Ionization of organic acids

The ionization of organic acids has been studied extensively and the results obtained are of the greatest value, as they furnish a means of comparing radicals as to their relative positive or negative nature. The extent to which an acid is ionized varies with the concentration of its solution. The comparison of the ionization of two acids is simplified by the fact that a mathematical relation exists between the extent of the ionization of an acid and its concentration, which is represented by the formula,... 

G.I. Almerindo, D.W. Tondo, and J.R. Pliego, Ionization of organic acids in dimethyl sulfoxide solution A theoretical ab initio calculation of the pKa using a new parametrization of the polarizable continuum model, J. Phys. Chem. A 108 (2004), pp. 166-171. 

This is a detailed review and evaluation of the enthalpies, Gibbs energies, entropies, and heat capacity changes accompanying ionization of organic acids. Included are eleven tables of data on various types of acids, including the carboxylic acids, phenols, anilinium ions, ammonium ions, the amino acids, barbituric acids, and several inorganic acids. The authors also discuss the interpretation of the data in terms of molecular considerations. The tabulated data refer to 25 "C and standard state conditions. There are 224 references. 

Hi) pH. Since pH determines the degree of ionization of organic compounds, it can also affect the rate of migration of these compounds. Increase in pH increases ionization of organic acids and a decrease in pH increases the ionization of organic bases. For an ampholyte such as an amino acid, which has both acidic and basic properties, both the above effects apply. (Figure 12.3)... 

In a series of organic acids of similar type, not much tendency exists for one acid to be more reactive than another. For example, in the replacement of stearic acid in methyl stearate by acetic acid, the equilibrium constant is 1.0. However, acidolysis in formic acid is usually much faster than in acetic acid, due to higher acidity and better ionizing properties of the former (115). Branched-chain acids, and some aromatic acids, especially stericaHy hindered acids such as ortho-substituted benzoic acids, would be expected to be less active in replacing other acids. Mixtures of esters are obtained when acidolysis is carried out without forcing the replacement to completion by removing one of the products. The acidolysis equilibrium and mechanism are discussed in detail in Reference 115. 

Serjeant, E. P., Dempsey, B. Ionization Constants of Organic Acids in Aqueous Solution, Pergamon, Oxford, 1979. 

Extractive alkylation is used to derivatize acids, phenols, alcohols or amides in aqueous solution [435,441,448,502]. The pH of the aqueous phase is adjusted to ensure complete ionization of the acidic substance which is then extracted as an ion pair with a tetraalkylammonium hydroxide into a suitable immiscible organic solvent. In the poorly solvating organic medium, the substrate anion possesses high reactivity and the nucleophilic displacement reaction with an alkyl halide occurs under favorable conditions. 

Johansson, P.-A. Gustavii, K., Potentiometric titration of ionizable compounds in two phase systems. 2. Determination of partition coefficients of organic acids and bases, Acta Pharm. Suecica 13, 407 -20 (1976). 

FIGURE 15.2 Common protein ionization methods used for MS-based proteomics. Two common ionization technologies are currently available for protein analysis. Top ESI volatilizes and ionizes peptides and proteins in solution. Bottom MALDI uses analytes that are co-crystallized in a matrix composed of organic acid on a solid support. A pulse of ultraviolet laser evaporates the matrix and analyte into gas phase, resulting in generation of single charge ions. 

Thus, on measuring a partition coefficient of HA, it is imperative to know which species is being measured, i.e., neutral (undissociated, HA) or charged species (A ). Mathematical procedures can be used to take into account the complicating equilibria, and partition coefficients can be calculated for both the nonionized and ionized species of organic acids. The difference in partition coefficient between the two species is approximately... 

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. 

Figure 5.16 Effect of pH on rejection of organic acids. Solute rejection increases at the pKa as the acid converts to the ionized form. Data from Matsuura and Sourirajan [46,47]...

As will be seen, the large amount of quantitative structure-activity relationship (QSAR) modeling that has been carried out for soil sorption has almost exclusively involved nonionic organic compounds. For strongly ionizing and inorganic chemicals, no QSARs are available. However, Bintein and Devillers (1994) developed a soil sorption QSAR that incorporated correction factors for ionization of weak acids and bases. 

Not all the hydrogen atoms contained in the molecules of organic acids ionize when in solution to give H+ ions. It is only the acid active hydrogens that can do this. 

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