Kinetics and Mechanism of Hydrolysis of Ruheman’s Purple in the Absence and Presence of Micelles in Aqueous Medium

Abstract

The mechanism of acid hydrolysis of Ruheman’s purple has been investigated and the effect of micelles on the rate has been examined. First order rate constant for the hydrolysis reaction decreases linearly with decreasing hydrogen ion concentration up to pH 3 and levels off thereafter with no reaction occurring above pH 7. Increasing ionic strength decreases the rate in accordance with the Bronsted-Christiansen equation. The salt effect in the dipole-dipole reaction is probably a reflection of the fast equilibrium steps involving ions preceding the rate-determining step. Solvents like ethylene glycol and dioxane have no effect, indicating that the rate-determining step of the reaction does not involve ionic species. A mechanism of the reaction has been proposed which is consistent with our observed results. In the presence of micelles the reaction rate was strongly inhibited by the cationic micelles of hexadecyltrimethylammonium bromide, whereas anionic micelles of sodium dodecyl sulfate showed a slight enhancement. The micellar data have been analyzed on the basis of available models.