Selective Activation of Reactant Molecules by Reversed Micelles

Article abstract of DOI:10.1021/j150603a013

The oxidations by Ce(IV) ion of the cations of acridine orange (AOH⁺) and its N-alkyl derivatives (AO⁺-(CH2)_nH) have been studied in water and in the reversed micellar system of cetylpyridinium chloride (CyPCl)/water/chloroform.The length of a substituting alkyl chain (n) is varied from methyl to octadecyl groups.In both media, the oxidation is found to proceed via the two-step mechanism: (1) AO⁺-(CH2)_nH + Ce(IV) -> I + Ce(III) (k₁) and (2) I + Ce(IV) -> P + Ce(III) (k₂), where I and P denote the one- and two-electron oxidized species, respectively.In water, the bimolecular rate constants, k₁ and k₂, do not vary very much with the change of n; k₁ = 7.0 X 10⁴-14 X 10⁴ M^-1 s^-1 and k₂ = 2.1 X 10⁴-5.9 X 10⁴ M^-1 s^-1 for n = 0-9 at = 6.0 X 10^-3 M, respectively.On the other hand, in the reversed micellar system, the first-order rate constant of step 1, k₁, has a sharp maximum at n = 1; k₁ = 1.9 s^-1 (n = 0) << 58 s^-1 (n = 1) > 18-3.8 s^-1 (n = 2-18). In contrast to this, the first-order rate constant of step 2, k₂, is little dependent on n; k₂ = 0.21-0.58 s^-1 for n = 0-18 at = 8.8 X 10^-3 M and = 5.6 X 10^-2 M.The remarkably high rate of the initial step for AO⁺-CH3 is rationalized in terms of its favorable orientation at the boundary between the micellar water phase and the surfactant layer.