Solvent dependence of the one‐electron reduction of substituted benzo‐ and naphthoquinones

Abstract

Potentials for the one-electron reduction of a series of substituted benzo-and naphthoquinones were measured in 10 aprotic solvents by cyclic voltammetry and used to consmct Hammett plots. Hammett reaction constants, p, were determined in each solvent and used as indicators of the solvent-mediated stabilization of the charged radical-anion product of the reduction reaction. Correlations of Hammett p values with solvent parameters suggest that the Lewis acidity of the solvent, in particular the solvent's ability to donate a partially positive proton, is a consistent predictor of the degree of charge stabilization of the quinone radical anion. Other mechanisms of charge stabilization, including solvent-solute charge transfer complexes and dipole-dipole interactions involving the molecular dipole moment (or an induced dipole moment) of the solvent, were found to be inconsistent predictors of the effect of solvent on one-electron quinone reduction in aprotic media.

Hydrogen bonding effects on the association processes between chloranil and a series of amides

“…1. In each case, the voltammogram, represented by the continuous line, corresponds to the electrochemical reduction of chloranil, which has been extensively studied in aprotic medium [28,29]. Two reversible consecutive electron transfer steps are observed.…”

Substituent and solvent dependence of the one-electron reduction of 5-substituted-N-methylisatins in aprotic solvents

“…Hammett q values can be used as indicators of how well solvents stabilize charged reaction products [19], such as the radical-anion product of the E 1 reduction of quinones [19]. To understand factors involved in solvent-mediated charge stabilization, we carried out correlations of measured Hammett q values with several types of solvent parameters: (i) parameters based on bulk properties of the solvent (dielectric constant, surface tension, coefficient of viscosity [27]); (ii) empirical solvent polarity parameters (Gutmann acceptor number [28], Hildebrandt solubility parameter, d [29], Dimroth and Reichardt E N T parameter [29], Kamlet-Taft p⁄ polarity index [30], and Swain A parameter [31]); and (iii) parameters associated with molecular properties of the solvent (charge on most positive hydrogen, molecular dipole moment, polarizability [27], and LUMO energy).…”

“…In addition, we determined best-fit Hammett substituent parameters for the N-methyl isatin system and used those sigma values to construct Hammett plots [18] and to measure Hammett reaction constants, q, in each of the 10 aprotic solvents. The Hammett q values are indicators of the relative charge stabilization [19] of the radical-anion products in each of the solvents.…”

In situ CCD video and voltammetric studies on enhanced cathodic peak observed at a hanging mercury drop electrode during consecutive two one-electron redox reactions in aprotic solutions

“…Similar CV measurements of various compounds were carried out at the HMDE and GC electrode and the obtained results are summarized in Table 1. The CV data obtained at GC electrode, typically the number of redox peaks and the peak potentials are in agreement with those reported previously [11,[24][25][26][27][28][29][30][31][32][33][34][35][36]. In every case, the values of I c1 p and I c2 p and hence the ratios of I c2 p =I c1 p were determined ( Table 1).…”

“…Furthermore, except for the case of O 2 , it appears that the value of I c2 p =I c1 p is larger for the redox couple with a larger value of potential difference ðDE c p Þ between the first ðE c1 p Þ and second ðE c2 p Þ reduction peaks. Previously, an enhanced I c2 p has been observed during the redox reactions of DPA [1], 1,1,4,4-tetraphenylbutatriene [4], 2-nitrobenzidine [5] and 1,9-dimethyldibenzo[b,f]pentalene [2,3] at the HMDE, while a smaller I c2 p than I c1 p at the solid plane (e.g., GC) electrode has been obtained during the redox reactions of BQ [32][33][34], MV 2+ [35], DNB [26,29], O 2 [36] and AB [27,31] in different aprotic solutions. For the smaller value of I c2 p , many causes such as the decrease in diffusion coefficient of anion radical (e.g., BQ À ) compared to that of the parent molecule (e.g., BQ) due to solvation or hydrogen-bonding and dimerization of anion radical, and the complexation between dianion (e.g., BQ 2À ) and parent molecule (e.g., (BQ-BQ) 2À ) have been proposed [9,36,37].…”