Acidity and Basicity Parameters

The two ac(id)ity parameters a and A. are strongly correlated (r j=0.937 using a set of 26 solvents for which both qnantities are known). Within the set, though there seems to be a difference in behavior between C-acids (chlorohydrocarbons, ketones, nitromethane and acetonitrile) on the one hand and 0-acids (alcohols, water, acetic acid) on the other. The slopes obtained when the values of a are plotted versus A. for the 0-acids and for the C-acids (Fig. 4.10) are probably the same within experimental error (1.0 0.2), but the points for the latter are displaced about 0.2 units of a downward. With caution, the correlations may allow estimation of a where values are not known. 

Catal (2001) records values of three new parameters for 190 solvents. They are SPP, believed to be a measure of pure polarity and polarizability uncontaminated by acidity, SB, a measure of basicity, and SA, a measure of acidity. SPP is based on the difference Av between the frequencies of the first absorption maxima of 2-(N,N-dimethylamino)-7-nitrofluorene (DMNAF, 3) and 2-fluoro-7-nitrofluorene (FNF, 4), according to Equation 4.4  

FIGURE 4.10 Acidity parameters Taft-Kamleta versus Swain sA. Circles are for O-acids and (filled) formamide crosses are for C-acids. Separate least-squares lines are fitted for O-acids and for C-acids. Data from Kamlet et al. (1983) and Swain et al. (1983), with corrections Reichardt and Welton (2011). 

SB is similarly based on the difference in frequency between 5-nitroindoline (NI, 5) and l-methyl-5-indoline (MNI, 6), according to Eqnation 4.7. (TMG is tetramethylguanidine.) 

The acid parameter, S A, is based first on the relation between the observed frequencies, measured in nonacidic solvents, of the stilbazolium betaine dye 7 (TBSB R=H) and the 0,0 -di-t-butyl derivative (DTBSB R = t-butyl) represented by Equation 4.8. 

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The purest measures of acid-base strength are gas-phase acidity and basicity parameters, where solvent effects are not applicable ... 

A further distinction that should be kept in mind is between those parameters that clearly pertain to solvents, such as those derived from bulk properties (refractive index, electrical permittivity, rate and equilibrium constants of reactions in the solvent, UV/visible or IR frequency shifts of solutes), and those that relate to molecules in solntion, such as those derived from enthalpies of reactions in dilute solution in an inert solvent. The donor number DN and Drago s E, E, C, Cg acidity and basicity parameters for Lewis acids and bases are examples of the latter kind. 

Table 6.3 Summary of the results for the van Oss-Good theory s acidic (+) and basic (-) parameters of the two polymeric surfaces, as calculated by various methods...

All the investigations cited so far have been in aqueous solution medium dependence of dissociation rate constants in binary aqueous solvents (cosolvents methanol, r-butyl alcohol, 1,2-ethanediol, 1,2,3-propanetriol, and sucrose) has been reported for the 4-cyanopyridine and 4,4 -bipyridyl penta-cyanoferrates(II). For water-rich media (IQi lD < 16.5), plots of logarithms of rate constants against mole fraction organic component are linear, against 1/D almost linear. Slopes differ greatly between cosolvents, but results can be correlated quite satisfactorily with a three-parameter equation incorporating acidity and basicity parameters and the excess function... 

Ash fusion characteristics are important in ash deposition in boilers. Ash deposition occurring on the furnace walls is termed slagging, whereas accumulation on the superheater and other tubes is termed fouling. A variety of empirical indexes have been developed (60,61) to relate fouling and slagging to the ash chemical composition through parameters such as acidic and basic oxides content, sodium, calcium and magnesium, and sulfur. 

In this equation E (R2) is the excess molar refraction, S (tt ) is the solute dipolarity-polarizabiUty, A (2a ) and B(2 3 ) are the solute H-bond acidity and basicity, respectively, and Vis the McGowan characteristic volume (in cm mol /100). The solute size, V, (molecule favors octanol) together with solute H-bond basicity, B, (favors water) are the dominating parameters of this equation. The use of Bo(2P ) resulted in equation... 

