E. Laviron

Electrochemical reactions with protonations at equilibrium: Part X. The kinetics of the p-benzoquinone/hydroquinone couple on a platinum electrode

Abstract The kinetics of the p-benzoquinone/hydroquinone Q/QH2 couple on a platinum electrode are analysed on the basis of the theory presented earlier (E. Laviron, J. Electroanal. Chem., 146 (1983) 15) for the nine-member square scheme when the protonations are assumed to be at equilibrium, using experimental data from the literature. The square scheme is of the NN type. The Tafel plots and the variations of the experimental apparent rate constants between pH 0 and 7 are in good agreement with the theoretical predictions. The heterogeneous rate constants found for the elemental electrochemical steps are as follow: Q Q−, kh3=1/6×10−3 cm s−1; QH.QH−, kh5=0.11 cm s−1; QH+QH., kh2⋍160 cm s−1; kh4 for the reaction QH2+.QH2 is in the range 0.5–4 cm s−1. Between pH 0 and 7, the reaction sequence during the reduction is, for the most part, successively H+e−H+e−, e−H+H+ e−, and e−H+e−H+ (reverse sequence during the oxidation).

Electrochemical studies of organometallic compounds : I. On the reversibility of the first reduction stage of titanocene dichloride

The 1e reduction of titanocene dichloride (η5-Cp2TiCl2) in various solvents has been examined in detail by polarography, voltammetry on a disk electrode, and linear potential sweep voltammetry. In all cases, the electron uptake is followed by a fast loss of Cl−, which is replaced by a neutral ligand (which can be a solvent molecule). In tetrahydrofuran, whose binding ability is weak, a pseudo reversible behaviour is observed due to a return of Cl− to the molecule. In dimethylformamide, which is strongly bonded, the reaction is irreversible. In pyridine, it can be reversible or irreversible according to the conditions.

The reduction mechanism of aromatic nitro compounds in aqueous medium: Part I. Reduction to dihydroxylamines between pH 0 and 5

Abstract The reduction of aromatic nitro compounds, RNO 2 , to their dihydroxylamines, RN(OH) 2 , was studied by cyclic voltammetry. It proceeds via electron-proton exchanges (nine-member square scheme). Between pH 0 and 5, the slow step is the reduction of the radical RNO 2 H . to the dihydroxylamine, first according to a H + e − (RNO 2 H . → RNO 2 H + 2 → RN(OH) 2 ) then to an e − H + (RNO 2 H . → RNO 2 H − → RN(OH) 2 ) process. The global path between pH 0 and 5 is probably first e − H + H + e − , then e − H + e − H + .

The reduction mechanism of aromatic nitro compounds in aqueous medium: Part V. The reduction of nitrosobenzene between pH 0.4 and 13

Abstract The electrochemical reduction of nitrosobenzene to phenylhydroxylamine has been examined in aqueous medium between pH 0.4 and 13, by polarography and by cyclic voltammetry. The results are analyzed using the theory of the 9-member square scheme with protonations at equilibrium (E. Laviron, J. Electroanal. Chem., 146 (1983) 15; R. Meunier-Prest and E. Laviron, J. Electroanal. Chem., 328 (1992) 33). A study of the variations of the apparent heterogeneous and surface rate constants shows that the sequences of addition of the electrons and protons are successively H + e − H + e − , e − H + H + e − and e − H + e − H + . The values of the elementary surface electrochemical rate constants deduced from our results are of the order of 10 9 s −1 , i.e. of the order of magnitude predicted by Brown and Anson (J. Electroanal. Chem., 92 (1978) 133). The elementary heterogeneous rate constants are much higher than predicted by the theory of the square scheme, which can be attributed to an increase in the apparent reversibility, owing to the occurrence of the ‘surface’ path, parallel to the heterogeneous path (cf. E. Laviron, J. Electroanal. Chem., 124 (1981) 19). The global 4e − reduction of an aromatic nitro compound to the corresponding hydroxylamine takes place via two successive 9-member square schemes linked by the dehydration of the intermediate dihydroxylamine; the reaction paths are determined.

