Kinetic study of the iron(iii) oxidation of some alkyl-substituted ferrocenes in water-in-oil microemulsions of AOT in heptane, isooctane and decane

Abstract

The kinetics of iron(III) oxidation of ferrocene and its 1,1′-dimethyl and 1,1′-dibutyl derivatives have been investigated at 20.0 °C in oil–AOT–water microemulsions by changing the organic solvent (heptane, isooctane and decane) and the molar ratio R = [H₂O]_ov/[AOT]_ov in the range 9.0–25.0, the overall AOT surfactant concentration being kept constant at 0.13 mol dm⁻³. In all cases studied the reaction rate decreases with increasing the parameter R and the overall rate constants are always smaller than those for the corresponding reactions in homogeneous aqueous solution. The kinetic data, interpreted by applying the pseudo-phase model to the microemulsions, indicate that the oxidation reaction takes place in the water pools and at the surfactant/water interface. The kinetic results allow the evaluation of the partition coefficients of the substrates between the oil and the aqueous phases. For a given oil, these partition coefficients increase as the reducing substrate becomes increasingly hydrophobic. The estimated incremental free energies of transfer of a methylene (or methyl) group of the substrate from water pools to the oils are independent of the type of the organic solvent used.