PKa values bases

Saurina, J., Hemandez-Cassou, S., Tauler, R., Izquierdo-Ridorsa, A., Spectrophotometric determination of pKa values based on a pH gradient flow-injection system. Anal. Chim. Acta... 

Estimated pKa Values Based on Rate Constants for N-Methylation and Bronsted Equation [Eq. (15)1°... 

Figure 3-2 Acid-base titration curve for hen lysozyme at 0.1 ionic strength and 25°C. O, initial titration from the pH attained after dialysis , hack titration after exposure to pH 1.8 A, hack titration after exposure to pH 11.1. The solid curve was constructed on the basis of "intrinsic" pKa values based on NMR data. From Kuramitsu and Hamaguchi5...

Base is chosen so as to favor enolate formation. Acidity of C-H bond must be greater (lower pKa value) than that of the conjugate acid of the base (C S table 1.1, pg 3)... 

Most organic compounds are bases, that is, they are capable of accepting a proton. The best-studied organic bases are the moderately strong ones, which will receive a proton in dilute aqueous solutions amines are the most important examples. The pKa value of the protonated base, referred to the infinitely dilute aqueous solution, is the usual measure of base strength, and the pH of the solution is a quantitative measure of solvent acidity, or ability to transfer a proton. 

The spectral changes which occur in increasingly acid solutions of polyaza-heterocycles may indicate a second ionization. This event, however, can readily be distinguished from dehydration by measuring the spectra in anhydrous dichloroacetic acid, provided that the pKa value for the anhydrous species is above 1. Anhydrous dichloroacetic acid has a Hammett acidity function (Hq) of — 0.9 (as determined using o-nitroaniline as the solute), and the ultraviolet spectrum of a base with a p > 1 would be that of the anhydrous cation in this 2 A. Albert and W. L. F. Armarego, J. Chem. Soc. 4237 (1963). 

It is the rapid increase in rates of hydration with increasing hydrogen ion concentration that prevents measurement with existing apparatus of the -pKa values of anhydrous bases such as pteridine. For example, at pH 1, hydration of the anhydrous cation is half-complete in 0.01 sec at 20°. Conversely, it is the comparative slowness of the reactions in near-neutral solutions that makes it possible, by adding acid solutions to near-neutral buffers, using the stopped-flow technique, to determine the p STa values of the hydrated species. 

Predicting Acid-Base Reactions from pKa Values 53... 

The larger the pKa value is, the weaker is the acid and the stronger the base. 

Charton s at value for S02Me merits comment in that it is based not on the behavior of aliphatic or alicyclic carboxylic acids but on the pKa values of 4-substituted quin-uclidinium ions 30 in water. This system has been advocated by Grob and Schlageter171 as... 

Charton s at values for SOPh and S02Ph are based on the pKa values for phenylsulfinyl- and phenylsulfonyl-acetic acids137, which we have already discussed in connection with the transmission of electronic effects by SO and S02 (Section III.C). These groups have also been the subject of a detailed study by Hogeveen and Montanari172, who measured the pKa values of some 3-phenylthio-, 3-phenylsulfinyl- and... 

The second problem involves the measurement of pKa values for carbonyl and thiocarbonyl derivatives. Grieg and Johnson (157) have pointed out that the measurement of pKa values for very weak bases (11) is an inaccurate and arbitrary process. Of particular difficulty for our purposes is the fact that different carbonyl derivatives may require different acidity functions. As a result of this situation, no attempt was made to make correlations of pKa data for carbonyl and thiocarbonyl derivatives with eq. (2). Because accurate pKa values can be measured for imines, these values were correlated with eq. (2), although the conformational problem remains. The imine sets were first studied by Charton and Charton (73), who correlated them with eq. (2). No correlations of data for carbonyl or thiocarbonyl derivatives with eq. (2) are extant in the literature. Bhaskar, Gosavi, and Rao (158) have reported that AG values for complex formation of substituted thioureas with iodine are a linear function of the Taft a values. Drago, Wenz, and Carlson (159) have reported similar results for complex formation between iodine and substituted amides. Oloffson (160) has reported a linear relationship between -AH for the complex of substituted N,N-dimethylamides with SbCls and the ffj constants. 

Steric factors and especially hindered rotations may change the pATa of an acid by up to two pAfa units as has been found for 2,6-disubstituted pyridines 2,6-di-t-butylpyridine [70k] is an unusual (Kanner, 1982) non-nucleophilic base which has a surprisingly low pKa value. When [70k] is compared to other 2,6-dialkylsubstituted pyridines [70] it is found to be the only disubstituted pyridine with a smaller pKa than pyridine itself (see Table 29). The exceptional behaviour of [70k] has been investigated intensively... 

Understanding the acidity and/or basicity of acids and bases in macrocyclic systems has an important connection to biological problems. It should help us to understand enzymes. In many proteins, drastically varying pKa values have been found for functional groups in the active site for example, the... 

Kuhn s carbanion analogues, [44 ], [45 ] and [46 ], have recently been synthesized, and the precursor hydrocarbons [44]-H, [45]-H and [46J-H dissociate into the respective anions in DMSO to show deep blue colours without any added base (Kinoshita et al., 1994). A fullerene anion, Bu Qb [47 ], has also been obtained as a stable carbanion (Fagan et al., 1992) its lithium salt has been isolated in the form Li [47 ]-4MeCN or Li [47"]-3-4THF. Several stable all-hydrocarbon anions of precursor hydrocarbons with low pKa values ( 7) are listed in Table 2, along with their oxidation potentials, ox-... 

From these equations it is possible to predict the effective lipophilicity (log D) of an acidic or basic compound at any pH value. The data required in order to use the relationship in this way are the intrinsic lipophilicity (log P), the dissociation constant (pKa) and the pH of the aqueous phase. The overaU effect of these relahonships is the effechve hpophilicity of a compound, at physiological pH, is approximately the log P value minus one unit of hpophilicity, for every unit of pH the pKa value is below (for acids) and above (for bases) pH 7.4. Obviously for compounds with mul-hfunchonal ionizable groups the relahonship between log P and log D, as weU as log D as a function of pH become more complex [65, 68, 70]. For diprotic molecules there are already 12 different possible shapes of log D-pH plots. 

The effect of temperature on acid or base pKa values cannot be reliably predicted [2, 17, 23]. For many nitrogenous bases, the pKa decreases by 0.1-0.3 for every 10 °C rise in temperature. For some carboxylic acids (e.g. acetic, benzoic, salicylic acids), the pKj remains essentially unchanged between 25 and 37°C. 

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