Constant acidity

Obviously, to model these effects simultaneously becomes a very complex task. Hence, most calculation methods treat the effects which are not directly related to the molecular structure as constant. As an important consequence, prediction models are valid only for the system under investigation. A model for the prediction of the acidity constant pfQ in aqueous solutions cannot be applied to the prediction of pKj values in DMSO solutions. Nevertheless, relationships between different systems might also be quantified. Here, Kamlet s concept of solvatochro-mism, which allows the prediction of solvent-dependent properties with respect to both solute and solvent [1], comes to mind. 

The ionisation ratio (/ = [SH+]/[S]) can be calculated from a knowledge of the acidity function (hj.) followed by the substrate, and the acidity constant of the conjugate acid. Thus, when I p i ... 

The acidity constant for an acid-base indicator was determined by preparing three solutions, each of which has a total indicator concentration of 5.00 X 10- M. The first solution was made strongly acidic with HCl and has an absorbance of 0.250. The second solution was made strongly basic and has an absorbance of 1.40. The pH of the third solution was measured at 2.91, with an absorbance of 0.662. What is the value of K, for the indicator ... 

The acidity constant for an organic weak acid was determined by measuring its absorbance as a function of pH while maintaining a constant total concentration of the acid. Using the data in the following table, determine the acidity constant for the organic weak acid. 

Solvent variation can gready affect the acidity of hydantoins. Although two different standard states are employed for the piC scale and therefore care must be exercised when comparing absolute acidity constants measured in water and other solvents like dimethyl sulfoxide (DMSO), the huge difference in piC values, eg, 9.0 in water and 15.0 in DMSO (12) in the case of hydantoin itself, indicates that water provides a better stabilization for the hydantoin anion and hence an increased acidity when compared to DMSO. 

A Hammett relationship of the form ApK = 5.8am has been proposed for 4-substituted pyrazoles (74TL1609) in order to explain the effect of 4-nitro ApK = 4.5, am = 0.71) and 4-diazo groups (Apiifa = 10.0, am = 1.76). The acidity constants of a series of pyrazolidine-3,5-diones have been determined (75AJC1583) and the 4- -butyl-1,2-diphenyl derivative phenylbutazone has a pK of 4.33. 

This procedure can now be repeated with a base D that is slightly weaker than C, using C as the reference. In this stepwise manner, a series of p determinations can be made over the acidity range from dilute aqueous solution to highly concentrated mineral acids. Table 8-18 gives pS bh+ values determined in this way for nitroaniline bases in sulfuric and perchloric acid solutions. This technique of determining weak base acidity constants is called the overlap method, and the series of p kBH+ values is said to be anchored to the first member of the series, which means that all of the members of the series possess the same standard state, namely, the hypothetical ideal 1 M solution in water. 

The rate of saponification of ethyl 2-thenoate, in contrast to ethyl 3-thenoate, was found to be considerably slower than predicted from the pKa of the acid, showing that the reactivities of thiophenes do not parallel those of benzene. The first explanation, that this was produced by a steric effect of the ring sulfur similar to the case in or /lo-substituted benzenes and in ethyl 1-naphthoate, could not be upheld when the same effect was found in ethyl 2-furoate. It was later ascribed to a stereospecific acid strengthening factor, involving the proper relation of the carboxylic hydrogen and the heteroatom, as the rate of saponification of 2-thienylacrylic acid was in agreement with that predicted from the acid constants. ... 

From measurements of acid constants, > " and rates of esteri-... 

The rates of alkaline hydrolysis of ethyl benzoate, ethyl 2-thenoate, and 3-thenoate are very similar. Also, the acid constants of benzoic acid (pAfl = 4.20) and 3-thenoic acid (pKa " 4.08) are of the same magnitude, whereas 2-thenoic acid (piCa = 3.49) is stronger. 

In the dilute aqueous solution normally used for measuring acidity, the concentration of water, H20], remains nearly constant at approximately 55.4 M at 25 °C. We can therefore rewrite the equilibrium expression using a new quantity called the acidity constant, Ka. The acidity constant for any acid HA is simply the equilibrium constant for the acid dissociation multiplied by the molar concentration of pure water. 

Stronger acids have their equilibria toward the right and thus have larger acidity constants, whereas weaker acids have their equilibria toward the left and have smaller acidity constants. The range of K.A values for different acids is enormous, running from about lO1 for the strongest acids to about 10-60 for the... 

Recall from the earlier discussion of acidity in Sections 2.7 through 2.11 that the strength of any acid HA in water can be expressed by an acidity constant, Ka. 

Table 17.1 Acidity Constants of Some Alcohols and Phenols...

