Vernon D. Parker

The reversible oxidation of aromatic cation radicals to dications. Solvents of low nucleophilicity

Abstract Reversible behavior for both electron transfers for oxidation of aromatic compounds to cation radicals and dications was observed in several common electrolytic solvents. Nitriles, nitro compounds and dichloromethane can all be rendered essentially nucleophile free for voltammetric purposes simply by conducting the voltammetric measurements over neutral alumina shortly after the mixture has been stirred. Solvents containing trifluoroacetic acid and the corresponding acid anhydride are not only useful for voltammetry but can also be used to prepare stable solutions of cation radicals and dications. Equilibrium constants for the disproportionation of cation radicals to dications were calculated from the reversible electrode potentials and the effect of changes in the solvent system on the equilibrium constants is discussed.

The anodic oxidation of hydroquinone in acetonitrile. On the question of a possible one electron intermediate

Abstract The oxidation of hydroquinone in acetonitrile takes place accompanied by the formation of two electron oxidation products as the first observable intermediates. In the presence of small amounts of added acid in acetonitrile the only observable product by rapid electrochemical techniques is monoprotonated benzoquinone. When voltammetry was conducted in the presence of only a neutral supporting electrolyte, the unprotonated quinone was observed as a transient product by rapid sweep cyclic voltammetry but undergoes reaction with anodically generated protons giving the protonated species observable by slow techniques. In the presence of 2,6-lutidine, only benzoquinone can be detected under either rapid or slow conditions. The validity of the above reaction scheme has been verified by cyclic voltammetry, coulometry with spectral analysis and rotating disk electrode voltammetry.

On the origin of voltammetric pre-peaks and cathodic potential shifts during anodic substitution reactions

Abstract During voltammetric pyridination of 9-phenylanthracene a pre-peak was observed, the height of which was in direct relation to the stoichiometric ratio of 9-phenylanthracene and pyridine. In order to determine whether or not the latter result could be taken as evidence for a nucleophile assisted electron transfer, a “non-assisted” mechanism was simulated. It was found that an ecec mechanism gave substantial cathodic shifts of the peak potential for oxidation of a substrate in presence of a reactant, and if the rate constants were sufficiently high not only a cathodic shift, but also a pre-peak was observed during simulation. Thus, the experimental observations cannot be used as evidence for an assisted mechanism.

The Effect of Structure and Solvation on the Thermodynamic Parameters for the Formation of Singly and Doubly Charged Ions of Organic Compounds in Solution

One of the most direct ways to determine the free energy change of a reaction in solution is to measure the corresponding reversible electrode potential. This places the study of electrode processes at a high level of importance in the search for understanding of the chemistry of reactive intermediates. Combined with gas phase ionization potential and electron affinity data, electrode potential measurements can provide information about fundamental processes such as solvation as well as bonded and non-bonded interactions between solution species. In this discussion we will focus on the formation of positive and negative radical ions as well as the corresponding doubly charged ions. A complete review of this topic is beyond the intended scope and the coverage reflects the research interests of the authors.