Dissociation constants, effect Table

A very interesting steric effect is shown by the data in Table 7-12 on the rate of acid-catalyzed esterification of aliphatic carboxylic acids. The dissociation constants of these acids are all of the order 1(T, the small variations presumably being caused by minor differences in polar effects. The variations in esterification rates for these acids are quite large, however, so that polar effects are not responsible. Steric effects are, therefore, implicated indeed, this argument and these data were used to obtain the Es steric constants. Newman has drawn attention to the conformational role of the acyl group in limiting access to the carboxyl carbon. He represents maximum steric hindrance to attack as arising from a coiled conformation, shown for M-butyric acid in 5. 

If a methyl group replaces a hydrogen atom on the carbon of the C==N bond across which addition of water occurs, a considerable reduction in the extent of water addition is observed. Conversely, the existence of such a blocking effect can be used as a provisional indication of the site at which addition of water occurs, while the spectrum and acid dissociation constant of the methyl derivative provide a useful indication of the corresponding properties of the anhydrous parent substance. Examples of the effect of such a methyl group on equilibria are given in Table IV. 

Superior antimicrobial activity in alkaline pH (seawater is always above pH 8), in the presence of nitrogenous organic matter, and due to lower volatility has been documented for bromine antimicrobials3 4. The pKa acid dissociation constants for HOC1 and HOBr are 7.4 and 8.7, respectively the dissociated acids are less effective antimicrobials4,5. Undissociated hypohalous acids are more effective because they are far better halogenating agents compared to the dissociated anion (hypohalite). Table 1 shows the effect of acid dissociation on antimicrobial performance in well-controlled laboratory experiments. 

Table 2.1 Effect of kon and koS values on the equilibrium dissociation constant Kd...

Because of the scarcity of electronic paramagnetic resonance data, and because of the frequent unreliability of the data from paramagnetism, boiling point elevation, spectrophotometry, and ortho-para hydrogen conversion, most published radical dissociation constants can be accepted only with reservations. An error of 50 % is not at all improbable in many cases. We are therefore not yet in a position to explain, or rather to test our explanations of, small differences in dissociation constants. Table I shows the values of K corresponding to various hexaarylethanes in benzene at 25°. Because of the order of magnitude differences in Table I, however, it is likely that some of the expected large effects, such as steric and resonance effects, exist. 

When solvated ions migrate within the electrolyte, the drag force applied by the surrounding solvent molecules is measured by solvent viscosity rj. Thus, in a solvent of lower viscosity, the solvated ions would move more easily in response to an applied electric field, as expressed by the Einstein—Stokes relation (eq 3). Solvents of low viscosity have always been considered the ideal candidates for electrolyte application however, their actual use was restricted because most of these solvents have low dielectric constants (Tables 1 and 2) and cannot dissociate ions effectively enough to prevent ion pairing. 

This preparative scheme leads to only 30% yield due to the side reactions between the meto-astatoaniline diazonium salt and astato-phenol, which cannot be eliminated even by continuous extraction of the product with n-heptane (167). All the astatophenols synthesized to date have been identified by either HPLC (99,104) or TLC (160,166,167). Their dissociation constants (KJ have been established from extraction experiments by measuring the relative distribution of compounds between aqueous borax buffer solutions and n-heptane as a function of acidity. On the basis of these derived values, the Hammett a-constants and hence the field (F) and resonance (R) effects have been estimated for these compounds (167) (see Table VI). The field effect for astatine was found to be considerably weaker than that for other halogens the resonance effect was similar to that for iodine (162). 

It is accepted that the acmal nucleophile in the reactions of oximes with OPs is the oximate anion, Pyr+-CH=N-0 , and the availability of the unshared electrons on the a-N neighboring atom enhances reactions that involve nucleophilic displacements at tetravalent OP compounds (known also as the a-effect). In view of the fact that the concentration of the oximate ion depends on the oxime s pATa and on the reaction pH, and since the pKs also reflects the affinity of the oximate ion for the electrophile, such as tetra valent OP, the theoretical relationship between the pATa and the nucleophilicity parameter was analyzed by Wilson and Froede . They proposed that for each type of OP, at a given pH, there is an optimum pK value of an oxime nucleophile that will provide a maximal reaction rate. The dissociation constants of potent reactivators, such as 38-43 (with pA a values of 7.0-8.5), are close to this optimum pK, and can be calculated, at pH = 7.4, from pKg = — log[l//3 — 1] -h 7.4, where is the OP electrophile susceptibility factor, known as the Brpnsted coefficient. If the above relationship holds also for the reactivation kinetics of the tetravalent OP-AChE conjugate (see equation 20), it would be important to estimate the magnitude of the effect of changes in oxime pX a on the rate of reactivation, and to address two questions (a) How do changes in the dissociation constants of oximes affect the rate of reactivation (b) What is the impact of the /3 value, that ranges from 0.1 to 0.9 for the various OPs, on the relationship between the pKg, and the rate of reactivation To this end, Table 3 summarizes some theoretical calculations for the pK. ... 

