Kinetic Studies of the Electron-Transfer Reaction in Iron(II) and Iron(III) Systems. V. The Reaction in Mixed Solvents of Methanol and Water

Abstract

Abstract The electron-transfer reaction between Fe(II) and Fe(III) species in mixed solvents of methanol and water was investigated by the use of Fe-59 as the radioactive tracer. The hydrolysis constant of Fe3+, KH, in the mixed solvent decreases gradually as the mole faction of methanol, xM, increases. At very high acidities, ionic association between Fe3+ and ClO4− ions occurs, thus forming an ion-pair in the mixed solvent. The apparent rate constant, kapp, of the electron-transfer reaction is a function with respect to xM; kapp does not vary much in the concentration range xM≤0.3, but it decreases as xM grows higher than 0.3. kapp is also dependent upon [H+] and in a linear relationship with [H+]−1. Thus, the two reaction paths, Fe2+-Fe3+ (k0) and Fe2+-FeOH2+ (kH), are deduced to be prevailing. Both rate constants, k0 and kH, decrease with an increase in xM. The constancy of kapp at 0≤xM≤0.3 is a result of compensation between the increasing KH value and the decreasing k0 and kH values. By taking into account the values of ΔHH\eweq=7.6 kcal/mol and ΔSH\eweq=−19.1 e.u. at xM=0.2, both of which do not differ greatly from those in pure water, the reaction is concluded to proceed through the hydrogen-atom-transfer mechanism in the mixed solvent, the inhibiting effect of methanol being due to the reduction of the hydrogen bondings between water molecules.