Lewis acids empirical measures

The measurement units of each parameter give a preliminary indication of the nature of these parameters, but for a more precise idea of their chemical and physical significance the reader is referred to the literature7-9. In the present context it is sufficient to bear in mind that most of these empirical parameters can be subdivided into parameters which measure the Lewis acidity (hence, the electrophilic power) and Lewis basicity (hence, the nucleophilic power) of a solvent. 

Whereas the number of Brpnsted acid sites can be easily determined,146 149 their acidity may vary depending on their position in the framework and also by interaction with Lewis acid sites. For these reasons and despite the various techniques tested, there are no general and reliable methods to measure the acidity of solid acids. The synthesis of active acidic zeolites used in industry still relies on a very empirical base, the most important character being their catalytic activity. 

Empirically measured parameters are additional solvent properties, which have been developed through the efforts of physical chemists and physical organic chemists in somewhat different, but to some extent related, directions. They have been based largely on the Lewis acid base concept, which was defined by G. N. Lewis. The concept originally involved the theory of chemical bonding which stated that a chemical bond must involve a shared electron pair. Thus, an atom in a molecule or ion which had an incomplete octet in the early theory, or a vacant orbital in quantum mechanical terms, would act as an electron pair acceptor (an acid) from an atom in a molecule or ion which had a complete octet or a lone pair of electrons (a base). Further developments have included the concepts of partial electron transfer and a continuum of bonding from the purely electrostatic bonds of ion-ion interactions to the purely covalent bonds of atoms and molecules. The development of the concept has been extensively described (see Ref. 11 for details). 

Donor number (or donicity), DN — is an empirical semiquantitative measure of nucleophilic properties (-> acid-base theories, subentry Lewis acid-base theory) of a solvent defined as the negative of the standard molar heat of reaction (expressed in kcalmol-1) of the solvent D with antimony pentachloride to give the 1 1 adduct, when both are in dilute solution in the inert diluent 1,2-dichloroethane, according to the reaction scheme ... 

Finally, an attempt was made to establish a measure of the electron-donating and electron-accepting power of organic solvents by means of infrared [72, 73] and F1 NMR measurements [73], Further empirical Lewis acid and base parameters will be discussed in Chapters 7.2... 7.5. A thorough and critical compilation of empirical solvent scales, including Lewis acidity and basicity scales, has recently been made for non-FlBD solvents [342],... 

Because of the rather localized negative charge at the phenoHc oxygen atom , the standard dye (44) is capable of specific HBD/HBA and Lewis acid/base interactions. Therefore, in addition to the nonspecific dye/solvent interactions, the betaine dye (44) predominately measures the specific HBD and Lewis acidity of organic solvents. On the other hand, the positive charge of the pyridinium moiety of (44) is delocalized. Therefore, the solvent Lewis basicity will not be registered by the probe molecule (44). If this solvent property is relevant for the system under study, other empirical measures of Lewis basicity should be used cf. Section 7.7. 

Mukerjee et al. [218] and Wrona et al. [219] have used the highly solvatochromic 71 transition energy of the stable 2,2,6,6-tetramethylpiperidine-l-oxide radical (TMPNO) for the development of a solvent polarity scale. So-called values, as empirical measures of solvent Lewis acidity, have been determined for 53 pure organic solvents and some binary solvent/water mixtures [219]. 

A more simplified but likewise sueeessful empirical two-parameter approach for the deseription of solvent effects has been proposed by Krygowski and Fawcett [113]. They assume that only specific solute/solvent interactions need to be eonsidered. These authors postulated that the solvent effeet on a solute property A can be represented as a linear funetion of only two independent but eomplementary parameters describing the Lewis aeidity and Lewis basicity of a given solvent. Again, for reasons already mentioned, the t(30) values were chosen as a measure of Lewis acidity. In addition, Gut-mann s donor numbers DN [26, 27] were chosen as a measure of solvent basicity cf. Table 2-3 and Eq. (7-10) in Sections 2.2.6 and 12, respectively). Thus, it is assumed that the solvent effect on A can be described in terms of Eq. (7-62) . 

Definition of hard/soft character is the result of empirical observations and trends in measured stability of complexes. For example, hard acids (such as Fe3+) tend to bind the halides in the order of complex strength of F > Cr > Br > I, and soft acids (such as Hg2+) in the reverse order of stability. However, as with any model with just two categories, there will be a grey area in the middle where borderline character is exhibited. This is the case for both Lewis acids and Lewis bases. Selected examples are collected in Table 3.2 below a more complete table appears later in Chapter 5. 

HSAB principle A method of classifying Lewis acids and bases (see acid) developed by Ralph Pearson in the 1960s. The acronym stands for hard and soft acids and bases . It is based in empirical measurements of stability of compounds with certain ligands. Hard acids tend to complex with halide ions in the order F->Cl->Br>I-... 

Gutimnn s Acceptor Number (AN). Mayer, Gutmann, and Gerger (112) have used infinite dilution P-nmr shifts of triethylphosphine oxide (52) as the basis for what they describe as Acceptor Number (AN), a quantitative empirical parameter for the electrophilic properties of solvents (the conversion factor is —5"" = AN/2.349). For protic solvents AN is intended to serve as a measure of HBD acidity for nonprotic solvents it is seemingly intended as a measure of Lewis-type acidity. Compared with a values which range from 33.5 to 41.3 for the aliphatic alcohols, AN values of representative non-HBD solvents are THF, 8.0 ethyl acetate, 10.8 DMSO, 19.3. Thus, the latter solvents are... 

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