It turns out that the CNE part accompanying CH dissociations not only is quantitatively important but also produces a remarkable effect (Table 12.7). Indeed, CNE compensates the major part, if not all, of the differences existing between sch energies 8ch + CNE is nearly constant for aU the C(sp )—H bonds, on one hand, and also for the C(sp )—H bonds, on the other hand. Hence, incorporating now the small (<0.05 kcal/mol) nonbonded interactions into this approximation, we rewrite Eq. (12.8) as follows... 

It is apparent that for the reactive extraction to be effective, the cephalosporins should be in dissociated form which is maintained by selecting appropriate pH values depending on the dissociation constant (pK ) of the molecules. Table 1 gives the pK values for various cephalosporin molecules. 

The neutral carboxyl group is not very effective in increasing the reduction rate of the complex. However, when the proton is removed from the carboxyl, the effect can increase and is greatest when the carboxyl ion is in a configuration favorable to chelation. Thus, the inverse (H+) path is not even observable for acid succinate in the same acidity range as that for which this path is important in the acid malonato reaction. The acid dissociation constants are known well enough so that the behavior difference between acid malonato and acid succinato can not be entirely ascribed to different acidities of the complexes. The results obtained with the acid malonate complexes, as reported in Table II, incidentally provide no support for the hypothesis (22) that electron transfer takes place by remote attack across hydrogen bonds. 

For the complex /ac,/ac-(H20)2(NH3)3Cr(0H)Cr(NH3)3(H20)25+ the first acid dissociation constant clearly shows that the singly deprotonated species must be hydrogen bond stabilized (Table XIX). However, the fact that the difference between the first and the second acid dissociation constants for this system is relatively small is consistent with stabilization also of the doubly deprotonated species (by two intramolecular hydrogen bonds) as shown in Fig. 15. A similar effect is expected for the cations (H20)5Cr(0H)Cr(H20)55+ and (H20)5Ir(0H)Ir(H20)55+. 

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). 

Solvent polarity influences the rate of copolymerization. Thus with increasing dielectric constant of the solvent, the copolymerization rate rises as a result of the increase in the dissociation constants of the active species. The apparent rate constant for the copolymerization of 2-hydroxy-4-(2,3-epoxypropoxy)benzophenone with phthalic anhydride, initiated by hexadecyltrimethylammonium bromide56), increases from 4.65 x 10 4 s 1 in o-xylene to 6.84x 10 4 s-1 in nitrobenzene. Hilt et al.S4) proposed a suitable model illustrating the effect of solvent polarity in the copolymerization of phthalic anhydride with ethylene glycol carbonate in a mixture of nitrobenzene and trichlorobenzene (Table 4). With increasing fraction of the more polar nitrobenzene, the rate of copolymerization increases. 

Table II gives the ratios of the 31P-NMR linewidths in the presence of calcium or magnesium for egg PC and DMPC vesicles. For egg PC this ratio is 1.0 at 10° and 20°C, indicating that calcium and magnesium have identical dissociation constants in their binding to egg PC at these temperatures. At 35°C the ratio is slightly less than 1.0. This difference, however, is small compared with the effects observed with saturated phosphatidylcholines.

In this chapter we have seen that acid dissociation constants are needed to calculate the dependence of apparent equilibrium constants on pH. In Chapter 3 we will discuss the calculation of the effects of ionic strength and temperature on acid dissociation constants. The database described later can be used to calculate pKs of reactants at 298.15 K at desired ionic strengths. Because of the importance of pKs of weak acids, Table 1.3 is provided here. More experimental measurements of acid dissociation constants and dissociation constants of complex ions with metal ions are needed because they are essential for the interpretation of experimental equilibrium constants and heats of reactions. A major database of acid dissociation constants and dissociation constants of metal ion complexes is provided by Martell, Smith, and Motekaitis (2001). 

TABLE 8.3 Effective Proton-Activity Dissociation Constants (KJ for Brpnsted Acids in Dimethylformamide (0.5 M TEAP). 

Recently the 2-naphthol system has been re-examined. From lifetime measurements at various pH values, Loken et al. (1972) and Ofran and Feitelson (1973) determined the rate constants (see Table 10). The effects of further ring substituents on the rate constants has been studied by Rosenberg and Brinn (1972) (see Table 10). Dissociation in the Si state is retarded by substituents which increase the electron density on the phenolic oxygen the electron-donating methyl group reduces k 1 while chloro- and bromo-substitution, as expected, increase it. 

The comparison of dissociation constants (K) of several phenols in aqueous solution with those of their nitro derivatives (Table 28) illustrates this effect of the nitro group. 

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