R. Meunier-Prest

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

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 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).

Theory of EC1, EC2, EC1E, EEC1 and irreversible surface reactions in polarography

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.

The reduction mechanism of the >CO group: Part 1. The electrochemical reduction of methyl isonicotinate in an aqueous medium

Abstract The theory of EC 1 , EC 2 , EC 1 E, EEC 1 (or EEC 1 E) reactions, as well as that for a slow electron transfer is presented for a reduction in the case of polarography in the following conditions: (a) all the species are strongly adsorbed and the adsorption is fast (at equilibrium); (b) the coverage is smaller than unity, all the molecules of the oxidized form reaching the electrode are instantaneously adsorbed and all the reaction products remain attached to the electrode; (c) adsorption obeys a Langmuir isotherm. Expressions for the polarogram, the half-wave potential and the wave height (for the EC 1 E reaction), are derived. They can be used to diagnose the reaction type, and to calculate the electrochemical or chemical rate constants. The kinetic diagram corresponding to each case is given. The results obtained for the EEC 1 , (EEC 1 E) reaction can be extended to the case of heterogeneous electrochemical reaction with a “volume” C 1 reaction.

Electrochemical reactions with protonations at equilibrium: Part 14. The cubic scheme

Abstract The electrochemical reduction of methyl isonicotinate NRCO(OMe) and its protonated form H + NRCO(OMe) has been examined in an aqueous medium between H 0 = 0 and pH 6.7. As shown by cyclic voltammetry and polarography, a global 2e − , 2H + reversible transfer is followed by two successive first-order chemical reactions and a 2e − , 2H + reduction (EC 1 C 2 E process). A mechanism is proposed, beginning with the 2e − , 2H + reduction of YRCO(OMe) (where Y is N or H + N) to the formal diionized forms YRC θ (OH 2 ) + (OMe) or YRC θ (OH)(OHMe) + , where the first chemical reaction C 1 is an internal proton transfer which yields the hemiacetal YRCH(OH)(OMe); this is analogous to the internal proton transfer occurring in the case of nitronic acids in acidic media. The variations of the rate constant of this process with pH (5.2 × 10 2 s −1 k 3 s −1 ) are analyzed. The second chemical reaction C 2 , which is much slower (rate constant k d −3 s −1 ), involves the loss of a MeOH molecule (analogous to a dehydration) to give 4-pyridine aldehyde, which is itself easier to reduce than 4-methyl isonicotinate.

The reduction mechanism of the >CO group —III. The electrochemical reduction of isonicotinic acid in an aqueous medium

Abstract A theoretical study of the 1e − , 1H + square scheme is presented for the case when the molecule possesses an electro-inactive protonation site other than the electro-active site. The results are valid for a surface reaction (adsorbed species, Langmuir isotherm) or for a heterogeneous electrochemical reaction (semi-infinite linear diffusion) with protonations taking place in solution near the electrode. It is assumed, as in the other papers of this series, that the protonations are fast (at equilibrium) and that the symmetry factors of the electrochemical reactions are equal to O.5. The reaction can be described by using a cubic scheme consisting of two planar square schemes linked by protonations. The cubic scheme can itself be preceded and/or followed by a protonation. The mathematical treatment shows that it is globally equivalent to a simple monoelectronic reaction, with an apparent rate constant k 2app , which depends on the p K a and the pH, and an apparent standard potential E r2 . The graph log k 2app = ƒ(pH) consists of rectilinear segments whose slopes depend on the number of protonations and deprotonations taking place before or after the electrochemical step. It can be shown very generally that, for a reduction, a prior protonation or posterior deprotonation gives a slope of −1 2 , whereas a prior deprotonation or a posterior protonation gives a slope of +1 2 . The reverse is true for an oxidation. The effects of successive reactions are additive, so that slopes of 0, ±1 2 , ±1, ±3 2 etc. can be obtained. Study of the graph log k 2app = ƒ(pH), with the help of the graph E r2 = ƒ(pH), allows the reaction sequence to be determined. The reaction sequence can be very complex, and the p K a values must be carefully assessed. The reaction path for the electro-active group can be determined by distinguishing the perturbations produced by the protonation of the electro-inactive site.

The E-DIM reaction with adsorption of the reactants surface vs. volume chemical reaction

The electrochemical reduction of isonicotinic acid NRCOOH and its protonated from H+NRCOOH has been examined in an aqueous medium between H0 = 0 and pH 6.7. As shown by cyclic voltammetry and polarography, a global 2e−, 2H+ reversible transfer is followed by two successive first-order chemical reactions and a 2e−, 2H+ reduction (EC1C2E process). A mechanism is proposed, beginning with the 2e−, 2H+ reduction of YRCOOH (where Y is N or H+N) to the formal diionized forms YRC−(OH)(OH2+), where the first chemical reaction C1 is an internal proton transfer which yields the hydrate YRCH(OH)2 this reaction, which is analogous to the internal proton transfer taking place in the case of nitronic acids in acidic medium has been shown previously to occur in the case of methyl isonicotinate. The rate constant of this process is 2 × 102 s−1 in neutral medium and 2 × 104 s−1 in acidic medium. The second chemical reaction C2 (dehydration), which is much slower (rate constant kd < 1.6 × 10−3 s−1), involves the loss of a H2O molecule to give 4-formylpyridine, which is itself easier to reduce than isonicotinic acid. The variations of the total current and the dehydration constant with pH are analyzed in detail, and compared to those observed for 4-formylpyridine.

The electrochemical reduction of 1,2-di( 4-pyridyl) ethylene in an aqueous medium

Abstract A theoretical study of the E-DIM reaction with adsorption of the reactants is presented. The electrochemical reaction E can have a heterogeneous (E h ) or surface (E s ) character, whereas the dimerization can occur in a volume near the electrode (DIM v ) or on its surface (DIM s ). Four paths are possible: E h DIM v , E h DIM s , E s DIM h or E s DIM s . The dependence of the variations of the characteristic parameters (e.g. E 1/2 in polarography or RDE voltammetry, E p in LSV), the measurement time (τ, RT/Fv ), the concentration c T or the surface concentration Г T is different in each case, and can thus be used to determine the nature and the sequence of the paths and to calculate the dimerization reaction rate constant.