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The electrochemical oxidation of a series of 2,3,4,5-tetrasubstituted pyrroles was studied in acidic, non-buffered or basic acetonitrile media. The processes were investigated at platinum electrodes by rotating-disc electrode voltammetry, cyclic voltammetry and controlled potential electrolysis together with visible absorption and e.s.r. spectroscopy. The optimal conditions are given for anodic generation of cation radicals, dications, nitrenium ions and pyrryl radicals. These four oxidized species have been observed during oxidation of 2,5-di(p-dimethyl-2,5-di-tert-butyl-3,4-diethoxycarbonylpyrrole have been analyzed.

Resume Le systeme d'equations qui relient entre elles les concentrations des six especes qui peuvent coexister apres l'oxydation electrochimique d'un compose comportant un groupe -NH- est etabli. Il est resolu dans le cas particulier de la dimethoxy-3,7 phenothiazine et l'on montre que la mesure des concentrations des especes en presence apres une oxydation monoelectronique de ce compose permet de determiner la basicite du milieu dans lequel l'oxydation a ete realisee. Ce resultat est utilise a l'etablissement d'une echelle de basicites relatives dans l'acetonitrile et a l'estimation de p K A CH 3 CN qui n'etaient pas connus.

This paper is the second in a series devoted to the electrochemical oxidation of diphenylamines and of coupling products obtained from them. Anodic processes with 5,10-diaryl 5,10-dihydrophenazines and tetraarylhydrazines in acetonitrile are studied by voltammetry and coulometry. Absorption and epr spectra of the corresponding cation radicals are described. This study gives the opportunity to discuss an anodic substitution involving a di-cation as the reactive intermediate. Aged solutions of electrogenerated hydrazinium radicals in neutral media and the acidic degradation of tetraarylhydrazines give a similar mixture the composition of which depends upon the nature of the para substituant on the aromatic ring.

Modulated specular reflectance spectroscopy was employed to identify reaction intermediates and products from model compounds undergoing stepwise one-electron electrode reactions following the EE mechanism with “nuances” represented by equations: A ± e ⇄ E 1 o B , B ± e ⇄ E 2 o C , A + C ⇄ k b k f 2 B The occurrence of the homogeneous redox reaction is readily detected by optical monitoring of either the B or C species during the reverse pulse in a double potential step chronoamperometric experiment. With kf>105 M−1 s−1, the absorbance of B goes through a maximum at the beginning of the reverse pulse for the anodic oxidation of N,N,N′,N′-tetraphenyl p-phenylenediamine and of 2,3,4,5-tetra-p- anisylpyrrole following an EE process in 0.1 M Et4 NClO4+CH3CN. The shape of this maximum is dependent upon the kf value as shown by theoretical curves obtained by a simulation procedure. Moreover, this maximum in the absorbance of B is still observed when the species B nearly completely disappears by a reversible chemical reaction in an EECr mechanism, where Cr represents the acid-base equilibrium between the dication C and the corresponding deprotonated form. Examples are given from the anodic process of N-aryl p-phenylenediamines and of 3,7-dimethoxyphenothiazine in neutral CH3CN medium which were also investigated by rotating-disc electrode voltammetry, cyclic voltmmetry and controlled potential coulometry

Resume Les processus d'oxydation electrochimique des diphenylamines p . monobustituees en milieu acetonitrile sont examines a l'aide du trace des courbes voltamperometriques et de voltammetrie cyclique ainsi que par la realisation de coulometries a potentiel controle couplees avec l'enregistrement des spectres d'absorption visible et de rpe . Selon le niveau d'acido-basicite du milieu, le radical cation primaire evolue vers un ou deux des composes de doublement suivants: la N , N ′-diarylbenzidine, la diaryl-5,10 dihydro-5,10 phenazine, la N , N , N ′-triaryl p . phenylenediamine ou la tetraarylhydrazine. L'observation des deux derniers produits necessite l'utilisation d'un milieu basique. La N , N ′-diarylbenzidine resulte du couplage C p — C p de deux radicaux cations Ar 2 NH +. tandis que la tetraarylhydrazine provient de la dimerisation de deux radicaux Ar 2 N o en presence d'une base forte. La dihydrophenazine et la N , N , N ′-triaryl p . phenylenediamine resultent probablement d'un couplage C p — N entre une molecule Ar 2 NH et l'ion Ar 2 N + correspondant. Le comportement anodique des diphenylamines diversement substituees permet de preciser les conditions de formation d'une dihydrophenazine.

