Allen J. Bard

The electrochemistry of organic compounds in aprotic solvents - methods and applications

The importance of aprotic solvents in studies of the electrochemistry of organic compounds is discussed and a general mechanism for the oxidation and reduction of aromatic hydrocarbons and related substances is presented. The effect of molecular structure on the electrochemical behaviour of organic compounds is illustrated by discussing the relationship between conformations of aryl-substituted anthracenes and dimethylaminophenyl-substituted ethylenes and their electrochemical properties. Techniques and methodology for electrochemical studies in nonaqueous solvents, including solution preparation and cell and electrode design, are briefly reviewed. INTRODUCTION The use of anhydrous aprotic solvents, such as N,N-dimethylformamide (DMF), acetonitrile (MeCN), and dichloromethane (CH2C12) as solvents for electrochemical studies started about fifteen years ago. Although many studies of the electrochemistry of organic compounds had been reported up to that time, most were carried out in partially or totally aqueous media. Indeed the second edition of Koithoff and Linganes Polarography has a chapter of only six pages on nonaqueous solvents and in a discussion of the polarography of organic compounds characterizes most reductions of these as irreversible. Although the earliest applications of aprotic solvents were aimed at increasing the solubility of the organic compounds, pioneering studies by Wawzonek, Hoytink, Peover, and their co-workers2 demonstrated that the use of these solvents simplified the electrochemical behaviour of organic substances by removing or decreasing the extent of chemical reactions coupled to the electron transfer step. Under these conditions many organic electrode reactions are electrochemically reversible so that correlations of electrode potentials with spectroscopic and molecular orbital (MO) data became possible and a molecular basis of electrochemical reactions of aromatic hydrocarbons was established. Moreover, aprotic solvents provided reduction and oxidation limits beyond those obtainable in aqueous solutions, so that a wider range of potentials became available for electrochemical studies. 379

Electrodeposition Techniques for Carbon Rod Flameless Atomic Absorption Analysis

Abstract Two techniques involving electrodeposition of Cu on a carbon rod (CR) prior to atomic absorption (AA) analysis are described. The in situ technique involves electrodeposition from ul samples contained in the well of the CR in an attempt to decrease matrix effects. The bulk electrolysis technique allows preconcentration of Cu on the CR from very dilute metal ion solutions. Analysis of Cu in solutions as dilute as 1.6 × 10−10 M (70 ml volume) appears possible.

Rotating Ring‐Disk Electrodes IV . Dimerization and Second Order ECE Reactions

Digital simulation techniques have been employed to compute the steadystate currents at the rotating ring-disk electrode (RRDE) as functions of a rate parameter exhibiting explicit dependence on concentration of parent species (A) and rotation rate and also as functions of the flux at the disk for those cases in which the disk generated product (B) undergoes a following dimerization EC reaction (2B -~ products) or a second order ECE reaction (B ~- A -* C, where C may be electroactive at the disk potential) and nuances thereof. Diagnostic criteria are given for using RRDE results to distinguish among possible reaction mechanisms and working curves are provided to allow calculation of the rate constants of the homogeneous reactions.

Electrohydrodimerization Reactions III . Rotating Ring‐Disk Electrode, Voltammetric and Coulometric Studies of Mixed Reductive Coupling of Dimethyl Fumarate in the Presence of Cinnamonitrile and Acrylonitrile in Dimethylformamide Solution

The electrohydrodimerization reaction of dimethyl fumarate in the presence of c innamonit r i le and acrylonitr i le was studied in dimethylformamide solution by rotating r ing-disk emctrode, voltammetric and coulometric techniques. At potentials where only dimethyl fumarate is electroactive, the rate and mechanism of decay of the dimethyl fumarate anion radical are only slightly per turbed from the results obtained in the absence of c innamoni t r i le and acrylonitrile. This is an indication that little or no cross-coupling was occurring. At potentials where both dimethyl fumarate and c innamoni t r i le are electroactive evidence for the occurrence of a solution oxidat ion-reduct ion reaction consuming c innamoni t r i le anion radical and dimethyl fumarate parent was obtained. In addition, the role of cis-trans isomerization of radical anions in electrohydrodimerization reactions is discussed. Differences in the cyclic vol tammograms of diethyl maleate in the presence of acrylonitr i le when compared with those obtained in the absence of acrylonitr i le and for dimethyl fumarate in the presence of acrylonitr i le are observed. We have previously reported (1) the use of rotat ing r ing-disk electrode (RRDE) voltammetry, cyclic vol tammetry, and coulometry in the elucidation of the mechanism of the electrohydrodimerization (electrolytic reductive coupling) of the three di-substi tuted olefins dimethyl fumarate (DF), c innamoni t r i le (CN), and fumaroni t r i le (FN) in anhydrous dimethylformamide (DMF) solution. Potent ia l -s tep chronoamperometry has also been employed to determine the electrohydrodimerization mechanism of diethyl fumarate (2). These studies showed that these hydrodimerizations proceed predominant ly via a one-electron reduction

Electrogenerated chemiluminescence. XII. Magnetic field effects on ECL in the tetracene—TMPD system; evidence for triplet—triplet annihilation of tetracene

Abstract The electrogenerated chemiluminescence (ECL) of the energy deficient system tetracene—N,N′-tetramethyl- p -phenylenediamine (TMPD) in 0.1 M TBAP—DMF solution shows an increase in intensity of up to 19% for magnetic fields of 7.5 kG. The results are interpreted as production of tetracene in the triplet state upon reaction of tetracene anion radical and TMPD cation radical followed by triplet—triplet annihilation.

