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 H_R, 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 h_R, = antilog₁₀(–H_R), is also nearly proportional to the HClO₄ concentration in these LiClO₄–HClO₄ solutions.