Kinetics 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   2 +and vitamin C, VC, in a model emulsion prepared by mixing octane, acidic (HCl) water and the non‐ionic surfactant hexaethyleneglycol monododecyl ether, C 12 E 6 , were investigated. Because emulsions are opaque, linear sweep voltammetry, LSV, was employed to monitor the reaction. Voltammograms of 16‐ArN   2 +in emulsions show two reduction peaks as in aqueous systems. The half‐life for the spontaneous decomposition of 16‐ArN   2 +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   2 +disappears almost immediately and a new reduction peak is detected at E p  = −0.25 V. Electrochemical titration of 16‐ArN   2 +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   2 + ] 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   2 +and VC − but did not allow the determination of the equilibrium constant for the DE formation. Copyright © 2008 John Wiley & Sons, Ltd.