Conductimetric studies in ketonic solvents. Part VI. The association behaviour of alkali-metal salts of substituted aromatic acids in ethyl methyl ketone

Abstract

Exceptionally large binary association constants, previously reported for alkali-metal salicylates dissolved in ketones are now found for other similar salts in ethyl methyl ketone. The ternary association of lithium salts is also observed. The data can be explained by assuming that potassium and sodium ions are completely desolvated in association with the anions whilst lithium ions are largely, but not completely desolvated. The association constants of hydroxycarboxylates are broadly consistent with the corresponding aqueous acid strengths but no simple correlation is possible for picrates and o-nitrobenzoates.ATTENTION has been drawn to the ion-association behaviour of alkali-metal salicylates in acetophenone, ethyl methyl ketone, and acetone at 25°.2 This contrasts with the ' normal ' behaviour typified by alkali-metal halides in non-aqueous solvents of moderate dielectric constant, in that (i) association constants are considerably greater than those found for salts having ' spherical ' ions, (ii) they increase by an order of magnitude in the sequence K < Na < Li, and (iii) lithium salicylate, besides having the highest association constants, shows ternary association, probably of the unilateral type. It was suggested that these observations are explicable if the anion is assumed to desolvate the alkali-metal cations, almost completely, in association.Measurements have now been extended to the alkalimetal salts of eight other organic acids, in dry ethyl methyl ketone at 25", selection being conditioned by the practicability of preparing the salts and the extent of their solubility in the solvent.Further to test the desolvation hypothesis the measurements on alkali-metal salicylates were also made at 15 and 35". 1-3. The data were fitted to the Fuoss-Kraus binary association equation to give A, and K a values for all the potassium salts studied and some of the sodium salts. In those cases where the extrapolation was too lengthy to give reliable internally consistent A, values these were derived indirectly from the corresponding alkali-metal iodide and tetraphenylboride , measurements by use of the Kohlrausch principle. To calculate Wooster unilateral ternary association constants (K31) for lithium salts the potassium ion conductance in ethyl methyl ketone, obtained from measurements on potassium tetraphenylboride, was subtracted from the value of A, for the potassium salt. This gave the conductance and Stokes radius of the anion A-enabling the conductance of the ternary ion, assumed to be Li2A+, to be estimated and hence a value for Fuoss-Kraus bilateral ternary association constants (K32) were based on RESULTS Conductivity measurements are shown in Tables