Pulse Radiolysis Studies. XIX. Solvent Effects in Electron Transfer and Proton Transfer Reactions of Aromatic Molecule Ions

Absolute rate constants were determined, by the pulse radiolysis technique, for the electron transfer reaction from an aromatic radical anion to a different neutral aromatic molecule: $${\mathrm{A}}_1{·}^{-}+{\mathrm{A}}_2={\mathrm{A}}_1+{\mathrm{A}}_2{·}^{-}$$ for a number of aromatic molecule pairs in ethanol, in ethylenediamine, and in diethylamine. The dependence of these rate constants upon the standard free energy for reaction of the pair and upon the dielectric properties of the solvent, as predicted by the theory of Marcus for electron transfer reactions, is found to support this theory, at least semiquantitatively, over a limited range. The effect of solvent on the rate of protonation of the biphenyl radical anion by an alcohol: $${\mathrm{C}}_{12}{\mathrm{H}}_{10}{·}^{-}+\mathrm{ROH}={\mathrm{C}}_{12}{\mathrm{H}}_{11}\mathrm{·+}{\mathrm{RO}}^{-}$$ was studied for a number of mixed solvent systems, namely, ethanol–ethylenediamine, ethanol–cyclohexane, ethanol–triethylamine, ethanol–diethylamine, ethylene glycol–ethylenediamine, and ethylene glycol–triethylamine. In the alcohol–amine systems the first order rate constant for the protonation reaction was drastically reduced in amine‐rich solution and increased in alcohol‐rich solution. The rate constant was increased at all concentrations of cyclohexane in the ethanol–cyclohexane system. These effects of the presence of a second solvent were attributed to the formation of a hydrogen‐bonded complex between the amine and the alcohol, which reduces the rate in these systems, and to the solvent structure breaking effect of the cyclohexane, which increases the rate.