Resume Les proprietes electrochimiques du di p -anisylnitroxyde et de deux radicaux piperidiniques en milieu acetonitrile ont ete examinees a l'aide de la voltamperometrie a des electrodes a disque de platine poli et de carbone vitreux, de la voltammetrie cyclique et de la coulometrie a potentiel contrǒle. Les processus cathodiques dependent de la teneur du milieu en donneurs de protons et la reaction d'oxydation conduit a des ions oxoammonium stables. Les hydroxylamines aromatiques donnent deux vagues anodiques avec des intensites due courant limite dependant de la basicite du compose etudie. En milieu basique, l'electrolyse a potentiel contrǒle au niveau de la premiere vague necessite un electron par molecule oxydee et represente la meilleure methode de preparation electrohimique des radicaux nitroxydes. Les spectres d'absorption et de RPE sont egalement donnes.

The association constants of α, β and γ-cyclodextrin complexes involving iodide, tri-iodide, and iodine species were measured by potentiometry. The equilibrium potential variations of a platinum electrode were obtained versus the cyclodextrin concentration in a solution containing iodide, tri-iodide, and iodine species.

This paper gives the general feaures about the electrochemical oxidation of diphenylamines and the compounds derived from them in acetonitrile medium. The anodic processes of N,N′-diarylbenzidines are investigated by voltammetry with the rotating disc electrode and cyclic voltammetry on platinum in acidic, neutral and basic medium. During controlled potential electrolysis, uv and visible absorption and epr spectroscopy indicate an association phenomena for the cation radical of N,N′-diphenylbenzidine in neutral medium. A structure is inferred for a new radical species formed from this radical in the presence of 2,6-lutidine.

Resume Les proprietes electrochimiques de quelques couples quinone-hydroquinone (Q/QH2) ont ete etudiees dans les solvants acetonitrile at nitromethane a laide des courbes voltamperometriques et de voltametrie cyclique ainsi que par la realisation de coulometries a potentiel controle couplees avec lenregistrement des spectres dabsorption UV-visible et de RPE La voltammetrie cyclique montre que la reversibilite du couple redox Q/QH2 est tres dependante de la nature des substituants et de lacidite de la solution. Ce comportement est lie a la stabilite de la forme protonee QH+ dont la formation est caracterisee par spectrophotometrie dans le cas de certaines quinones portant des groupes methoxyle. Une etude plus detaillee de la dimthoxy-2,6 p-benzoquinone et de la tetramethoxy-3,3′,5,5′ p-biphe noquinone illustre ce role de QH+ dans les processus bielectroniques des couples Q/QH2 en presence de HClO4. Dans ces conditions, le second compose e fournit le premier exemple de systeme Q/QH2 thermodynamiquement reversible en milieu organique. Les radicaux QH+.2, detectes dans certains cas, paraissent resulter principalement de reactions redox se produisant en solution

Resume Cet article presente une etude detaillee de l'oxydation electrochimique d'une serie de diphenylamines p.p′-disubstituees en milieu acetonitrile neutre non tamponne ou en presence de bases de forces variees. Les processus sont examines par le trace des courbes voltamperometriques et de voltammetrie cyclique a l'electrode de platine poli ainsi que par la realisation d'electrolyses a potentiel controle couplees aux enregistrements des spectres d'absorption visible et RPE. La tetramethoxy-2,2′,4,4′ diphenylamine s'oxyde en un radical cation stable en milieu neutre tandis qu'en milieu lutidine-2,6 il se forme l'ion nitrenium moyennement stable dont l'evolution conduit a un compose a structure dihydrophenazinique qui a ete isole. Les modes de degradation des radicaux cations des diphenylamines di p.substituees dependent de la nature des substituants et de la force de la base utilisee. Les bases fortes conduisent aux tetraarylhydrazines tandis que les bases pyridiniques et parfois l'eau assurent une transformation en diaryldihydrophenazine.

Oxidation des Hydroxylamins (I) mit Bleitetraacetat in Methylenchlorid liefert, wie sich ESR-spektroskopisch nachweisen last, das Aminoxyl-π-Radikal (II), dessen Zersetzung zu p-Nitrodiphenylamin (III) sowie zum N-Oxid des (N-p-Nitrophenyl)-p-benzochinonimins (IV) fuhrt.