Acid-dissociation constant carboxylic acids

Both the carboxyl and the mercapto moieties of thioglycolic acid are acidic. Dissociation constants at 25°C are for pR, 3.6 pi, 10.5. ThioglycoHc acid is miscible ia water, ether, chloroform, dichloroethane and esters. It is weakly soluble ia aHphatic hydrocarbons such as heptane, hexane. Solvents such as alcohols and ketones can also react with thioglycolic acid. 

The acid dissociation constants of some representative carboxylic acids are tabulated in Table 4. 

Another approach to evaluating cri was taken by Roberts and Moreland,who defined inductive substituent constants in terms of the acid dissociation constants of 4-substituted bicyclof2.2.2]octane-l-carboxylic acids, 3. 

The least squares value for the p constant obtained by this procedure is +6.2 it wiU be obviously subject to change as more meta and epi substituents become available. Only the cata-NO group was excluded from the above plot because it causes a strongly enhanced resonance effect in nucleophilic substitution (Section IV,C, l,a) and an anomalous effect of uncertain origin in the dissociation of carboxylic acids. It can be assumed that the reaction constant for 4-chloro-... 

Grunwald, E. Berkowitz, B. J., The measurement and correlation of acid dissociation constants for carboxylic acids in the system ethanol-water. Activity coefficients and empirical activity functions, j. Am. Chem. Soc. 73, 4939 -944 (1951). 

Bacarella, A. L. Grunwald, E. Marshall, H. R Purlee, E. L., The potentiometric measurement of acid dissociation constants and pH in the system methanol-water. pKa values for carboxylic acids and anilinium ions, j. Org. Chem. 20, 747-762 (1955). 

The pronounced electron-withdrawing nature of the 1,2,5-thiadiazole system is also evidenced by strong carbonyl electrophilic activation and by enhancement of carboxy acidity. The acid dissociation constants of thiadiazole acids, discussed in Section 4.09.4.1, fall in the range 1.5-2.5. The 1,2,3-thiadiazole carboxylic acids are easily decarboxylated at 160-200 °C. This reaction has been used for the synthesis of monosubstituted derivatives as well as the parent ring and deuterated derivatives <68AHC(9)107>. An efficient bromo-decarboxylation of 3-amino-1,2,5-thiadiazole-carboxylic acid has also been reported <70BRP1190359>. 

Singlet excited state acid dissociation constants pK can be smaller or greater than the ground state constant pK by as much as 8 units. Phenols, thiols and aromatic amines are stronger acids upon excitation, whereas carboxylic acids, aldehydes and ketones with lowest >(71, ) states become much more basic. Triplet state constants pKr are closer to those for the ground state. Forster s cycle may be used to determine A pK =pK —pK) from fluorescence measurements if proton transfer occurs within the lifetime of the excited molecule. 

The standard notation for successive acid dissociation constants of a polyprotic acid is Kt, K2, K2, and so on, with the subscript a usually omitted. We retain or omit the subscript as dictated by clarity. For successive base hydrolysis constants, we retain the subscript b. The preceding examples illustrate that Kal (or K ) refers to the acidic species with the most protons, and Kbl refers to the basic species with the least number of protons. Carbonic acid, a very important diprotic carboxylic acid derived from COz, is described in Box 6-4. 

At low pH, both the ammonium group and the carboxyl group are protonated. At high pH, neither is protonated. Acid dissociation constants of amino acids are listed in Table 10-1, where each compound is drawn in its fully protonated form. 

The inductive effect of one carboxyl group is expected to enhance the acidity of the other. In Table 18-4 we see that the acid strength of the dicarboxylic acids, as measured by the first acid-dissociation constant, K1, is higher than that of ethanoic acid (Ka = 1.5 X 10-5) and decreases with increasing number of bonds between the two carboxyl groups. The second acid-dissociation constant, K2, is smaller than Ka for ethanoic acid (with the exception of oxalic acid) because it is more difficult to remove a proton under the electrostatic attraction of the nearby carboxylate anion (see Section 18-2C). 

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... 

Table 3.11 Acid Dissociation Constants of Some Representative Carboxylic Acids...

Buxton GV, Sellers RM (1973) Acid dissociation constant of the carboxyl radical. Pulse radiolysis studies of aqueous solutions of formic acid and sodium formate. J Chem Soc Faraday Trans 1 69 555-559... 

Fig. 2 Theoretical influence of pH on the ratio of equilibrium constants FCapp/- pep and Kapp/Kest for peptide (solid line) and ester (dotted line) bond formation, respectively. pKi and pK2 are the acid dissociation constants of the carboxyl and amino groups, respectively, the values used for the calculation (pi i = 3.75, pK2 = 7.75) are representative of an internal peptide bond. Theoretical values calculated using the following equations Kapp/ffpep = 1/(1 + [H+]/fCi)(l + K2/[H+D with Kpep = [peptide]/[RC02-][R NH3+], Kapp/ffest = 1/(1 + Ki/[H+]) with Kest = [ester]/[RC02H][R OH]...

Amino acids (and peptides) are characterized not only by their acid dissociation constants (pff,s), but also by an isoelectric point (pi), which is the pH at which the amino acid or peptide has no net charge. For a simple amino acid with only an a-carboxyl and an a-amino group the pi is determined by a balance between the tendency of the proto-nated amino group to lose a proton and the un-protonated carboxyl group to bind a proton. It can be readily shown that this pH is... 

Fig. 1. The Relation Between the Logarithm of the Acid Dissociation Constant, pKi, in Series of Substituted Fatty Acids, and the Reciprocal of the Number of Carbon Atoms, d, between the Substituent and the Carboxyl Group.

Available from Pharmacia Biotech as Immobiline, these are "derivatives of acrylamide with different dissociation constants. For acidic Immobiline (with pK 3.6 or 4.6), carboxylic acid is chosen as a functional group for the bases, tertiary amino groups are selected. When different pK values of Immobiline are mixed in appropriate proportions, together with acrylamide, bisacrylamide, TEMED, and ammonium persulfate, the buffering groups responsible for ereating the pH gradient become covalently attached to the polyacrylamide base. 

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