The reduction mechanism of aromatic nitro compounds in aqueous medium. Part IV. The reduction of p-nitrobenzophenone between H0 = −5 and pH 14

Abstract The electrochemical reduction of 4-nitrobenzophenone RNO2 has been examined in aqueous medium on a Hg electrode between H0 = −5 and pH 14, by polarography and cyclic voltammetry. It occurs in three main steps: 2e− reduction to dihydroxylamine, which dehydrates to give the nitroso compound, itself reducible (2e−) to the hydroxylamino form. The results were analyzed using the theory of the nine-member square scheme with fast protonations [E. Laviron, J. Electroanal. Chem., 146 (1983) 15]. The phenomena observed constitute an epitome of those occurring during the reduction of organic compounds in aqueous medium, as far as two main factors, adsorption and protonations, are concerned. In acidic medium (pH

The use of polarography and cyclic voltammetry for the study of redox systems with adsorption of the reactants. Heterogeneous vs. surface path

1), the first 2e− reduction appears as heterogeneous, although it occurs via the adsorbed species. The apparent heterogeneous electrochemical rate constant is about 105 larger than the elementary rate constants. For pH & > 1, adsorption is much stronger, and the reactions are of a purely surface nature, in polarography and a fortiori for any value of the sweep rate in cyclic voltammetry. The calculated values of the elementary surface electrochemical constants, 7.8 × 108 s−1, are of the order of magnitude predicted by Brown and Ansons formula. The first 2e− stage is controlled by the electrochemical reaction up to H0 ≈ −2, and by dehydration for H0(pH) larger than about 0. The order of addition of the electrons and protons at each pH is determined. The global 4e− reduction is of the ECE type up to pH ≈ 10. A novel feature of the reduction of aromatic nitro compounds in alkaline medium has been brought out; basically a 1e− reversible wave, followed by a 3e− irreversible wave, should be observed. However, the influence of the dehydration causes the two waves to merge, producing a single 4e− wave (EECE process).

Electrochemical reactions with protonations at equilibrium: Part XIII. Experimental study of the homogeneous electron exchange in quinone/dihydroquinone systems

The use of polarography and linear-sweep voltammetry (LSV) for the study of a redox reaction O + ne ⇔ R when both O and R can be adsorbed (Langmuir isotherm) is examined, on the basis of a rigorous theoretical treatment presented earlier for a rotating disk electrode (r.d.e.) (E. Laviron, J. Electroanal. Chem., 124 (1981) 19 and J. Electroanal. Chem., 140 (1982) 247). In aqueous medium on a mercury electrode, the reaction practically always occurs via the adsorbed species (surface redox reaction). However, two cases can be distinguished, according to whether the rate of desorption of the product of the reaction (in polarography) or of the adsorbed reactant (in LSV) is large or small when compared with the duration of the measurement (τ in polarography, RTnFv in voltammetry). In the first case, the reaction appears as heterogeneous, with an apparent rate constant khm, which is much larger than the normal constant kh, and which can be determined by using the classical theories for a heterogeneous reaction. In the second case, the reaction has a “surface” character, and the electrochemical surface rate constant ks can be determined by using the appropriate theories. The domain for each reaction can be represented by using adsorption diagrams lotτ or logv vs. log(bObR)12 (bO, bR; adsorption coefficients). The advantages of using polarography and cyclic voltammetry rather than r.d.e. voltammetry for the study of the above systems are discussed; they are theoretical (non-steady-state nature of the methods) as well as experimental (use of the dropping mercury electrode).

The reduction mechanism of aromatic nitro compounds in aqueous medium Part 3. The reduction of 4-nitropyridine-N-oxide between H0 = −6 and pH 9

