Specific interactions of trifluoroacetate ion and trifluoroacetic acid in nitrobenzene

Abstract

The conductances of solutions of triethylammonium trifluoroacetate (Et 3NHCOOCF 3) in nitrobenzene have been measured in order to study the ionic association. ▪ Using the Fuoss liner relationship [1] for solutions covering the concentration range 10 −4−10 −3 M, the association constant K A has been evaluated (K A = 3·10 5 dm 3 mol −1). This constant is two orders of magnitude greater than that observed for triethylammoniun picrate in the same solvent [2] (HPic and CF 3COOH have the same aqueous pK a's values). For salt concentrations ranging form 10 −3 to 5.10 −3 M deviations occur in that the Fuoss relationship shows downward curvature. This is interpreted as due to formation of triple ions of the type (Et 3HN, CF 3COO, Et 3NH) +. In this range of concentrations such triple ions to not appear to a great extent in nitrobenzene when the bond between the ion and the ion pair is purely electrostatic. However in the present case, the triple ion can be stabilized by hydrogen bonds: ▪ The constant for the formation of the triple ions has been evaluated using the method of Wooster [3]. Upon addition of an excess of CF 3COOH to the salt solutions a large decrease of the association constant K A, and of the mobility Λ 0 is observed. This is interpreted as due to the formation of the homoconjugate anion (CF 3COOHOOCCF 3) −. This 1:1 symmetric complex is characterized by a formation constant of 10 5 dm 3 mol −1 which is three orders of magnitude greater than the formation constant of most homoconjugate cations in nitrobenzene. The study has been extended to other substituted acetic acids in order to determine the influences of the acidity on the formation of these different types of hydrogen bonds.