Acid-base parameters, quantitative

Drago and Wayland proposed a quantitative systan of acid-base parameters to account for reactivity by explicitly including electrostatic and covalent factors. This approach uses the equation — A// = + QCg where is the enthalpy of the reaction A + B AB... 

Hammett equation(s) 78, 93, 148ff., 151 f., 153ff., 167f., 190, 193, 196, 297, 299, 308, 312, 375, 381, 392, see also Dual substituent parameter, and Quantitative structure-reactivity relationships Hammond postulate, in additions of nucleophiles to diazonium ions 157 Hard and soft acids/bases principle (Pearson) 49, 54, 109... 

Drago and co-workers Introduced an empirical correlation to calculate the enthalpy of adduct formation of Lewis acids and bases ( 5). In 1971, he and his co-workers expanded the concept to a computer-fitted set of parameters that accurately correlated over 200 enthalpies of adduct formation ( ). These parameters were then used to predict over 1200 enthalpies of interaction. The parameters E and C are loosely Interpreted to relate to the degree of electrostatic and covalent nature of the Interaction between the acids and bases. This model was used to generalize the observations involved in the Pearson hard-soft acid-base model and render it more quantitatively accurate. 

Next, a quantitative model, referred to as the E and C equation, is presented for predicting and correlating the enthalpies of adduct formation. The use of this equation and the interpretation of the parameters which result is discussed. Exceptions to the correlation are considered and the valuable insight about intermolecular interactions that can be gained by firmly establishing these exceptions is demonstrated. The parameters we obtain and valid transformations of these parameters are considered in the light of both the HSAB model and Donor Strength model of acid-base chemistry. Both of these concepts are shown to be at best incomplete models of coordination. The relationship between our parameters and the a—q Hammett parameters is quantitatively demonstrated. 

These qualitative explanations, whether they be hard-soft or ionic-covalent or Class A-Class B, all suffer from the arbitrary way in which they can be employed. All Lewis acid-base type interactions are composed of some electrostatic and some covalent properties, i.e., hardness and softness are not mutually exclusive properties. Predictions are straightforward when dealing with the extremes, but with other more ambiguous systems, one can be very arbitrary in explaining results and the predictive value is impaired. What is needed is a quantitative assessment of the essential factors which can contribute to donor strength and acceptor strength. Proper combination of these parameters should produce the enthalpy of adduct formation. Until this can be accomplished, one could even question the often made assumption that the strength of the donor-acceptor interaction is a function of the individual properties of a donor or acceptor. 

When this conjugation occurs, the level of active (corrosive) acid is substantially decreased. No simple quantitive correlation has been shown between the acidity (pKa) of acids in hydrocarbon formulation and low polar solvents (Coetzee, 1967). Acid-base interaction with and without proton transfer (PT) (BH+A B...(HA)m) has been related to acid and base enthalpies of reaction (Pawlak and Bates, 1982), the infrared carbonyl stretching band and gradual appearance of the asymmetric COO band (Lindeman and Zundel, 1972 Magonski and Pawlak, 1982), changes in pH (Kuna et al., 1982 Pawlak et al., 1982), NMR proton chemical shifts (Magonski and Pawlak, 1982), and dipole moments (Sobczyk and Pawelka, 1979). These parameters depend upon the acid-base strength of the partners, ApKa(PT) the difference between the pKa(acceptor) and pKa(donor) on the water scale (Sobczyk, 2001). 

In this case, the Lewis acid-base approach has been assumed to account for all non-DLVO interactions. The exponential-decay expressions [Eq. (27)] deriving from the work of Pashley and Quirk [51], or other quantitative expressions for non-DLVO interactions, similarly could have been inserted. A major advantage of the Lewis acid-base approach is that all parameters in the expression can be determined a priori, whereas the exponential-decay... 

Titration — A process for quantitative analysis in which measured increments of a - titrant are added to a solution of an - analyte until the reaction between the analyte and titrant is considered as complete at the - end point [i]. The aim of this process is to determine the amount of an analyte in a -> sample. In addition, the determination can involve the measurement of one or several physical and/or chemical properties from which a relationship between the measured parameter/s and the concentration of the analyte is established. It is also feasible to measure the amount of a - titrand that is added to react with a fixed volume of titrant. In both cases, the -> stoichiometry of the reaction must be known. Additionally, there has to be a means such as a -> titration curve or an - indicator to recognize that the -> end point has been reached. The nature of the reaction between the titrant and the analyte is commonly indicated by terms like acid-base, complexometric, redox, precipitation, etc. [ii]. Titrations can be performed by addition of measured volume/mass increments of a solution,... 