Abstract The rate of the electron (and proton) exchange reaction , where BQ and BQH 2 are benzoquinones and dihydrobenzoquinone, and XQ and XQH 2 their corresponding monosubstituted forms (substituents CH 3 , C 6 H 5 , or Cl) is studied as a function of pH. A W-shaped curve is obtained for log k f and log k b , whose segments have slopes of ±1 (or 2); the curves parallel that predicted for log k app ( k app being the apparent isotopic rate constant for the system BQ + BQH 2 ⇄ BQH 2 + BQ) on the basis of the theory of the homogeneous nine-member scheme with fast protonations (E. Laviron, J. Electroanal. Chem., 169 (1984) 29) and that of the homogeneous e − – e − exchange (E. Laviron, J. Electroanal. Chem., 148 (1983) 1). The sequence of proton and electron exchange is discussed in detail. It can be concluded in particular that when the pH increases (from H 0 = −2 to pH 7) the global rate for the BQ/BQH 2 system is controlled successively by the reproportionation reaction BQH + + BQH 2 ⇄ BQH + BQH + 2 , by the isotopic reactions BQ + BQ − ⇄ BQ − + BQ and BQH 2 + BQH + 2 ⇄ BQH + 2 + BQH 2 , and by the reproportionation reactions BQH − + BQ ⇄ BQH + BQ − and BQ 2− + BQ ⇆ 2 BQ − .

Electrochemical reactions with protonations at equilibrium: Part XV. The 2e−, 2H+ bi-cubic scheme

Abstract The electrochemical reduction of 4-nitropyridine has been examined in aqueous medium between H 0 = −6 and pH 9.6, by polarography and by cyclic voltammetry. It occurs in three main steps: (i) the nitro compound is reduced (2e − ) to the dihydroxylamine; (ii) the dihydroxylamine dehydrates to give the nitroso compound; (iii) the nitroso compound is reduced (2e − ) to the hydroxylamine. Analysis of the first 2e − stage, using the theory of the bicubic scheme with fast protonations as discussed by Meunier-Prest and Laviron (J. Electroanal. Chem. 328 (1992) 33) leads to the conclusion that the addition of the second electron is rate determining. A study of the variations in the apparent heterogeneous rate constant, from H 0 = −6 to pH 2, and of those of the surface rate constant, from pH 4 to 9.6, allows the sequence of addition of the electrons and protons to be determined; this sequence is complex, because of the protonation of the pyridine nitrogen, but it can be established that, on the electroactive site proper, the sequence is e − H + e − H + for all acidity values. The global reaction is of the electrochemical-chemical-electrochemical (ECE) type, and as such, has been analysed in the framework of the theory of Nadjo and Saveant (J. Electroanal. Chem., 48 (1973) 113). The case of 4-nitropyridine is exemplary, because the figurative point can be situated in each of the four main zones of the kinetic diagram, according to the acidity. The mechanism established here for 4 -nitropyridine should be that of all aromatic nitro compounds. This excludes in particular a pre-protonation of the nitro group. The values of the elementary surface electrochemical rate constants deduced from our results are of the order of 10 9 s −1 , i.e. of the order of magnitude predicted by Brown and Anson (J. Electroanal. Chem., 92 (1978) 133). The elementary heterogeneous rate constants are much higher than predicted by the theory of the bicubic scheme, which can be attributed to an increase in the apparent reversibility, owing to the occurrence of the “surface” path, parallel to the heterogeneous path (cf. the work of Laviron J. Electroanal. Chem., 124 (1981) 19).

Electrochemical studies on organometallic compounds

A theoretical study of the 2e−, 2H+ reaction is presented for when the molecule possesses an electroinactive protonation site other than the electroactive site. The results are valid for a surface reaction (adsorbed redox system, Langmuir isotherm) or for a heterogeneous electrochemical reaction (semi-infinite linear diffusion) with protonations taking place in solution near the electrode. As in the other papers of this series, it is assumed that the protonations are fast (at equilibrium), that the symmetry factors of the electrochemical reactions are equal to 0.5, and that the disproportionation and dimerization reactions can be neglected. The reaction can be described by using a bi-cubic scheme, consisting of two cubes linked by one of their edges. In a certain pH range, the reaction appears as a direct 2e−, 2H+ reaction; the system behaves as a simple reaction with two successive electron exchanges. Two apparent electrochemical rate constants, which depend on the individual electrochemical rate constants, on the pH, and on the difference between the redox potentials can be defined. The graphs of the variations of the logarithm of the apparent rate constants are made up of successive rectilinear segments with slopes of 0, ± 14, ± 12, etc. The reaction path (order of addition of protons and electrons) can be very complex, and the analysis requires a careful examination of all the factors involved. The case where a further protonation takes place has been examined. The changes produced are analysed.