Ionization constants, anomalous

Iminothioamides, isothiazoles from, 108 Indole, 2,3-dehydro-JV-methyl-, 143 Ionization constants, anomalous, 6-7 covalent hydration, effect on, 5-7 Isoquinoline, activation of, 313, 314, 318, 344... 

Thus, a methyl group placed at the site of hydration decreases the proportion of the hydrated species and, hence, shifts both the ultraviolet spectra (cf. Fig. 2A and B) and the ionization constant of the substance towards normality. A valuable means for locating the site of hydration, therefore, is to introduce a methyl group in various likely places until the anomalous spectrum is lost and the spectrum of the predominantly anhydrous species restored. The effect of such a methyl group on the pjfiT value is also revealing because a decrease in the amount of the hydrated species causes a decrease in the p value,... 

These linked equilibria can give an anomalous pKa for the ionizing group. The apparent ionization constant, A(aPP) is defined by... 

Probably the most reliable value for the first ionization constant of isophthalic acid is that determined by Ang in 1958103. When the pKa value of 3.70 (water, 25 °C) is combined with a value of 4.205 for benzoic acid and a statistical correction is applied for the presence of two ionizable groups, the value of 0.204 for am is obtained. It is generally believed among physical organic chemists that CO2H should differ but little from CC Me in net electron-attracting ability and therefore the above am value is anomalously low by at least 0.1 unit (see Section IV.A). The explanation of this is not clear. There does not appear to be any information available for the first ionization of terephthalic acid of reliability comparable with that for isophthalic acid. 

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