Coulometric titration with electrogenerated +2 tin: Determination of iodine, bromine, and various oxidants via iodometry

The use of electrolytically generated +2 tin as a coulometric titrant has been studied. In a 3 to 4M sodium bromide and 0.2N hydrochloric acid solution, containing 0.2M stannie chloride, the generation of stannous ion is 100% efficient up to a current density of 80 mA/cm2. With this medium iodine and bromine can be titrated coulometrically with errors of about ±0.3% or less. The titration is well suited for the iodometric determination of strong oxidants. Employing stannous ion and bromine for direct and reverse titrations, quinone and hydroquinone have been titrated.

Resistive effects in thin electrochemical cells: Digital simulations of current and potential steps in thin layer electrochemical cells

Summary A digital simulation technique has been used to treat electrochemical reactions in thin layer electrochemical cells in which non-uniform current densities will result because of high electrolyte resistance Calculations have been applied to chronopotentiometric, chronoamperometric, and chronocoulometric experiments. Current densities and potentials as a function of the electrode length are given for a variety of conditions.

Electrogenerated Chemiluminescence XI . Electrochemistry and Electrogenerated Chemiluminescence in Scintillator Dye Melts

Melts of sc int i l la tor compounds which are solid at room tempera ture , such as 2 ,5-diphenyloxazole (PPO) and 2 ,5-diphenyloxadiazole (PPD) conta ining severa l qua te rna ry ammonium salts as suppor t ing electrolytes , were examined as solvents for e lec t rochemical and e lec t rogenera ted chemi luminescence (ECL) studies. Al though the resistance of these solutions was high, cyclic vo l t ammet ry of 9 ,10-diphenylanthracene and rubrene in PPO showed the product ion of s table radica l anions and cat ions for both compounds, and the ECL emission was essent ia l ly the same as the fluorescence emission of these compounds in PPO. ECL of rubrene was also observed in mol ten phenanthrene, PPD, th ianthrene , and p te rpheny l , as wel l as in mix tu res of PPO wi th 1or 2 -methy lnaphtha lene . There has been much interest in recent years in the appl icat ion of nonaqueous solvents in e lec t rochemis t ry (1). Many of these solvents have the advan tage of r a the r wide potent ia l l imits before the solvent i tself or the suppor t ing e lec t ro ly te undergoes reduct ion or oxidat ion at an iner t electrode; these solvents are also often less l ike ly to react wi th e lec t rogenera ted species. Most solvents employed are l iquid at room t empera tu re [e.g., N,N-d ime thy l fo rmamide (DMF), acetoni t r i le (ACN)] while severa l l iquefy at low tempera tu res (SO2, NH3). Only a few studies of organic systems have employed as solvents compounds which are solid at room tempera tu re ; these include d imethylsu l fone (rap 127~ (1,2) , mol ten e thy lammonium chloride (3), and the A1CI~-KC1 mol ten salt sys tem (4). Recent studies involving aprot ic solvents include e lec t rogene ra ted chemiluminescence (ECL) of aromat ic compounds, whe re e lec t rogenera ted radica l anions (R T)

Chronopotentiometric deposition and stripping of silver, lead and copper at platinum electrodes

Abstract The effect of pre-treatment (pre-oxidation, pre-plating, or platinization) of a platinum electrode on the chronopotentiometric deposition and stripping of silver, lead and copper in various media is considered. Depositions were found to occur less reversibly at a pre-oxidized electrode. Evidence for retention of some deposited metal, even at potentials considerably more positive than the reversible stripping potential is presented and possible mechanisms for this retention are suggested.

The Electrochemistry of the Liquid Crystal N‐(p‐Methoxybenzylidene)‐p‐n‐butylaniline (MBBA) The Role of Electrode Reactions in Dynamic Scattering

The electrochemical reduction and oxidation of MBBA were studied by cyclic voltammetry and by controlled potential coulometry. The reduction in DMF leads to the formation of the radical anion which decomposes with a half‐life of about 4 sec, apparently by both ec and ece pathways. The oxidation product was unstable on the time scale of this study. The possible importance of electrode reactions in interpreting the dynamic scattering phenomenon in nematic liquid crystals is suggested.