The extraction time has been observed to vary linearly with polymer density and decreases with smaller particle size [78,79]. The extraction time varies considerably for different solvents and additives. Small particle sizes are often essential to complete the extraction in reasonable times, and the solvents must be carefully selected to swell the polymer to dissolve the additives quantitatively. By powdering PP to 50 mesh size, 98 % extraction of BHT can be achieved by shaking at room temperature for 30 min with carbon disulfide. With isooctane the same recovery requires 125 min Santonox is extractable quantitatively with iso-octane only after 2000mm. The choice of solvent significantly influences the duration of the extraction. For example talc filled PP can be extracted in 72 h with chloroform, but needs only 24 h with THF [80]. pH plays a role in extracting weakly acidic and basic organic solutes, but is rarely addressed explicitly as a parameter. 

The model that utilized regression analysis was one that built upon previous work by the same authors [36,39]. In this case, the dataset was expanded to 125-129 drugs and the number of assessed descriptors increased to 210. Models for acidic and basic compounds were developed separately as well as a model using all compounds, and the advantages of analyzing acids and bases separately were minimal. Mean-fold errors were generally around 1.8. Descriptors that dominated the models included lipophilicity, fraction anionic or cationic, surface electrostatic potential, and parameters specific to aliphatic carbons and fluorine. 

Commercially viable systems for the decolorisation of spent dyebaths can be based on hydrogen peroxide treatment initiated by UV radiation. A representative selection of six disulphonated monoazo acid dyes and two disazo disulphonated types was exposed for various times in a pilot-scale photochemical reactor of this kind. The controlling parameters were dye structure, pH, peroxide dosage and UV light intensity [39]. In a wider survey of the response of various classes of dyes to the combination of UV radiation and hydrogen peroxide, viable candidates for further in-plant treatment trials were the water-soluble reactive, direct, acid and basic classes. On the other hand, water-insoluble colorants such as disperse and vat dyes did not appear to be viable [40]. 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Extensive collections of pK values are available in the literature, e.g., [98-101]. It is also possible to predict pK values for a broad range of organic acids and bases using linear free energy relationships based on a systematic treatment of electronic (inductive, electrostatic, etc.) effects of substituents which modify the charge on the acidic and basic center. Quantitative treatment of these effects involves the use of the Hammett Equation which has been a real landmark in mechanistic organic chemistry. A Hammett parameter (a), defined as follows ... 

A distinct minimum in NiO solubility is observed in plots of log (NiO solubility) versus basicity (-log aM2o), which can be demarcated into two branches corresponding to acidic and basic dissolution. Acidic dissolution is represented by a straight line with a slope of+1, and a NiO solubility that decreases with an increase in aM20- Basic dissolution is represented by a straight line with a slope of to either -1 or -V4, corresponding to Equations (6-9) and (6-10), respectively. The CO2 partial pressure is an important parameter in the dissolution of NiO in carbonate melts because the basicity is directly proportional to log Pcc>2 An MCFC usually operates with a molten carbonate electrolyte that is acidic. 

One of the parameters in the broad class of liquid adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. The acidity of zeolitic adsorbent is normally affected by the zeolite Si02/Al203 molar ratio, the ionic radii and the valence of the cations exchanged into the zeolite. In this contribution, Sanderson s model of intermediate electronegativity of zeolitic adsorbent acidity (SjJ can be calculated as a representation of the strength of the adsorbent acidity based on the following equation ... 

One of the parameters in the broad class of equilibrium-selective adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. ZeoUtes can be ion-exchanged with a variety of metal cations... 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

H-bond acidity and basicity respectively v, p, a, b and c are the multilinear regression parameters. This equation was later modified [59] leading to a new Equation (5.7) ... 

