Acid-base strength quantitative measures

As discussed above, there have been few systematic studies in which the acid or basic strength of materials relevant to catalysis has been correlated on a quantitative scale. The utility of microcalorimetric measurements of the heats of adsorption of various molecules is evident. These measurements can be used to determine the acid or basic strength of surfaces and establish their effect on the catalytic behavior of the materials. If we desire to control these acid-base properties to tailor and improve catalysts for existing processes and to design improved catalysts for new catalytic processes, a quantitative scale of the acid-base interactions is required. Appropriate correlations, perhaps involving electronegativity scales, would allow the prediction of the acid-base strength of the surface sites which can then be related to the catalytic activity of those sites. Additional research in this area is required. 

The DN scale has historical merits. It was among the first attempts to establish Lewis base strength quantitatively. It was widely used in the field of solution, coordination and organic chemistry to describe the Lewis basicity of solvents. However, after 40 years of use, convergent critical analyses have pointed out the serious limitations of DN. They concern (i) the choice of SbCls as a reference Lewis acid, (ii) the sample of Lewis bases studied, (iii) the quality of calorimetric measurements and (iv) the domain of validity of the scale. 

Most organic compounds are bases, that is, they are capable of accepting a proton. The best-studied organic bases are the moderately strong ones, which will receive a proton in dilute aqueous solutions amines are the most important examples. The pKa value of the protonated base, referred to the infinitely dilute aqueous solution, is the usual measure of base strength, and the pH of the solution is a quantitative measure of solvent acidity, or ability to transfer a proton. 

In this part of the chapter, we develop a quantitative measure of the strengths ol weak acids and bases. We then use this information to explore how acid strength is related to molecular structure. 

Quantitative measurements of the strength of an acid are best based on electrometric measurements of the pH of the partly neutralized solution. Conductivity measurements are likely to give ionization constants that are too high due to the presence of conducting impurities. Estimates of acid strength from rate data are also somewhat unreliable... 

The major disadvantage of the HSAB principle is its qualitative nature. Several models of acid-base reactions have been developed on a quantitative basis and have application to solvent extraction. Once such model uses donor numbers [8], which were proposed to correlate the effect of an adduct on an acidic solute with the basicity of the adduct (i.e., its ability to donate an electron pair to the acidic solute). The reference scale of donor numbers of the adduct bases is based on the enthalpy of reaction. A//, of the donor (designated as B) with SbCb when they are dissolved in 1,2-dichloroethane solvent. The donor numbers, designated DN, are a measure of the strength of the B—SbCb bond. It is further assumed that the order of DN values for the SbCb interaction remains constant for the interaction of the donor bases with all other solute acids. Thus, for any donor base B and any acceptor acid A, the enthalpy of reaction to form B A is ... 

A major development in the study of EGB s is the recently reported measurements of rates of protonation by acids of known pK. The correlation of such rates with pK, the Bronsted relationship, also enables bases of determined pK to be used in the measurement of kinetic acidities of weak acids. This quantitative approach will eventually lead to the optimisation of reaction conditions for preparative reactions by providing data which can be used to match the acid/base pairs more exactly. In many organic reactions involving bases the base chosen is stronger than is strictly neccessary and consequently such reactions are often complicated by side reactions such as condensation reactions and isomerisations. The advantage of an EGB of moderate strength has been seen in the vitamin A preparation described in Scheme 18, where the facile cisftrans isomerisation is avoided. 

What then is the difference between an acid and an electrophile, or between a base and nucleophile No great difference until we try to use the terms in a quantitative sense. For example, if we refer to acid strength, or acidity, this means the position of equilibrium in an acid-base reaction. The equilibrium constant Ka for the dissociation of an acid FIA, or the pKa, is a quantitative measure of acid strength. The larger the value of Ka or the smaller the pKa, the stronger the acid. 

These are qualitative conclusions. We ought to be able to use the acidity and basicity constants—which are measures of acid and base strength— to express them quantitatively. 

But die idea is clear The strength of an acid can be quantitated by knowing how well it transfers a proton to some standard base. Moreover die extent to which it transfers a proton to the standard base could be compared with die extent to which other acids transfer a proton to die same standard base. Thus, by using a single base, we could not only measure the strength of an acid but also compare acids quantitatively. 

To make it simple, let us choose a molecule as die standard base. The acid strength will be a measure of how well the acid transfers a proton to that molecule acting as a base, and we can quantitate the acid strength by measuring the amount of die acid diat is ionized in die presence of that base. Thus die ratio of the amount of acid which has transferred a proton to the base compared to the amount of acid which has not transferred a proton to the base is a direct measure of die strength of diat acid. Water has been chosen as the standard base molecule (in... 

More recently, a quantitative scale for Lewis acidity based on fluoride ion affinities was calculated using ab initio calculations at the MP2/B2 level of theory.26 Due to its high basicity and small size, the fluoride ion reacts essentially with all Lewis acids thus the fluoride affinity (or reaction enthalpy) may be considered as a good measure for the strength of a Lewis acid. An abbreviated pF scale is given in Table 1.3. This scale was used recently by Christe and Dixon112 for estimating the stability of salts of complex fluoro anions and cations. The pF value represents the fluoride affinity in kcal mol 1 divided by 10. 

Laurence, C., Gal, J. (2010). Lewis Basicity and Affinity Scales Data and Measurement. New York John Wiley Sons. Includes a discussion of the various acid-base theories and quantitative scales for judging the strengths of Lewis acids and bases. 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

Far fewer quantitative measurements have been made of Lewis acid strength compared to that of Brpnsted acids.A simple table of Lewis acidities based on some quantitative measurement (e.g., that given for Brpnsted acids in Table 8.1) is not feasible because Lewis acidity depends on the nature of the base and any solvent that can function as a base. For example, lithium perchlorate functions as a weak Lewis acid in ether. " Qualitatively, the following approximate sequence... 

The measurement of the acidity of solid acid surfaces has been the focus of a vast number of studies. The most commonly used techniques are Hammett titrations, chemisorption of bases and TPD. Extensive discussions of diese methods and their shortcomings are available in the literature [4], The use of adsorption calorimetry makes it possible to determine quantitatively the surface acidity and the acid-strength distribution of solid acids. Surface acid-base properties of catalytic solids can also be studied by base desorption using TG [71]. 

Another quantitative measure of acidity is the proton affinity (PA), which was originally introduced to describe the basicity of molecules in their gaseous state. The PA is an intrinsic property of a molecule, defined in the absence of a solvent. Therefore, the PA seems to be a more appropriate parameter for the characterization of acidity or basicity (i.e., the acid or base strength) of surface OH groups than the pl[Pg.139]

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