Dynamics of cetyltrimethylammonium bromide-mediated reaction of phenylsulfinylacetic acid with Cr(VI): Treatment of pseudo-phase models

Abstract

The influence of cetyltrimethylammonium bromide, CTAB, on the oxidative decarboxylation of phenylsulfinylacetic acid, PSAA, and several meta-and para-substituted PSAAs by Cr(VI) was investigated in 95 % H2O–5 % CH3CN medium. The rate profile displayed a peculiar trend with an initial rate increase at low CTAB followed by sharp rate inhibition at higher CTAB con­centrations. The initial rate acceleration could be explained by strong binding of SO42- on the positively charged micellar surface. The specific partitioning of PSAA in the micellar phase by hydrophobic interaction and the oxidizing species HCrO3+ in aqueous phase by electrostatic repulsion accounted for the rate retardation at higher CTAB concentrations. The Hammett plot with dif­ferent substituted PSAAs showed excellent correlation affording negative ρ value, which supports the proposed mechanism involving the intermediate formation of sulfonium cation. The obtained ρ value in CTAB medium was found to be slightly lower than that in aqueous medium. Quantitative analysis of the rate data for the inhibition shown by CTAB was performed using the Menger–Portnoy and the Piszkiewicz pseudo-phase models. The binding cons­tant for PSAA with micelles was evaluated from the Piszkiewicz cooperative model.