THERMODYNAMICS OF SUBSTITUTED RHODANINE .1. TEMPERATURE, MEDIUM AND SUBSTITUENT EFFECTS ON THE DISSOCIATION-CONSTANTS OF 3-BENZAMIDORHODANINE AND ITS SUBSTITUTED DERIVATIVES

The dissociation constants of 3-benzamidorhodanine and its substituted derivatives have been determined potentiometrically and spectrophotom etrically. The data are discussed in terms of the electronic character of the substituents and of the change in temperature and medium of th e solution. The pK(H) values have been found to increase with increasi ng electron-repelling nature of the substituents and with decreasing i onic strength and dielectric constant of the medium. The resulting lin ear Hammett plots of pK(H) versus the Hammett constant sigma values in dicates the coplanarity of the molecules of the investigated compounds . The dissociation constants have been found to depend on the nature o f the solvent as well as on its content. The evaluated thermodynamic p arameters indicate that the dissociation processes are non-spontaneous , endothermic, and entropically unfavourable. These functions are affe cted by the electronic character of the substituents.