Acidity constant estimation

During the conductometric measurements it was also observed that for various acids the equivalent conductivity became constant at higher concentrations, which can be explained67 by the formation of their homoconjugate ions HX2 = X" H+ X" for a number of acids we estimated by means of the method of French and Roe68 the formation constants concerned ... 

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

In 20% dioxan-water (Milsden and Cohen, 1972). The reference reaction is the formation of phenyl acetate from phenol and acetic acid at 25° (rate constant estimated at 1.5 x 10 10 dm3 mol-1 s 1). These authors very high rate constants for the lactonization of compounds B.2.23-25 (data in parentheses) which lead to much quoted EM s in the region of 10 M, appear to be too high by several orders of magnitude (Caswell and Schmir, 1980)... 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

The major absorption in the 31P n.m.r. spectrum of an equimolar solution of penta-phenoxyphosphorane and sodium phenoxide in DMF-acetonitrile is due to the hexaphenoxyphosphate anion, as predicted from the low equilibrium constant estimated for equation (2) (page 35).27 Catechol and phosphorus oxychloride in refluxing benzene gave the spirophosphorane(108), which with triethylamine gave the salt (109).45 On the basis of its 31P chemical shift in DMF solution, (108) was formulated86 as the free six-co-ordinate acid (110), but it seems probable that DMF is... 

Bordwell129 developed a method of estimating relative bond dissociation energies (BDE) for families of acids, HA, by combining equilibrium acidity constants, pXeA, with the oxidation potential of their conjugated bases, A, both measured in DMSO ... 

Chemical/Physical. Hydrolyzes in water forming methanol and hydrobromic acid. The estimated hydrolysis half-life in water at 25 °C and pH 7 is 20 d (Mabey and Mill, 1978). Castro and Belser (1981) reported a hydrolysis rate constant of 3 x 10 Vsec or a half-life of 26.7 d. Forms a voluminous crystalline hydrate at 0-5 °C (quoted, Keith and Walters, 1992). 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Availability of Experimental Data Methods for Estimation of Acidity Constants... 

Estimation of Acidity Constants The Hammett Correlation Illustrative Example 8.2 Estimating Acidity Constants of Aromatic Acids and Bases Using the Hammett Equation... 

P 8.1 Estimation of Acidity Constants and Speciation in Water of Aromatic Organic Acids and Bases... 

The electrochemical behavior of dimers containing the MoJ unit has been reported for several derivatives. Oxidation of Mo2(S04)J in acid solution formed Mo2(S04)4" (E1/2 = 0.22 V versus SCE), but rapid decomposition of the oxidized product ensued with a rate constant estimated from cyclic voltammetry to be in the range of 10-3 —10-1 sec-1 (75). Reproducibility of the electrochemical behavior depended on acid concentration, electrode history, and sweep rate and was generally indicative of complex processes not amenable to rigorous interpretation. 

A clearer indication of the absolute and relative contributions of field, resonance, and polarizability effects to the acidity of the various compounds can be obtained by calculating the individual PpOF, pRoR, and pj a terms for each acid rather than just focusing on the p , pj, and p° values, respectively. These terms are summarized in Table 18 for the compounds with Y-groups with unknown substituent constants (Y = C=CH, CH=NH, and CH=S), these terms were calculated based on approximate substituent constants estimated as described in reference 118. 

The acidity constants of protonated ketones, pA %, are needed to determine the free energy of reaction associated with the rate constants ArG° = 2.3RT(pKe + pK ). Most ketones are very weak bases, pAT < 0, so that the acidity constant K b cannot be determined from the pi I rate profile in the range 1 < PH <13 (see Equation (11) and Fig. 3). The acidity constants of a few simple ketones were determined in highly concentrated acid solutions.19 Also, carbon protonation of the enols of carboxylates listed in Table 1 (entries cyclopentadienyl 1-carboxylate to phenylcyanoacetate) give the neutral carboxylic acids, the carbon acidities of which are known and are listed in the column headed pA . As can be seen from Fig. 10, the observed rate constants k, k for carbon protonation of these enols (8 data points marked by the symbol in Fig. 10) accurately follow the overall relationship that is defined mostly by the data points for k, and k f. We can thus reverse the process by assuming that the Marcus relationship determined above holds for the protonation of enols and use the experimental rate constants to estimate the acidity constants A e of ketones via the fitted Marcus relation, Equation (19). This procedure indicates, for example, that protonated 2,4-cyclohexadienone is less acidic than simple oxygen-protonated ketones, pA = —1.3. 

We can consider decarboxylation reactions in terms that are analogous to those in proton transfer reactions the reactivity of the carbanion in carboxylation reactions is analogous to internal return observed in proton transfer reactions from Bronsted acids. Kresge61 estimated that the rate constant for protonation of the acetylide anion, a localized carbanion (P A 21), is the same as the diffusional limit (1010 M s1). However, achieving this rate is highly dependent on the extent of localization of the carbanion. Jordan62 has shown that intermediates in thiazolium derivatives are also likely to be localized carbanions, which implies that protonation of these intermediates could occur at rates approaching those of other localized carbanions. 

Fluorimetric titration in water was applied to estimate acidity constants of some well-known antihypertensive drugs containing in their structure an NH-unsubstituted tetrazole fragment governing their acid properties pKa 3.15 (losartan 29), 4.70 (irbesartan 83), and some others <2001MI477>. 

Through lack of an unambiguous method for direct determination, the acidity constants of carbon acids have, for many years, been estimated from the rate of proton abstraction by means of rate-equilibrium relationships. Thus, Bell (1943) (see also Hibbert, 1977) estimated the acetaldehyde, acetone and acetophenone acidity constants (19.7,20.0 and 19.2, respectively) by assuming that the rate constants for proton abstraction from several mono- and dicarbonyl compounds to a single base (A-) with pAHA = 4.0 obey a Bransted equation in its differential form (47). By taking curvature into account, the... 

The rates within each protein were then standardized relative to that of leucine. The order of relative racemization rates is presented in Table II. Relative rates are very similar among the various proteins except for aspartic acid and glutamic acid in wheat gluten. This situation is discussed below. (The relative rate constants estimated for the second region of the casein curves in Figure 2, using the 3-hour and 24-hour points, is k(asp) k(p>he) k(glu) k(ala) k(leu) = 4.0 3.0 2.5 2.5 1.0.)... 

Because the system exists essentially completely as the thiol isomer, a carbon-acid acidity constant for ionization starting with the thio-keto form as the initial state, QJ, could not be measured, and a keto-enol equilibrium constant, ATe, could not be determined. A lower limit for can nevertheless be estimated on the assumption that 5% of the keto isomer would have produced a detectable signal in the H NMR spectrum of the enol form. Because no such signal was seen, must be greater than 20, which makes pK less than —1.3. The relationship = KeQJ then leads to > 1.1 x 10 M, pQ <2.1. 

The rate constants of initiation of styrene by triflic acid are estimated to be roughly k,- 10 M sec-1 at 0° C in CH2CI2 this is 10,000 times smaller than the corresponding propagation rate constant under similar conditions [17]. However, it does not take into account the higher order kinetics in acid [134]. Initiation of more nucleophilic monomers is faster, with ki = 103 M l-sec-1 for a-methylstyrene [21] and kj = 5104 M -sec l for p-methoxystyrene [23], as determined by stopped-flow methods at ambient temperature in CH2CI2 and C2H4CI2. 

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