1052
J. Phys. Chem. 1981, 85,1052-1057
Substituent Effects on Specific Interactions of Acetate Ions and Acetic Acid Molecules 2. Pawelka’ and M. C. Haulalt-Pirson” Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 8-3030Heverlee, 8e/gium (Received: October 15, 1980)
From conductance measurements of a series of triethylamine-acetic acid solutions in nitrobenzene, the following association constants were determined: KAofor the ionic association Et,”+ + RCOO- =+ Et3NHOOCR, KT for the formation of a unilateral triple ion Et3NHOOCR + Et3NH’ ((Et3NH)zOOCR)+,and K1- for the homoconjugation RCOO- + HOOCR + (RCOO)zH-. Log KAo and log KT were found to follow a linear relation with aqueous pK, values of the corresponding acid: ionic association increases with increasing basicity of the acetate ion. This shows that ion pairs and ion triplets are not only stabilized by electrostatic forces but also by hydrogen bonds. The homoconjugation of the aliphatic carboxylate anions seems to increase with decreasing acidity of the corresponding acid for the unsubstituted and chloro-substituted acetate anions, whereas the trifluoroacetate anion presents a greater homoconjugation than the di- and trichloroacetate anions. A good linear correlation is found between the standard free energy of homoconjugation of acetate and the Gutmann donor number of the solvent. This shows that solvation of the neutral acid molecule is a determining factor in the solvent effect on anionic homoconjugation. Introduction Recently we have turned our attention to the homoconjugate cations (-NHN-)+ and to the influence of substitution on the formation of these “symmetric” hydrogen-bonded complexes.2 The equilibrium constants we determined in nitrobenzene for the formation of the pyridinium-pyridine complexes K,+
RPyH+ + RPy e(RPyHPyR)+ (1) show that homoconjugation increases with increasing basicity of the pyridine. This was tentatively explained by considering the potential energy curve of the proton in the (-NHN-)+ system. According to the explanation we proposed, it is expected that homoconjugation of the anions KiC
A- + AH e(AHA)(2) will increase with decreasing acidity of the conjugated acid. There is in the literature considerable evidence for the existence of anion-acid symmetric complexes in polar solvents like acetone and a~etonitrile.~-* However, few works report values of equilibrium constants for a series of acids and these are often derived from different experimental methods. Gordon4determined the relative formation constants of the dibenzoate ion (ArCOO)zH- in acetonitrile using a distribution method. Kolthoff and Chantooni6 estimated values for the equilibrium constant K A-...HA for a series of meta- and parasubstituted benzoic acids employing conductance titrimetry and potentiometry. In both works the constants are found to increase with increasing acidity of the benzoic acid. Recently Sadek et al? determined potentiometrically the homoconjugation constants KHA;for acetic, chloroacetic, (1) On leave from the University of Wroclaw, Poland, 1979-1980. (2) M. C. Haulait-Pirson and M. De Pauw, J.Phys. Chem., 84,1381 (1980). (3) P. Bryant and A. Wardrop, J. Chern. SOC.,895 (1957). (4) J. E. Gordon, J. Phys. Chern., 67, 19 (1963). ( 5 ) J. F. Coetzee and G. P. Cunningham, J. Am. Chem. SOC.,87,2534 (1965). (6) I. M. Kolthoff and M. K. Chantooni, Jr., J. Phys. Chem., 70,856 (1966). (7) 2. Pawlak and L. Sobczyk, Adu. Mol. Relaxation Processes, 5,99 (1973). (8) M. M. Davis, NBS Monogr. (US.), No. 105 (1968). (9) T. Jasinski, A. A. El-Harakany, F. G. Halaka, and H. Sadek, Croat. Chem. Acta, 51, 1 (1978).
dichloroacetic, and trichloroacetic acids in acetonitrile. They found contrary behavior, the homoconjugation decreasing as the acid strength increases. Using the same technique Pawlak et al.l0 also measured the KW2-constants for acetic and dichloroacetic acids in acetoconitrile but found a greater homoconjugation for the stronger acid. These authors also note the formation of RCOO(HOOCR)2-complexes, this last complex being more stable, the weaker the acid. From this brief review, it appears that no final conclusion on the systematic dependence of stability of the homoconjugate anion on acidity can be drawn. In the present work, conductance measurements were made in nitrobenzene by use of solutions of triethylamine with acetic, chloroacetic, dichloroacetic,trichloroacetic, and trifluoroacetic acids. The method we use requires a great number of conductance measurements but allows accurate determination of ionic association constants and homoconjugation constants, and provides evidence of formation of unilateral triple ions. Our purpose is, among other things, to derive some information on the influence of substituents on the formation of different hydrogen-bonded complexes for which the equilibrium constants can be determined. Experimental Section Apparatus. Details of the measuring apparatus were identical with those described in the previous papers2 Materials. Nitrobenzene (Fluka puriss) was distilled from activated alumina under reduced pressure. The residual conductivity does not exceed 3 X lo4 ohmv1cm-’. Triethylamine (Fluka) was distilled from CaHz and stored over molecular sieves (4A). Acetic acid (Aldrich) was distilled from Pz05.Dichloroacetic acid (Aldrich) was dried with MgS04 and then distilled. Trifluoroacetic acid (Aldrich) was refluxed with an then distilled from Pz05. The liquid acids were stored over molecular sieves (4 A) in a drybox. Chloroacetic acid and trichloroacetic acid (Aldrich) were twice recrystallized from benzene and dried over P205in a vacuum desiccator. Tetrabutylammonium trifluoroacetate was prepared by the method of Clark and Emsleyll and stored over PzOs in a vacuum desiccator. (10) 2. Pawlak, 2. Szponar, and C. Dobrogowska, Rocz. Chem., 48,501 (1974). (11) J. H. Clark and J. Emsley, J. Chem. SOC.,Dalton Trans., 1125 (1974).
