Solvent Effect of Methanol on the Rate of Formation Reaction of Monochloroiron(III) Complex in Aqueous Solution

Abstract The rate constants of the formation reaction of FeCl2+ complex from Fe(III) and Cl− ion were measured in water-methanol mixed solvents by use of a stopped-flow spectrophotometer. The apparent rate constant k is dependent upon the acid concentration of the medium, indicating that the reaction can proceed through two parallel paths, Fe3++Cl− (k1) and Fe(OH)2++Cl− (kH). Both k1 and kH increase with the increase in the mole fraction of methanol in the medium, xM. Since kH is much larger and is consequently much more accurately determined than k1 considerations were mainly made on the nature of kH. According to the dependence of kH on the dielectric constant of the medium, it was found that the activation free energy of the Fe(OH)2++Cl− reaction consists of the electrical and non-electrical parts and that the dimension of the activated complex corresponds approximately to the distance of the outer-sphere approach between the reacting species. These facts are therefore in favor of an outer-sphere mechanism in the anation reactions of iron (III) by simple anions.