Like other Bronsted-Lowry acids discussed in Section 2.7, carboxylic acids dissociate slightly in dilute aqueous solution to give H30+ and the corresponding carboxylate anions, RC02. The extent of dissociation is given by an acidity constant, Ka. 

Table 22.1 Acidity Constants for Some Organic Compounds...

Acidity constant, Ka (Section 2.8) A measure of acid strength. For anv acid HA, the acidity constant is given bv... 

Appendix A Nomenclature of Polyfunctional Organic Compounds A-1 Appendix B Acidity Constants for Some Organic Compounds A-8 Appendix C Glossary A-10 Appendix D Answers to In-Text Problems A-30... 

Sometimes the acidity constant (acidity dissociation constant, KJ is employed for the conjugate acid of the amine used. Under these circumstances ... 

The linear correlation of log[NO+]/[HN02] with HR yields an acidity constant pATR = -7.8 in H2S04 corresponding to equal concentrations of NO+and HN02 at about 57% H2S04. ... 

The subscript R in pA R indicates that the acidity constant KR is based on the ifR acidity scale. 

As in the case of diazotization by N203 (Sec. 3.1), either the formation of XNO or the nitrosation of the amine (or of the aminium ion) may be rate-limiting. Under most experimental conditions the second alternative applies. If a steady-state concentration of XNO exists (which is however, not always the case) the reaction system of Schemes 3-26 and 3-27 yields the rate equation shown in Scheme 3-29 if it is the amine base (ArNH2) that is nitrosated. Xa is the acidity constant of the conjugate acid (ArNH3). 

Luchkevich et al. (1986, Table 6) demonstrated that for the three isomeric nitro-benzenediazonium ions and their (Z)-diazohxydroxides the acidity constants can be determined by ultraviolet spectrophotometry, by potentiometry, from the kinetics of reaction with hydroxide ions, from the (Z) (E) isomerization kinetics, and from the kinetics of azo coupling reactions. These independent methods gave surprisingly consistent results. ... 

Each of the curves in Figure 5-3 exhibits two or three pH regions in which the slope of the logarithmic plot is approximately —1, with intermediate regions where the slope is small or zero. Lewis and Hanson (1967) showed that in the case of (E,)-4-nitrobenzenediazoate the portion of the curve with slope —1 at relatively high pH was consistent with the acidity constant K3 of the (E )-diazohydroxide determined either by titration or spectrophotometrically, the relevant results being (by... 

As suggested by Roberts and Moreland many years ago (1953), the acidity constants of 4-substituted bicyclooctane-l-carboxylic acids provide a very suitable system for defining a field/induction parameter. In this rigid system the substituent X is held firmly in place and there is little possibility for mesomeric delocalization or polarization interactions between X and COOH (or COO-). Therefore, it can be assumed that X influences the deprotonation of COOH only through space (the field effect) and through intervening o-bonds. On this basis Taft (1956, p. 595) and Swain and Lupton (1968) were able to calculate values for o and crR. 

Hammett (1937) observed that the value of the constant op for a nitro group as obtained from the acidity constant of 4-nitrobenzoic acid (crp = 0.78) was applicable, for example, to the rate of hydrolysis of ethyl 4-nitrobenzoate, but not to the acidity constants of 4-nitrophenol and 4-nitroanilinium ion. These two acidities were compatible only with significantly higher constants denoted by op (1.21 and 1.27, respectively). This was (correctly) interpreted as resulting from the donor-acceptor interaction represented by the mesomeric structures 7.7. 

The effect of the diazonio group on the reactivity of benzene derivatives was studied quantitatively by Lewis and Johnson (1959) by measuring the acidity constants of 3- and 4-diazoniobenzoic acid, of the 4-diazonio-anilinium ion, of 3- and 4-diazo-niophenol, and of 4-diazoniophenylacetic acid. The results are given in Table 7-3. 

Quantitative studies based on kinetic measurements using strongly electrophilic diazonium ions and, as coupling components, 1-naphthol, 2-naphthol-6-sulfonic acid, and resorcinol in aqueous acid were made by Sterba and coworkers (Kropacova et al., 1970 Kavalek et al., 1970 Sterba and Valter, 1972 Machackova et al., 1972a). In a typical case (2,6-dichloro-4-nitrobenzenediazonium ion and 1-naphthol) the dependence of the logarithm of the measured rate constant (ks) on pH was linear with a slope of 1. At pH < 1, however, a practically constant value of ks was obtained. The measured rate constants therefore correspond to Scheme 12-62, in which the first term relates to the reaction of the naphthoxide ion and the second to that of the undissociated naphthol Ka is the acidity constant of 1-naphthol. 

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