0022-3654/81/2085-1052$01.25/00 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 8, 1981 1053
Interactions of Acetate Ions and Acetic Acids
Procedure. We did not succeed in crystallizing triethylammonium salts of the various acetic acids. Consequently, stock salt solutions were obtained by adding triethylamine and the corresponding acid to nitrobenzene in equivalent amounts in the cases of trifluoroacetic acid and trichloroacetic acid, and with a small excess of triethylamine in the cases of dichloro- and chloroacetic acids. Under these conditions the salt concentration may be considered as being equal to the acid concentration. By dilution we obtained different salt solutions with concentrations lying between and 5 X mol L-l. This procedure for preparing salt solutions is based on consideration of titration curves performed in nitrobenzene. The general contours of the curves we obtained are very similar to those obtained by Bryant and Wardrop3 on titrating the same acids by triethylamine in acetone and acetonitrile. For the 1:ltriethylamine-trichloroacetic acid system in nitrobenzene, the conductivity tended to drift with time toward lower values. This fact was already observed by Bryant and Wardrop during the titration of this acid by Et3N in the region of the equivalence point and is attributed to further interaction of this acid with tertiary bases.12 For acetic acid, the excess of amine necessary to react completely with the acid is so large that the residual conductivity of the triethylamine-nitrobenzene mixture represents more than 10% of the conductivity of the triethylamine-acid-nitrobenzene solution. Under these circumstances, it was not possible to obtain reliable conductance values for triethylammonium trichloroacetate and triethylammonium acetate solutions. As shown in the next section, the determination of the homoconjugation constant is based among other things on conductance measurements of salt solutions in the presence of excess of free acid. In these cases, the salt solutions were prepared by adding equivalent amounts of amine and acid to nitrobenzene containing a large excess of acid and by diluting with the same nitrobenzene-acid mixture. The salt concentration is then considered to be equal to the amine concentration. No conductivity drift was observed for triethylamine-trichloroacetic acid solutions containing a large excess of acid. On the other hand, the residual conductivity of the acid-nitrobenzene solution only represents 1% (or less) of the conductivity of the corresponding more dilute salt solution. All stock solutions and dilutions were made in a nitrogen-filled drybox. All measurements were carried out at 25 f 0.01 "C.
Results a n d Their Treatment The overall ionic association constant KA characterizing the equilibrium between ions (conducting species) and ion pairs (nonconducting species) is defined as [ion pairs] KA = (3) [ions]2y+2 where [ions] represents the positive ion concentration which is equal to the total concentration of the free and complex anions. y+ is the activity coefficient. From the change of the association constant KA of a salt upon addition of ligands, it is possible to get some information about the stoichiometry of the ion-ligand complexes and to evaluate the addition constants of the ligand to the ions. Huyskens et al.13 have proposed a general relation between KA and ligand concentration [L]. In the ~
~~~~~~
(12) (a) Silberstein, Berichte, 17, 2664 (1884); (b) Verhoek, J. Am. Chern. Soc., 56, 571 (1934). (13) J . Macau, L. Lamberts, and P. Huyskens, Bull. SOC.Chim. Fr., 2387 (1971).
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0038 ACy:/F[zJ
Flgure 1. The function F ( Z ) l h plotted vs. ACY,~/F(Z)for triethylammonium trifluoroacetate (filled circles, left-hand ordinate, lower abscissa) and for triethylammonium picrate (open circles, right-hand ordinate, upper abscissa) in nitrobenzene.
particular case in which only one complex of 1:l stoichiometry between the anion and an acidic ligand is formed, the dependence of KA on ligand concentration [L] is given by
R KAo/KA = 1 + K,-[L] (4) where KAo and KA refer to the association constant in the absence and in the presence of a given amount of ligand, respectively, and K1- is the equilibrium constant for the addition of a ligand molecule to the anion Ki
A- + L eA--L (5) The KA values can be easily determined by using the appropriate conductance equation relating the experimental conductances A to the salt concentrations C. When the association is appreciable, like that for hydrogen-bonded ions, the limiting conductance &, and the ionic association constant KA may be obtained by using the linear relationship of Fuoss14 F(Z)/A = l / A o + A C Y + ~ K ~ / ( F ( Z ) A ~(6) ~) where F(Z) is a mobility function described and tabulated by Fuoss. The activity coefficient y+ can be calculated by means of the Debye-Huckel equation. Consequently, a plot of F(Z)/A against ACykz/F(Z) should result in a straight line with intercept equal to l/h0and slope equal to K A /no2. Salt concentrations and experimental equivalent conductances for the systems Et,N-CF3COOH, Et3NCClSCOOH, Et3N-CHClzCOOH, Et,N-CH&ICOOH, and Et3N-CH3COOHin nitrobenzene without and with various excesses of triethylamine or of the corresponding acid are available as supplementary material (see paragraph at end of text regarding supplementary material). The conductance measurements made with solutions containing equimolar amounts of acid and amine or a small excess of amine with respect to acid lead to the determination of KAO, the actual ionic association constant of the triethylammonium acetates in nitrobenzene. Indeed, the small quantity of excess amine does not change the dielectric constant and the viscosity of nitrobenzene by more than a few percent. On the other hand, formation of homoconjugate cations of the type (EhN)zH+is negligible in this solvent. We do not observe significant changes in the conductivity of triethylammonium picrate upon addition of 0.1 M free Et3N to nitrobenzene. The conductance measurements made by using solutions containing a given (14) R. M. Fuoss, J. Am. Chem. SOC.,57, 488 (1935).
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