Ionization constants, of carboxyl

Ionization constants of czs-3-substituted acrylic acids have been correlated with the Hammett equation by Hogeveen (58) and by Charton (60). Charton has correlated ionization constants for a number of other c/s-vinylene sets with the Hammett equation (60). Charton and Charton have correlated some cw-vinylene sets with the extended Hammett equation [eq. (2)] (73). Sufficient data are available for twelve sets of cis-vinylene equilibria, of which four sets represent ionization constants of hydroxy compounds (sets 12-1 to 12-4) and eight sets represent ionization constants of carboxylic acids (sets 12-5 to 12-12). All sets have been correlated with eq. (24) and eq. (2). Sets studied are reported in Table XII. Results of the correlations are reported in Table XIII. Sets designated A were correlated with eq. (24), sets designated B were correlated with eq. (2). In the case of the second ionization constant of 2,3,5,6-tetrahydroxy-l,4-benzoquinone (set 12-3), it is uncertain which hydroxyl group ionizes therefore, the value for X = OH was excluded from the correlation. All of the sets 12-1 to 12-4 gave significant correlations with both eq. (24) and eq. (2),... 

Thus, a values for XCH2 groups may be defined free of resonance effects. Values of a for groups other than XCH2 will contain a resonance effect if they are defined from the ionization constants of carboxylic acids. 

The basis of the extiathermodynamic approach is simple. Consider any property such as the heat of formation. The effect, for instance, of an amino group in benzene is assumed to be the same as its effect in any aromatic compound (e.g. toluene or any of the xylenes). The principle is best illustrated for the ionization constant of carboxylic acids in a number of solvents ... 

B. Pullman (1) In these days a proposal for the mechanism of acid-base ionization must be quantitative in order to be significant. Authors who relate the ionization constants of carboxylic acids. O... 

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... 

None of the other reactions so far discussed involve interaction between a pair of charged species. This is but another instance of the electrostatic effect shown by Kirkwood and Westheimer to be responsible for the disparity between the first and second ionization constants of dibasic acids, for the effect of the carboxylate ion on the basicity of an a-amino acid, and for the difference in reactivity of ionic compounds compared with analogous nonionic species in acid- or base-catalyzed reactions. ... 

Acid dissociation constants have also been measured for several pyrrole carboxylic acids238-243 (Table X) and attempts have been made, by taking the reaction constant p for ionization as unity, to extrapolate the a values obtained from the ionization constants of... 

Photon correlation spectroscopy, carried out under very dilute conditions, has unambiguously demonstrated the expansion of carboxylic emulsion polymers at high pH, but it may not always be useful in predicting properties of practical interest. Of special concern is the apparent decrease in the intrinsic ionization constant of surface carboxyls at very low concentration. Since most uses of emulsion polymer occur at high concentrations, the measurement of particle-particle interactions is of great practical importance (21J. It has been found that the sedimentation and viscometric techniques closely reflect viscosity changes in latexes at much higher solids. Extension of the PCS approach to more concentrated systems is underway but not without problems (22). 

Comparison of the ultraviolet absorption and ionization constants of aminopyrazine, methylaminopyrazine, and dimethylaminopyra-zine indicates that aminopyrazine exists as such and not in the tautomeric imino form.147 Aminopyrazines, like their pyridine analogs, form diazonium salts, which readily decompose to the corresponding pyrazinones. For example, nitrous acid treatment of aminopyrazine318 and 2-aminopyrazine-5-carboxylic acid210 gives the corresponding pyrazinones in 30 and 59% yield, respectively. The diazonium salt from aminopyrazine cannot be converted into bromo-pyrazine under the conditions of the Sandmeyer reaction. 

On the other hand, the acid pK of the carboxylic group of the substrate is 5.5, and the pK s for ring protonation of the azulene system and for O protonation of the COOH group are known to be negative (application of the H0 scale, see Vol. 2, p. 358). Consequently, the coefficient with a value of 3.8 x 10-2 mole. I"1 in the denominator of eqn. (13) cannot be an ionization constant of the substrate, and it must have another meaning. An explanation of the experimental findings is offered on the basis of the mechanism... 

Using the data on the ionization constants of the 5-substituted 2-carboxylic acids 30 and 31 (69T5815 72JCS(P2)1738), the transmission of total electronic effects, was shown to increase in the sequence benzene < thiophene < selenophene, tellurophene < furan < pyrrole (Table XXII). 

As mentioned in Section IV.A, Nelson and Yang [494] have proposed a surface complexation model to describe the effect of pH on adsorption equilibria of chlorophenols on activated carbon. To account for the well known suppression of uptakes as pH increases, they introduce the ionization constants of separate acidic (A) (e.g., carboxyl) and basic (B) (e.g., pyrone) functional groups on the carbon surface ... 

In discussing the permanent polarization of single bonds we cannot help wondering how far along a carbon chain this factor exerts its influence. From an examination of the ionization constants of the aliphatic chloroacids we see (Table 2) that two methylene groups interposed between the carboxyl group and the carbon to which the chlorine atom is attached almost destroy the effect of the permanent polarization of the carbon-chlorine bond upon the carboxyl group. 

Table 3 Ionization constants of some carboxylic acids in water at 25°C.

Alkyl groups have little effect on the acidity of a carboxylic acid. The ionization constants of aU acids that have the general formula C H2 +iC02H are very similar to one another and equal approximately 10 (p/fa 5). Table 19.2 gives a few examples. 

The most characteristic property of phenols is their acidity. Phenols are more acidic than alcohols but less acidic than carboxylic acids. Recall that carboxylic acids have ionization constants of approximately 10 (pK 5), whereas the A a s of alcohols are in the... 

Ives, Linstead and Riley 4 have made extensive studies on the effect of a double bond in the carbon chain of aliphatic acids. In general, the unsaturated acids are stronger than the saturated ones. However, the effect of the double bond decreases as it is removed from the carboxyl. These effects are shown in the results obtained by these authors quoted in Table I. The ionization constants of corresponding unsatu-... 

The ionization constants of the tellurophene-2-carboxylic acid and a number of 5-substituted derivatives have been determined in water at 25° by using a potentiometric procedure.63 The thermodynamic pK, values are summarized in Table XXI. 

Ionization Constants of 5-SuBSTnvrHD Tellurophenb-2-Carboxylic Acids in Water at 25° ... 

In considering the structures of the organic acids, it is evident that the a-carbon atom (the one combined directly with the carboxyl group) influences the ionization constant of the acid to the greatest extent. A classification of the acids will be given which depends upon the direction of the valences by which this a-carbon atom is combined with the other atoms in the molecule, or its valence, or its oxidation potential. This divides the acids into four general classes which may be formulated as follows ... 

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