A quantitative description of the influence of the solvent on the position of chemical equilibria by means of physical or empirical parameters of solvent polarity is only possible in favourable and simple cases due to the complexity of intermolecular solute/solvent interactions. However, much progress has recently been made in theoretical calculations of solvation enthalpies of solutes that can participate as reaction partners in chemical equilibria see the end of Section 2.3 and references [355-364] to Chapter 2. If the solvation enthalpies of all participants in a chemical equilibrium reaction carried out in solvents of different polarity are known, then the solvent influence on this equilibrium can be quantifled. A compilation of about a hundred examples of the application of continuum solvation models to acid/base, tautomeric, conformational, and other equilibria can be found in reference [231]. 

Different researchers tried to establish a correlation between acid-base properties of compounds, in particular, of oxides, and their thermodynamic parameters, in order to range them in a definite row and to get a scale representing quantitative characterization of acid-base properties. 

The Gutmaim s Acceptor Number (AN) was proposed [Gutmann, 1978] as a quantitative empirical parameter of solvent hydrogen bond acidity based on P-nmr shifts of thiethylphosphine oxide at infinite dilution, calculated as AN = -6 " 2.349. 

Another disadvantage of the experimental acidity scales is the absence of serious quantitative data because, as a rule, a number of equilibria are reciprocally affected in the studied solutions. In this case, the estimation of some equilibrium constants based on one point is impossible. Therefore, more precise data on the acidity scales may be obtained by establishing equilibrium constants of acid-base reactions and the effect exerted by the cation and anion composition of an ionic melt on them. The regularities obtained on the basis of these parameters will help us to treat some aspects of the problem in question more correctly. 

The effect of metal ions or metal chelates on the rate of hydrolysis of salicyl phosphate is difficult to evaluate quantitatively, for several reasons. Salicyl phosphate itself undergoes intramolecular acid-base-catalyzed hydrolysis in a series of reactions, each of which is pH dependent and has its own rate constant. Moreover, salicyl phosphate reacts with metal ions or chelated metal ions to give a variety of metal chelates, some of which are mixed ligand chelates. The hydrolysis reaction, however, does not take place via all of these chelates. From a careful study of the solution equilibria involved and the effect of various solution parameters on the rate of hydrolysis of salicyl phosphate, the following conclusions have been reported (105). 

Various conceptual DFT-based reactivity indices in association with some new parameters are successfully employed in the development of stronger QSAR/QSTR models [332]. Deeper correlations of the toxicity of different classes of organic compounds like chlorinated benzenes [333], polychlorinated biphenyls [312, 334—336], and benzidine [337] at DFT level of theory are reported. The toxicity of the polychlorinated biphenyls as well as benzidine is itrfluenced by its electron affinity and planarity. The interactions of the chlorinated benzo-derivatives and benzidine with other biomolecules like nucleic acid/base pairs or aryl hydrocarbon hydroxylase (AHH) receptors are primarily of charge-transfer type, which can be quantitatively assessed from Parr to Pearson formula [254] and can be given as... 

As noted in the preceding section, IGC is an excellent tool for measurements of surface properties and of acid base interaction potentials of macromolecular solids (2,10,16). In this section the importance of acid-base interactions relative to the adhesion of PUs is considered in greater detail. IGC has been applied to a series of PU adhesives and to selected polymer substrates, allowing quantitative measurements to be made of the acid/base (electron donor-acceptor) interaction parameters applicable to the surfaces of these materials. Acid base pair-interaction parameters for substrate/PU combinations have been calculated. The bond characteristics of polymer/PU combinations have been measured, in part by conventional lap-shear procedures and in part, by the more recent constrained blister detachment method [11, 12]. Possible relationships between bond properties and acid base interactions have been considered, and a comparison of the two adhesion tests has been made. 

Any attempt to relate acid/base interactions between adhesives and substrates to the bond properties of the relevant pair depend on a quantitative evaluation of a pair interaction parameter. There are no broadly accepted theoretical guidelines to such an evaluation, but a pair interaction parameter, I p, can be defined by... 

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