879. A polarographic investigation into the redox behaviour of quinones: the roles of electron affinity and solvent

Abstract

One-electron reduction potentials, El, of thirty quinones in apmtic conditions have been determined. A comparison with published molecularorbital calculations shows that electron addition takes place to the lowest unoccupied molecular orbital. The El values are linearly related to the electron affinity of the quinones. Intramolecular hydrogen bonding leads to an enhanced electron affinity in some a-hydroxy-quinones and apparently also in an unconjugated hydroxy-quinone. Although some El values for hydroxylic solvents parallel those for aprotic solvents, there are marked deviations for halogeno-and cyano-quinones. It is tentatively suggested that changes in hydrogen bonding with the hydroxylic solvent account for this.IN connection with studies of polyquinone and charge-transfer systems,2 it was desirable to find a readily measured parameter which would describe the electron affinity of the quinone and closely reflect changes in the relevant energy level brought about by substitution in the quinone nucleus. Whereas conventional redox potentials (at zero pH) for the reaction, Quinone + 2e + 2H+ Quinol, have proved of considerable practical value in indicating the oxidising or reducing power of a system, and have been successfully correlated with the gain in resonance energy in passing from an unsubstituted quinone to its quinol, there are notable anomalies, in particular for halogens, when substituents are introduced.s Also the use of these conventional redox potentials as a measure of the electron affinity of the molecule has not been very successful. Since Hoijtink and his co-workers showed that one-electron redox potentials measured in virtually aprotic conditions for a series of aromatic hydrocarbons are linearly related to the energies of the lowest unoccupied molecular orbitals, we have determined the potentials, El, corresponding to the reaction, Quinone + e Quinone-, for a series of orthoand para-quinones and substituted quinones, mainly in acetonitrile, and investigated their correlation with the calculated energies of the lowest unoccupied molecular orbitals. We then hoped to interpret the redox behaviour of quinones in terms of their electronic energy and environmental factors. To this end, a set of El values has been determined in hydroxylic solvents and compared with the results in acetonitrile.

Some electrochemical and chemical properties of methoxatin and analogous quinoquinones.

“…The collected precipitate was washed with water and recrystallized from methanol (MeOH) to yield pure, orange-brown HI (0.78 g): mp 258-62°C (dec.) [lit. (10) 20.16% N. The mass spectrum of the derivative also was consistent with the desired product.…”

Potential Mechanism for Pentachlorophenol-Induced Carcinogenicity: A Novel Mechanism for Metal-Independent Production of Hydroxyl Radicals

“…This type of reaction between semiquinone radicals and H 2 O 2 has been previously proposed by Koppenol and Butler (39), who suggested that if a quinone/ semiquinone couple has a reduction potential of between -330 and +460 mV, it can theoretically bring about a metalindependent Fenton reaction. It was suggested that such reactions are thermodynamically feasible and do not require metal ions for catalysis (39,40), which might be the case in this study, where the reduction potentials of the quinone/semiquinone couples for 2-chloro-, 2,5-dichloro-, tetrafluoro-, tetrabromo-, and tetrachloro-1,4-benzoquinone are -100, +60, +200, +240, and +250 mV, respectively (41). These values are within the suggested range of -330 to +460 mV.…”

Electron Transfer Processes Between Hydroquinone and Hausmannite (Mn3O4)

“…The one-electron reduction potentials of quinone in aprotic solvent, measured polarographically from the half-wave potential (vs. saturated calomel electrode) were 0.54 and 1.23 V (Peover, 1962). The equilibrium constants for hausmannite and manganite (Bolt and Bruggenwert, 1976) The appearance of semiquinone anion radical indicates that the reduction of the hausmannite was a one-electron transfer process.…”