The Effect of Substituents on the Acid Strength of Benzoic Acid. I. In Methyl Alcohol.

Abstract

The purpose of this work is to investigate, by an experimentally consistent method, the effect of substituents on acid strength. The acids chosen for study were the monosubstituted benzoic acids. These acids have the advantages that much theoretical work has been done on the subject of benzene derivatives, and that the geometrical relationships between the groups in the benzoic acids are relatively simple. Ideally, a thorough investigation of relative acid strengths and the effect of substituents on acid strengths would involve the determination of the thermodynamic quantities free energy, entropy, and heat of reaction in a number of solvents. In aqueous solution and in dioxane-water and water-methyl alcohol mixtures, a part of this task has been completed by Harned and his coworkers (18) for the aliphatic-carboxylic acids by measurement of 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.

Linear Free Energy Relationships.

“…log k = a + bY + cyz (80) Equation 80 where f" is the degenerate activity coefficient of A(tbutyl chloride) and f*" is the degenerate activity coefficient of the transition state for its ionization in the solvent i.…”

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

“…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.…”

“…which is the equilibrium constant for the reaction Az + B0 ^Ao + Bz (7) This equilibrium constant will be less sensitive to changes in ionic strength, since there are equal numbers of equally charged ions on both sides. The effect of the nature of the solvent will be much less, and the róles of the dielectric constant of the medium and the structure of the acids will be relatively more important.…”

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

“…Weight per cent of dioxane............... The experimental method was described in an earlier paper (5), and the measure of the precision of the results was estimated at 0.01 unit in log Ka.Bo•…”

“…This necessitates a knowledge of the dissociation constants of the acids in the medium employed. Table 1 summarizes the determination of the dissociation constant of benzoic acid by measurements against hydrochloric acid in a suitable concentration cell (5). These values are at µ = 0.05 (mostly lithium chloride).…”