the reactant and in a thoroughly dried medium, no deposit of a polymeric material onto the electrode surface was electrogenerated. At low scan rates and under the same conditions, the oxidation of BBO5O5 gave rise to two closely spaced, irreversible processes. The radical cations involved in each step were thought to be highly reactive towards nucleophilic species present in solution. At high scan rates, a single reversible one-electron transfer was observed, the formal potential of which varied with the nature of the electrolyte cation in the same way as for BO5O5. The heterogeneous charge-transfer rate constants estimated for BO5O5 and BBO5O5 were compared with the values predicted by the Marcus theory. As for BO5O5, attempts to electropolymerize BBO5O5 were unsuccessful.     

Electrochemical determination of naltrexone on the surface of glassy carbon electrode modified with Nafion-doped carbon nanoparticles: Application to determinations in pharmaceutical and clinical preparations

A new sensitive and selective electrochemical sensor was developed for determination of naltrexone (NAL) in pharmaceutical dosage form and human plasma. Naltrexone is an opioid antagonist which is commonly used for the treatment of narcotic addiction and alcohol dependence. A voltammetric study of naltrexone has been carried out at the surface of glassy carbon electrode (GCE) modified with Nafion-doped carbon nanoparticles (CNPs). The electrochemical oxidation of naltrexone was investigated by cyclic and differential pulse voltammetric techniques. The dependence of peak currents and potentials on pH, concentration and the potential scan rate was investigated. The electrode characterization by electrochemical methods and atomic force microscopy (AFM) showed that CNPs enhanced the electroactive surface area and accelerated the rate of electron transfer. Application of the modified electrode resulted in a sensitivity enhancement of more than 20 times, relative to the bare GCE, in detection of NAL and a considerable negative shift in peak potential was achieved. Two linear dynamic ranges of 1–10 μM and 10–100 μM with a detection limit of 0.1 μM was obtained in phosphate buffer of pH = 3. Differential pulse voltammetry as a simple, rapid, sensitive and selective method was developed for the determination of NAL in dosage form and human plasma without any treatments. No electroactive interferences were found in biological fluids from the endogenous substances and additives present in capsules.     

Gold nanoparticle arrays for the voltammetric sensing of dopamine

Gold (Au) nanoparticles immobilized on an amine-terminated self-assembled monolayer (SAM) on a polycrystalline Au electrode were successfully used for the selective determination of dopamine (DA) in the presence of ascorbate (AA). Well-separated voltammetric peaks were observed for DA and AA at the nano-Au electrode (Au nanoparticle-immobilized electrode). The oxidation potential of AA is shifted to less positive potential due to the high catalytic activity of Au nanoparticle. The reversibility of the electrode reaction of DA is significantly improved at the nano-Au electrode, which results in a large increase in the square-wave voltammetric peak current with a detection limit of 0.13 μM. The coexistence of a large excess of AA does not interfere with the voltammetric sensing of DA. The nano-Au electrode shows excellent sensitivity, good selectivity and antifouling properties.     

Integration analysis of the cyclic voltammograms of the electrode reaction in a diffusionless system

A method of integration analysis of the cyclic voltammogram of the electrode reaction in a diffusionless system is presented. The direct results of the integration analysis are the concentrations of the oxidized and reduced form of the electroactive substances, the forward and backward reaction rates. From these two rates, electrochemical parameters can be estimated according to Butler–Volmer kinetics. The method presented can be applied to the cases in which the cyclic voltammetric oxidation–reduction peak potential separation is greater than only 20 mV. The application of the method to the experiment was satisfactory.     

Galvanostatic and potentiostatic fluorination of 2-indanone, 1-indanone and 1,3-indandione in Et3N · 4HF medium. Adsorption effects on yield and product selectivity

Selective electrochemical fluorination (SEF) of 1-indanone, 2-indanone and 1,3-indandione were carried out in Et₃N · 4HF ionic liquid. Cyclic voltammetric measurements indicated that the ionic liquid undergoes oxidation below 2 V on platinum electrode while the background limit is extended up to 2.3 V on glassy carbon electrode. Well defined voltammetric peaks were observed for all the three compounds on glassy carbon electrode. Significant absorption effects were also noticed. Preparative electrolysis indicated lower selectivity for all the three compounds under galvanostatic conditions when compared to potentiostatic conditions. Both under galvanostatic and potentiostatic conditions, 2-indanone, which exhibits weak adsorption, gave 1-fluoro-2-indanone with high selectivity. 1-Indanone exhibits higher anodic oxidation potential around 2 V. 1,3-Indandione exhibited strong intermediate or product adsorption during voltammetric measurements. The yield and selectivity were much lower for these two compounds under galvanostatic conditions. Quite interestingly in both these cases, the selectivity close to 70% for the monofluorinated compound could be achieved under potentiostatic conditions by passing up to 6 F/mole of electric charge. Prevention of formation of over oxidation products under potentiostatic conditions may be the main cause for this improvement as suggested by chronoamperometric measurements.