Polarography and kinetics of the thermochromic system of dehydrodianthrone

Abstract

A polarographic and voltammetric study of dehydrodianthrone I in aqueous 75–80 % phenol revealed that the two species present have different reactivities : the stable form A is reduced irreversibly, whereas the thermochromic green form B is reduced reversibly, both to dianthranol VI. Equilibrium constants were determined from the absorption spectra at different temperatures. From the kinetic currents of reduction of B the rate constants for the thermochromic change A → B were evaluated; ΔH^≠= 14.5 kcal/mole, ΔS^≠=–20 cal/mole deg. On the basis of these kinetic parameters and the rules governing the electrode kinetics, a number of previously suggested structures for the form B were ruled out. The twisted conformation X is considered to be the most probable structure of the green form B of I.