Kinetic and Mechanism of the reaction between 4-hexadecylbenzenediazonium ions and vitamin C in emulsions: Further evidence of the formation of diazo ether intermediates in the course of the reaction

The kinetics and mechanism of the reaction between hydrophobic 4-hexadecylarenediazonium ions, 16-ArN and vitamin C, VC, in a model emulsion prepared by mixing octane, acidic (HCl) water and the non-ionic surfactant hexaethyleneglycol monododecyl ether, C₁₂E₆, were investigated. Because emulsions are opaque, linear sweep voltammetry, LSV, was employed to monitor the reaction. Voltammograms of 16-ArN in emulsions show two reduction peaks as in aqueous systems. The half-life for the spontaneous decomposition of 16-ArN in the emulsion was estimated as t_1/2 = 14.5 h at T = 25 °C. Upon addition of VC to the system, the first reduction peak of 16-ArN disappears almost immediately and a new reduction peak is detected at E_p = −0.25 V. Electrochemical titration of 16-ArN shows that the new peak corresponds to the formation of a 1:1 adduct. The i_P(E_p = −0.25 V) values can be linearly correlated with [16-ArN] and the observed rate constants, k_obs, were determined by fitting the (i_p, t) data to the integrated first order equation. The variation of k_obs with [VC] follows a saturation kinetics profile, consistent with the formation of an intermediate in a pre-equilibrium step. All the evidence is consistent with a reaction mechanism comprising two competitive pathways, the spontaneous D_N + A_N mechanism and the unimolecular decomposition of a transient diazo ether (DE) formed in a pre-equilibrium step. The data allowed estimations of the interfacial rate constant for the reaction between 16-ArN and VC⁻ but did not allow the determination of the equilibrium constant for the DE formation.