Benzoic acid, ionization constants table

The ionization of hydroxy-substituted benzoic acids is also affected by chelation. If mtramoleculeir H bonding is possible, the carboxylic acid ionization constant is raised due to the stabilization of the anion (265, 114, 273). This is Ulustrated in Table 5-VII. 

Monofunctioning acids and bases A small discrepancy exists between the pK values calculated in the Tables of Chapter 2 and the standard values in the literature. For example, the pKa value for benzoic acid, calculated in Table 2.2 IS 4 16 0 01 at 20°C, (similarly we obtained 4 16 0 01 at 25 X), whereas the most highly regarded values are in the range 4 18 to 4 20 at 25" (Kortiini, etal., 1961). The reason for this discrepancy is that our calculations, up to this point, have included both concentration and activity terms, and our results for benzoic acid are mixed pKg values. These mixed constants, which we call K, are constant only for the concentration of ionized species at v. hich the pH was measured and stand in contrast to equation (31) in which all the terms are expressed as activities. The standard values for benzoic acid, on the other hand, have been calculated using activities throughout and the thermodynamic constant, KI, is entirely independent of concentration (see p. 2 and Appendix H). 

Table 19 3 lists the ionization constants of some substituted benzoic acids The largest effects are observed when strongly electron withdrawing substituents are ortho to the carboxyl group An o nitro substituent for example increases the acidity of benzoic acid 100 fold Substituent effects are small at positions meta and para to the carboxyl group In those cases the values are clustered m the range 3 5-4 5... 

We take the view of McDaniel and Brown that the Hammett substituent constants should be defined by Eq. (7-22). Table 7-1 lists many of these constants based on the ionization of meta- and para-substituted benzoic acids. 

For sets nos. 1, 2, and 3 of Table XXVII, eq. (1) appears to hold for ionization of ortho substituted benzoic acids (f =. 048 —. 058), with Kj = Pi I= 1.6 . 1. This result is reasonable for field effects transmitted only throu the molecular cavity i.e., the lines of force do not pass through appreciable solvent of high dielectric constant (the solvent is presumably excluded by the close proximity of the CO2H center and the substituent) (36). It is further of interest that eq. (1) fails for the ionization of ortho substituted benzoic acids in solvents of high OH content (sets nos. 4, 5, and 6 of Table XXVII). 

C). The value of a may be determined from its definition in terms of the ionization constant if the appropriate benzoic acid derivative has been measured. Such a values may then be used to determine p values for other reactions, and these p values in turn lead to the possibility of determining new o values. Equation 7.4.24 implies that a plot of log KA or log kA versus oA should be linear for a given series of reactions involving the same reactive groups. Extensive experimental evidence attests to this relation. Table 7.2 contains some values that have been reported in the literature. Extensive tabulations... 

Strong et al. [8] have determined the ionization constant of benzoic acid in H2O as a function of ternperamre by conductance methods. Their data are listed in Table 20.3, with the pK based on a hypothetical standard state of 1 mol dm. Temperature was controlled to +0.002°C. 

TABLE 20.3. Ionization Constants of Benzoic Acid as a Function of Temperature... 

McDaniel, D.H., Brown, H.C. (1958) Extended table of Hammett substituent constants based on the ionization of substituted benzoic acids. J. Org. Chem. 23, 420. 

Table 7-1. Hammett Substituent Constants Based on Ionization of Benzoic Acids"...

A vast amount of research has shown that the Hammett relationship holds for hundreds of sets of reactions. (Ionization of 40-odd p-substituted benzoic acids, for example, is one set.) By use of just two tables—one of a constants and one of p constants— we can calculate the relative Keq s or relative rates for thousands of individual reactions. For example, from the a value for W-NO2 ( + 0.710) and the p value for ionization of benzoic acids in water at 25" (+1.000), we can calculate that Ka for m-nitrobenzoic acid is 5.13 times as big as the Ka for benzoic acid. Using the same acid-catalyzed hydrolysis of benzamides in 60% ethanol at 80° (-0.298), we can calculate that m-nitrobenzamide will be hydrolyzed only 0.615 as fast as benzamide. 

McDaniel, D.H. and Brown, H.C. (1958). An Extended Table of Hammett Substituent Constants Based on the Ionization of Substituted Benzoic Acids. J.Org.Chem., 23,420-427. 

Table 19.3 lists the ionization constants of some substituted benzoic acids. The largest effects are observed when strongly electron-withdrawing substituents are ortho to... 

The equilibrium ionization constants KA for selected p-substituted benzoic acids including benzoic acid, their pKA values, and Hammett s constants are collected in Table 2. The plots of KA x 10s and also pKA against a for these acids are shown in Fig. 4. The data in Table 2 and the plots in Fig. 4 demonstrate their linear relationships. As K,a increases, pK decreases, and a increases. 

TABLE 6. Apparent ionization constants for silyl-substitued benzoic acids at 25 ... 

The value of the substituted derivative can be calculated from that of the parent compound (p ), the substance class-specific constant p, which is a measure of the sensitivity of the reaction to substituent effects with respect to the ionization of benzoic acids (p = 1), and tabulated (e.g. Perrin, Dempsey and Serjeant, 1981 Hansch, Leo and Taft, 1991) Hammett cror Taft cr values (Table 1.3). 

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