Maria Gabriella Severin

Self-protonation effects in the electrochemical reduction mechanism of p-nitrobenzoic acid

Abstract The electrochemical reduction mechanism of p -nitrobenzoic acid (NBA) in DMF has been investigated by cyclic voltammetry, polarography and controlled potential macroscale electrolysis. The stoichiometry of the reaction occurring at the first cathodic wave involves 0.8 Faraday and yields 0.2 moles of p -hydroxylaminobenzoic acid and 0.8 moles of the conjugate base of NBA, per mole of substrate consumed. This overall reaction is consistent with a mechanism involving four electron steps and four proton transfers from NBA to its basic reduction intermediates (self-protonation mechanism). Comparison with the voltammetric behaviour of the methyl ester of NBA supports the mechanism and allows to single out the species responsible for the successive reduction waves of NBA. The kinetic analysis of the self-protonation process in the conditions of cyclic voltammetry, allows to assign the rate determining steps of the overall reaction and to evaluate the rate constant of the proton transfer from NBA to the anion radical formed by reversible one electron transfer to the latter.

Kinetic analysis of a slow electron transfer coupled with a father—son reaction

Abstract The electrode reaction mechanism involving interaction of the products of a slow, rate-determining electron transfer with the parent molecule (father—son reaction) has been examined theoretically under the conditions of cyclic voltammetry. The kinetic analysis indicates how to determine the correct values of the kinetic parameters for both the heterogeneous charge transfer and the homogeneous chemical step. The irreversible cathodic reaction of diphenylmethylphenylsulphide in anhydrous DMF provides a good example of this mechanism, since the diphenylmethyl carbanion resulting from the irreversible two-electron reduction undergoes proton transfer from the parent molecule (self-protonation mechanism).

Study on a proton transfer reaction between electrogenerated carbon bases and parent nitrogen acids

Abstract The electroreduction of secondary 2-bromoisobutyramides in dipolar aprotic solvent affords carbanions in an overall two electron process. The fast proton transfer between the electrogenerated base and the NH acid starting material (self-protonation) has been studied by cyclic voltammetry at the mercury electrode. Measurements of the second-order rate constant of the self-protonation step have been performed at 0°C for a series of ten bromoamides. The results are discussed on the basis of the effect of the substituent at nitrogen on the acid—base properties of both α-carbon and nitrogen atoms.

Bimolecular kinetics according to a stochastic analysis of reactant dynamics

A stochastic representation of the reversible bimolecular process A+B⇌AB is introduced on the basis of the nearest-neighbor distribution. It leads to a description of the reactant pair dynamics under the action of its interaction potential, without introducing any boundary condition or sink function. In this way it becomes evident that reaction processes are particular manifestations of the molecular dynamics. The analysis of the eigenvalues of the time evolution operator allows one to identify the conditions for a well-defined time scale separation between the slow kinetic processes and the fast equilibration of the unbound pair. Correspondingly the rate equations for the reversible bimolecular kinetics are recovered from the long time behavior of the nearest-neighbor distribution. By means of asymptotic methods, analytical approximations are derived for the rate coefficients and their concentration dependence. This allows a simple rationalization of the effects of the interaction potential between the r...

Protonation kinetics of anionic intermediates in the electrochemical reduction of triphenylethylene: a disproportionation mechanism

The voltammetric behaviour of triphenylethylene (T) in dimethylformamide in the presence of traces of proton donor indicates that the radical anion (T–˙) is stable, the dianion (T2–) is rapidly protonated, and the resulting carbanion (TH–) is a relatively strong base. In the presence of added water, T–˙ decays via disproportionation followed by protonation of T2–. This was indicated by both homogeneous and voltammetric kinetics. In fact, in homogeneous conditions, the rate of decay is second-order in [T–˙], enhanced by water, and inhibited by T. Voltammetric kinetics further support the proposed mechanism showing that, under particular conditions, the reaction is rate-limited by the disproportionation step. Alternative mechanisms can be disregarded on the basis of both kinetic and thermodynamic data.

AN ELECTROCHEMICAL DETERMINATION OF RELATIVE PKA OF WEAK ACIDS IN N,N-DIMETHYLFORMAMIDE

The self-protonation reaction undergone by α-bromoamides upon electroreduction in N,N-dimethylformamide is employed to get relative pKa data on a series of NH acids. The results are compared with the pKas of the same acids in dimethylsulfoxide.