Due to their better biomimetic properties, phospholipids have been proposed as an alternative to 1-octanol for lipophiiicity studies. The use of immobilized artificial membranes (lAM) in lipophiiicity determination was recently reviewed and we thus only briefly summarize the main conclusions [108]. lAM phases are silica-based columns with phospholipids bounded covalently. lAM are based on phosphatidylcholine (PC) linked to a silica propylamine surface. Most lipophiiicity studies with lAM were carried out using an aqueous mobile phase with pH values from 7.0 to 7.4 (log D measurements). Therefore, tested compounds were neutral, totally or partially ionized in these conditions. It was shown that the lipophiiicity parameters obtained on I AM stationary phases and the partition coefficients in 1-octanol/water system were governed by different balance of intermolecular interactions [109]. Therefore the relationships between log kiAM and log Poet varied with the class of compounds studied [110]. However, it was shown that, for neutral compounds with log Poet > 1, a correspondence existed between the two parameters when double-chain lAM phases (i.e., lAM.PC.MG and IAM.PC.DD2) were used [111]. In contrast, in the case of ionized compounds, retention on lAM columns and partitioning in 1 -octanol / water system were significantly different due to ionic interactions expressed in lAM retention but not in 1-octanol/water system and due to acidic and basic compounds behaving differently in these two systems. 

In the development of this descriptive method, the retention of almost 100 components (ranging from nentral hydrocarbons, analytes with donor acidity, and basicity to strong acids and bases) had been stndied with varions mobile phases, nnbnffered and bnffered at pH 2.8 and pH 7. By empirical mathematical treatment, the following colnmn parameter was evalnated ... 

The roles of carbocations in commercially important hydrocarbon transformations are still not perfectly understood. The same can be said for carbocations in biological systems. Significant questions concerning reactivity still need to be explained. Why do so many reactions of carbocations show constant selectivity, in violation of the reactivity-selectivity principle Is it possible to develop a unified scale of elec-trophilicity-nucleophilicity, in particular one that incorporates these parameters into the general framework of Lewis acidity and basicity. Finally, quite sophisticated synthetic transformations are being developed that employ carbocations, based upon insights revealed by the mechanistic studies. 

As outlined in Section 1.3, the solvent acidity and basicity have a significant influence on the reactions and equilibria in solutions. In particular, differences in reactions or equilibria among the solvents of higher permittivities are often caused by differences in solvent acidity and/or basicity. Because of the importance of solvent acidity and basicity, various empirical parameters have been proposed in order to express them quantitatively [1, 2]. Examples of the solvent acidity scales are Kosower s Z-values [8], Dimroth and Reichard s Er scale [1, 9], Mayer, Gutmann and Gergefs acceptor number (AN) [10, 11], and Taft and Kalmefs a parameter [12]. On the other hand, examples of the solvent basicity scales are Gut-... 

The acid-base properties of a mixed solvent is also an important factor influencing the behavior of solutes. Thus, the parameters of the acidity and basicity of mixed solvents have been studied to some extent [35], Figure 2.10 shows the donor numbers of mixtures of nitromethane and other organic solvents. Because ni-tromethane has very weak basicity (DN= 2.7), the addition of small amounts of basic solvents (HMPA, DMSO, pyridine) increase the donor number remarkably. 

Taft used log10 Kc for H—A = 4-fluorophenol to define the pXeB scale of basicities through pXeB = log10 Kc. Abboud and Bellon used their own experimental data to define parameters quantitatively describing both HB acidity and basicity, log10 Kc being given by a bilinear form of these descriptors. Later on, this approach was applied by Abraham,... 

The solubility parameter approach was subsequently expanded from three to four terms with the division of the hydrogen-bonding parameter into acidic and basic solubility parameters to quantify electron-donor and electron-acceptor properties [48,49], However, the expansion of these solubility parameter terms did not make the equation any easier to use foptfiBi prediction of solubility in cosolvent systems. 

During the last 15 years, Abraham and his co-workers have established a set of five descriptors for the general description of logarithmic partition coefficients by linear regression. Their so-called linear free energy relationship (LFER) descriptors E, S, A, B, and V are effective parameters for the polarizability, polarity, hydrogen-bond acidity and basicity, and volume of the solute molecules, respectively [113-116]. They are mainly derived from experimental refraction and partition coefficients of the solutes. 

At present the dye techniques are very useful and economical but are somewhat approximate. Advances in use of indicator dyes for measuring surface acidity and basicity may be expected to include a two-parameter measure of acid or base strength similar to the E and C equation of Drago, and the use of fluorescent indicators for colored solids. 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

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