A stopped-flow investigation of the rate of formation and disappearance of the 4,4′,4″-trimethoxytriphenylcarbonium ion in dioxan–water mixtures

Abstract

First-order rate constants and equilibrium extents of conversion are recorded for the formation of the coloured carbonium ion from 4,4′,4″-trimethoxytriphenylmethanol in 0·1M- and 1·0M-HCl solutions in several dioxan–water mixtures, and in HClO4 and LiClO4–HClO4 solutions in the 40 and 60 vol. % dioxan mixtures. The Gibson stopped-flow technique was used for the kinetic measurements.The acidity function HR, like H0, passes through a maximum near the middle of the dioxan–water range, and the rate of attainment of equilibrium passes through a minimum. A minimum is also observed in the rate of approach to equilibrium as the HClO4 concentration is increased at a fixed dioxan–water ratio; but this minimum disappears when the total electrolyte concentration is kept at 1M by additions of LiClO4. In the latter case the rate constant for the ion formation is proportional to the HClO4 concentration and the rate constant for the back reaction is practically independent of it. The function hR, = antilog10(–HR), is also nearly proportional to the HClO4 concentration in these LiClO4–HClO4 solutions.