The Effect of Substituents on the Acid Strength of Benzoic Acid. III. In Ethylene Glycol and n-Butyl Alcohol.

Abstract

This paper is a report of the potentiometric determination of the relative acid strengths of the substituted benzoic acids in the solvents ethylene glycol and «-butyl alcohol. The paper also includes a discussion of the results presented for the solvents methyl alcohol (10) and ethyl alcohol (11), together with the results available for aqueous solution (5). experimental partThe experimental method has already been described (10) and the only difference noted in the application of the method was that longer times were necessary for the establishment of electrode equilibrium in «-butyl alcohol. In addition, an electrode would occasionally become poisoned and have to be replaced. The poisoning of an electrode showed itself by an appreciable potential difference between two electrodes in the same solution, the e.m.f. usually changing with time. It was found that gold-on-glass electrodes prepared by the method of Newberry (23) gave more reproducible results in «-butyl alcohol, since these electrodes could be cleaned by heating to redness in the flame of an alcohol lamp. All measurements were carried out at 25°C.Ethylene glycol was purified by the method of Akerlof (1). The glycol was allowed to stand over powdered, freshly calcined calcium oxide for several days. The calcium oxide was removed by filtration and the ethylene glycol distilled through a Widmer column. The boiling point for each distillation was very constant. The distillation temperature was 90.0°C. at 6 mm. of mercury and 95.5°C. at 11 mm.The index of refraction was nl°= 1.4321. The purity, calculated from a density determination on the assumption that the only impurity was water (20), was 99.93 per cent.«-Butyl alcohol was purified by the method of Mason and Kilpatrick 1 This paper was abstracted from the dissertation presented by John Habersham Elliott to the Faculty of the Graduate School of the University of Pennsylvania in partial fulfilment of the requirements for the degree of Doctor of Philosophy, April, 1940. It was read at the Ninety-ninth Meeting of the American Chemical Society, which was held in Cincinnati, Ohio, April, 1940.

Symposium on the Thermodynamics of Electrolytic Dissociation: The Effect of Dielectric Constant on Acid-Base Equilibria.

“…Kirkwood and Westheimer have evaluated DE by considering the ions and molecules involved as ellipsoidal cavities of low dielectric constant with the charges along the focal line, and Westheimer (33) has recently evaluated log KAxBo for the para-substituted benzoic acids in aqueous solution. The calculations and comparison with experiment for the alcohols as solvent have been given elsewhere (9,22,23). Figure 3 shows that the observed relative acid strengths are more sensitive to the macrodielectric constant than the predictions of theory.…”

“…An examination of the results for the twenty-three acids shows that equation 21 holds in all the pure solvents of dielectric constant > 24, the only exceptions being the ortho acids in the solvent ethylene glycol. Table 3 gives the slopes and intercepts of equation 21, as determined by the method of least squares for all the experimental results in the four solvents (5,7,8,9,10,22,23). Fig.…”

The Effect of Substituents on the Acid Strength of Benzoic Acid. IV. In Dioxane–Water.

“…Table 1 summarizes the determination of the dissociation constant of benzoic acid by measurements against hydrochloric acid in a suitable concentration cell ( 5 ) . For comparison the best lines through the experimental data in the pure solvents are given (6). The ratio to the thermodynamic constant is K,(0.05) K , That the ratio K J K , increases markedly for the same molarity of lithium chloride at low dielectric constants of the mixture is evident from the results of Lynch and La Mer (8).…”

“…On the assumption that log KAlBo is independent of electrolyte concentration in these solutions, log , is plotted against the reciprocal of the dielectric constants of the electrolyte-free media in figures 1 to 5. For comparison the best lines through the experimental data in the pure solvents are given (6). It is evident that the equation log , log (1) is not applicable to the dioxane-water mixtures (10).…”

Dependence of the Substituent Effect on Solvent Properties

“…Substituent effects, described traditionally by substituent constants (SC), also depend on a solvent, or more generally, on a kind of medium applied to their estimations. − An application of the Hammett substituent constants , σ to the reference reaction for SC, i.e., acid–base equilibria of substituted benzoic acids measured in various solvents, reveals that the reaction constant, ρ, describing a sensitivity of the reaction to the substituent effects (SE) strongly depends on the solvent used . ρ-Values for the above-mentioned reaction is 1.00 for water (by a definition), 1.14 for ethanol, 1.02 for methanol, 0.966 for n -butanol, and 2.4 in 85.1 wt % N -methylacetamide in water . Moreover, the estimation of the reaction constant for gas phase data shows a tremendous increase of its sensitivity on the substituent effect, with ρ = 5.6 .…”