Role of the carboxy proton in heterogeneous electron transfer to o-nitrobenzoic acid

Heterogeneous electron transfer (ET) to o-nitrobenzoic acid has been investigated in NN-dimethylformamide containing tetrabutylammonium perchlorate. Although primary anion radicals undergo fast homogeneous decay, voltammetric measurements allowed the ET rate constant to be determined at various temperatures. These rate constants are abnormally low for an aromatic nitro derivative, even ortho-substituted, as also shown by comparison with the ET rate constant for methyl o-nitrobenzoate, which is at least two orders of magnitude higher. However, the e.s.r. spectrum demonstrates the presence of a hydrogen bond in o-nitrobenzoic acid anion radical, which is not present in the parent molecule, showing that the ET process entails significant structural change. The latter rather than solvent reorganisation around the charge of the anion-radical, localised by the ortho effect, is believed to provide the major contribution to the charge-transfer activation free energy.

Compared selectivities of redox-catalyzed and direct electrochemical processes Part 2. Reactions in which product selection involves competition between dimerization and deactivation followed by further reduction (or oxidation)

Abstract Possible changes in selectivity between redox-catalyzed processes and direct electrochemical reactions are discussed for a typical reaction scheme where product selection involves competition between a dimerization step and a reduction (or oxidation) pathway in which the reductant (or oxidant) is the catalyst on the one hand and the intermediate resulting from the first electron transfer on the other. This second reduction (or oxidation) pathway may or may not be controlled by the transformation of the first electron transfer intermediate into a second intermediate that undergoes electron transfer. In the former case, changes in selectivity between the two types of electrolysis conditions arise solely from the space dependence of the reactant and intermediate concentrations. They usually favor the reduction (or oxidation) pathway in a redox-catalyzed process rather than the direct reaction. In the converse situation where the transformation of the first electron transfer intermediate is reversible and acts as a pre-equilibrium to the second electron transfer, one source of possible changes in selectivity also results from space dependence of the reactant and intermediate concentrations. However, it tends to play an opposite role, favoring the reduction (or oxidation) pathway in the direct reaction rather than the redox-catalyzed process. This effect is amplified if, as is usually the case, the catalyst is a weaker reductant (or oxidant) than the first electron transfer intermediate of the direct electrochemical reaction.

Electrochemical reduction mechanism of rotenone. Part 2. Effect of proton donors

The effect of proton donors on the electrochemical reduction mechanism of rotenone (I) in acetonitrile and on the yield and nature of the electrolysis products has been examined. In the presence of acetic acid one-electron reduction of the carbonyl group leads to the formation of the neutral ketyl radical which dimerizes to the corresponding pinacol (a new derivative in the rotenoid series) as established by 13C n.m.r. analysis. On the basis of 1H n.m.r. spectra a trans-12a-H, 12-OH configuration is proposed for this compound. If phenol is used as proton donor, rotenol (or its reduction products) is obtained as well as the pinacol. The latter can be formed through keto-enol isomerization of the ketyl radical and/or rotenone itself catalysed by the conjugated base of the proton donor which is formed during the electrolysis.

Decay of electrogenerated dianions of acidic indenes: an ‘indirect’ self-protonation mechanism

The electrode reduction of 1,2,3-triphenylindene, giving rise to the corresponding radical anion and dianion in two successive one-electron steps, was investigated in dimethylformamide with the aim of clarifying the role of the acidic substrate in the decay of the electrogenerated bases. At –50 °C the direct protonation of the radical anion by indene (self-protonation) was too slow to be effective, and the proton transfer from indene to the dianion did appear kinetically unfavourable with respect to solution electron transfer between the same species (comproportionation). However, dianion protonation by an excess of water present in the solution can induce, through the action of OH– ions, the substrate to transform into its conjugate base. The effect resembles that predicted for a self-protonation mechanism and can be termed an ‘indirect’ self-protonation process. The proposed reaction scheme was confirmed by comparison of cyclic voltammetric responses with those derived by mathematical computation. The rate-constant values for the relevant steps, obtained by the fitting of experimental and computed data, are consistent with the corresponding values determined for similar compounds.