293. Studies of the ortho-effect. Part II. The dissociation constants of some o-substituted acids

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

“…The explanation advanced (43) is that in the para series hydrogen ion arises to some extent from the methylene group; this is possible in view of the well-known reactivity of methylene in phenylacetic acid (and in like compounds), and the isolation, by Opolski and Zwislocki (137), of salts of the postulated formula C2H6OCOCH=C6H4=NOOM Moreover, it is noteworthy that the ortho-substituted acids, unlike the para-substituted acids, are not stronger than the meta-substituted acids, although electromeric disturbances should operate from the ortho-position as easily as from the para-position, so as to facilitate ionization of the acid. The suggestion offered by Dippy and Lewis (45,46) is that nitroxyl in the ortho-substituted acids (phenylacetic (V), /3-phenylpropionic, and cinnamic (VI)) is capable of chelating with hydrogen of the methylene group along the lines proposed by Sidgwick and Callow (156) for onitrotoluene.…”

An Extended Table of Hammett Substitutent Constants Based on the Ionization of Substituted Benzoic Acids

“…The dissociation constants obtained by Dippy and co-workers are the most extensive set available and these values were obtained by conductance methods. [26][27][28][29][30][31][32][33][34] Accordingly, it appears reasonable to continue to use values obtained by Dippy and co-workers where such values are available. Thermodynamic dissociation constants obtained by other workers may be used as a check on the reliability of the data.…”

Dissociation constants of toluic acids in aqueous solution at different temperatures and study of related thermodynamic parameters

“…It has already been mentioned that earlier workers (7,8,14) determined the ionization constant values of isomeric toluic acids by the colorimetric method. Modified photoelectric colorimetry (15,20,25) and conductometry ( 21) have also been applied Table III.…”

“…An apparent value of p Ka corresponding to mH+' may be calculated from eq 6, which is obtained by combining eq 1 and the limiting pA:a' = 2Sf(pd0)1/2-log my*'(m2 + « *') (mr wH+') (6) law; thus, pKa may be obtained by extrapolation of the values of pKa vs. ionic strength. The observed values of pKa for the three isomeric toluic acids at the seven different temperatures could be expressed as a function of the absolute temperature T(=t°C + 273.15) by an equation of the form suggested by Harned and Robinson (22), namely pKa = / ~ß + yT (7) The choice of the above form has also been preferred in a very recent paper by Bates, Roy, and Robinson (2). The standard thermodynamic quantities AG°, AH0, AS0, and ACP°f or the dissociation process are related in simple fashion to the constants of eq 7.…”