Electron-Sharing Ability of Organic Radicals. VII. Dissociation Constants of a Series of Organic Acids and Amines in Water, Methanol and Ethanol

Abstract

A recent discussion of the role of the solvent in acid-base equilibria was given by Hall' at the Cincinnati symposium on non-aqueous solutions. Opinion regarding the solvent influence on the physical properties of the solute may be divided into two divergent schools. The summary of Bronsted2 would indicate that the solvent influence is a constant and additive factor a t least for acids and bases in alcoholic solution, C~n a n t ,~ however, considers that the simplest relationships between the structures of compounds and the free energy changes of their reactions will be found in the gaseous state.It is apparent that an attempt to compare the relative affinities of organic radicals by measuring the electrical properties of polar groupings attached to these radicals cannot be successful if the solvent influence on the free energies of the polar groupings is erratic, As previously stated,4 it would be expected that the variations in the electrical properties of any polar grouping could be expressed as it. function of the electron-sharing ability of the organic radical attached to it. If the polar grouping formed a complex with the solvent, this complex would be expected to be of a characteristic structural type for that particular polar grouping and solvent. The free energy of the polar grouping with its solvent complex would still be a function of the electron-sharing ability of the organic radical, since this is the only variable factor affecting the polar grouping. Erratic or irregular influences due to the solvent would be expected only if the radical contained atoms other than the polar grouping which united with the solvent. If these latter complexes did not themselves dissociate to form electrically charged bodies which would interfere with the measurements for the polar grouping, disturbing influences would be expected only if the second union were located with but four5 or less atoms between it and the polar grouping. (1) Hall, C h m . Rcu., 6, 191 (1932). (2) Bransted, ibid., 6, 291 (1928). (3) Conant, Ind. Eng. Cham., 94,466 (1932). (4) Hixon and Johnis, THIS JOURNAL, 49, 1788 (1927).(5) This statement is based on the fact that the inhence of the most negative radicals seems to be abaorbed completely before ttensmisnion through four carbon atoms.

The Dissociation Constants of Monocarboxylic Acids; their Measurement and their Significance in Theoretical Organic Chemistry.

“…In this connection it should be noted that Wooten and Hammett (174) have shown that with metaand para-substituted benzoic acids the values of log Kr for butanol and water when plotted yield a straight line (actually, these workers believe that their results as a whole are best interpreted along the lines already discussed by Schwarzenbach and Egli (153a), who take into account, not only the dielectric constant of the solvent for the acid, but the distance separating the reacting group and the substituent and the component of the group dipole acting in this direction). Furthermore, the data provided by Halford (73), Bright and Briscoe (26), and Hixon (67) for alcoholic solutions, and by Griffiths (70) for chlorobenzene solutions (obtained by an indicator method) give, in each case, an order which is precisely the same as that derived from aqueous solutions (after excluding systems which are recognized as abnormal); and Kolthoff, Lingane, and Larson (113a) go so far as to predict the dissociation constants of acids in alcohols from a knowledge of the constants for aqueous solutions.…”

Potentiometric Electrode Systems In Nonaqueous Titrimetry

“…The dissociation constants of a series of acids and amines have been measured in ethanol and in methanol. Both platinized and palladized electrodes were used, the latter being more suitable for compounds containing the benzene ring (64,65).…”

“…This affords a larger interface than the capillary openings of a commercial calomel electrode and is claimed to eliminate erratic potentials. A calomel electrode containing sodium chloride in absolute methanol has been used as a reference in studies of dissociation constants in methanol (64). For titrations of tobacco alkaloids in a benzene-chloroform-acetic acid medium Cundiff and Markunas (37) found that more satisfactory titration curves were obtained if the aqueous potassium chloride in the calomel electrode was replaced with methanolic potassium chloride.…”

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

“…The agreement between the methods is again emphasized. There are available no values at µ = 0.05, and the values at µ = 0.01 (15) and at various other ionic strengths (1) are not comparable, as the constant de-…”

“…Very little has been done with this method in anhydrous solvents; the cells employed by Harned and his coworkers are not applicable to the substituted benzoic acids, since these substances are reduced by hydrogen, and the use of the quinhydrone electrode leads to difficulties, due to the interaction of the quinhydrone and the silver-silver halide electrode. There remains the use of concentration cells (17,30) and other cells involving liquid junctions (1,15).…”