Lewis affinity

A few scales of Lewis affinity and some spectroscopic scales of Lewis basicity (see below) have been constructed by carrying out the reaction A - - B AB on a dilute solution of the acid in pure, liquid base as solvent. This pure base method will be studied in Chapter 4. It gives solvent basicity scales which are not strictly equivalent to solute basicity scales measured on a dilute solution of the acid and the base in an inert solvent. 

The ECT model has been little used in the analysis of gas-phase ion chemistry. In contrast, the ECW model is generally found helpful in many fields of solution chemistry and biochemistry, as shown in several reviews [77, 173, 177-179] and in a book [6] and by its introduction in many textbooks of (mainly inorganic) chemistry (e.g. ref. [180]). The ECW model is particularly useful for showing that there is no inherent one-parameter order of Lewis affinity. Factoring and rearranging Equation 1.121 (with W = 0) lead to [179]... 

The mathematics of the Lewis acid/base concept is that of a data matrix of m rows and n columns. Data are complexation constants, as logA" or AG. Each row corresponds to a Lewis acidity scale towards a reference base B° (/ = 1 to m) and each column corresponds to a Lewis basicity scale towards a reference acid A°j (J = I to n). For a rigorous treatment, the data measured in different media cannot be mixed in the same data matrix. In the matrix measuring Lewis affinity, the data are complexation enthalpies. There are extrathermodynamic relationships (isoequilibrium relationships or enthalpy-entropy compensation law) which allow transformations between blocks of the affinity and basicity matrices. In the principal component analysis of Lewis basicity, this justifies, somewhat, the mixing of affinity columns and basicity columns in a unified basicity-affinity matrix. 

The first calorimetric measurements on the reaction of SbCb with Lewis bases in solution seem to have been made in 1963 by Olofsson [1], Later, this author published a series of papers between 1963 and 1973 on the enthalpy of complexation of SbCls with numerous carbonyl bases [2-11], and also a few ethers [11, 12], methanol [12], water [13] and nitrobenzene [11], Gutmann extended this series to a host of different Lewis bases [14,15] and proposed, in 1966, the concept of donor number (DN) [16-18] to express quantitatively the Lewis basicity of soivents. Although the DN scale has been proposed and extensively used [19-21] as a solvent parameter, it relies on measurements made on dilute solutions of bases. It is, therefore, a solute scale and not a solvent scale of Lewis affinity. 

The existence of numerous theoretical studies on the Lewis affinity of BF3 and on the nature of the dative bond in BF3 complexes, generally performed at high theoretical levels, illustrates that the choice of BF3 as a reference Lewis acid for constructing an affinity scale of Lewis bases is well founded, not only from the experimental but also from the computational point of view. Indeed, the electronic structure of BF3 is very simple, since this trigonal planar molecule contains only four first-row atoms. Calculations on the thermodynamics of BF3 complexes can significantly improve our knowledge of Lewis affinity, as illustrated below. 

It is possible to compute the BF3 Lewis affinity, at 0 and/or 298 K in the gas phase, of bases that are difficult to study experimentally, such as weak carbon n bases [41,42, 73], water [74] and diaminocarbenes [75]. 

Nevertheless, the values given in Table 5.25 may be useful for comparison with other Lewis affinity scales and with the spectroscopic scales presented later. Their comparison with theoretical diiodine affinities may also help in choosing the best calculation method(s) in halogen-bonding studies. 

Table 6.1 Some typical gas-phase cationic Lewis affinity and basicity scaies that can be established from literature data. Only leading references are given. The stoichiometry column refers to the type of adduct (one- or two-ligand adduct) that was studied.

A significant difference of 4.4 1.0 kJ mol (that is, a 21% relative differenee) is found between the enthalpies measured in CCU and in cyclohexane. It appears that the diiodine affinities measured in alkanes must not be mixed with those measured in CCI4, as was unfortunately done in the Drago EC analysis of Lewis affinity. Moreover, the experimental value to be compared with the diiodine affinity of DMSO computed in vacuo by quantum chemical methods is the value measured in cyclohexane. Taking into account the solvent effect of cyclohexane, a good calculated value should be at least —25.3 ( 0.8) kJ mor. ... 

In summary, ligands tend to diminish the affinity of the substrate for the Lewis-acid catalyst as well as the extent of activation by this catalyst, once the ternary complex is formed. Only a few examples of ligand-accelerated catalysis " have been described... 

In contrast to the situation in the absence of catalytically active Lewis acids, micelles of Cu(DS)2 induce rate enhancements up to a factor 1.8710 compared to the uncatalysed reaction in acetonitrile. These enzyme-like accelerations result from a very efficient complexation of the dienophile to the catalytically active copper ions, both species being concentrated at the micellar surface. Moreover, the higher affinity of 5.2 for Cu(DS)2 compared to SDS and CTAB (Psj = 96 versus 61 and 68, respectively) will diminish the inhibitory effect due to spatial separation of 5.1 and 5.2 as observed for SDS and CTAB. 

A second question involves the influence of ligands on the rate and selectivity of the Lewis-acid catalysed Diels-Alder reaction in water. In Chapter 3 we have demonstrated that nearly all the ligands studied induce a significant decrease in the affinity of the catalyst for the dienophile. This effect is accompanied by a modest reduction of the rate of the Diels-Alder reaction of the ternary dienophile -catalyst - ligand complex. 

Chapter 5 also demonstrates that a combination of Lewis-acid catalysis and micellar catalysis can lead to accelerations of enzyme-like magnitudes. Most likely, these accelerations are a consequence of an efficient interaction between the Lewis-acid catalyst and the dienophile, both of which have a high affinity for the Stem region of the micelle. Hence, hydrophobic interactions and Lewis-acid catalysis act cooperatively. Unfortunately, the strength of the hydrophobic interaction, as offered by the Cu(DS)2 micellar system, was not sufficient for extension of Lewis-acid catalysis to monodentate dienophiles. 

The XeF+ cation forms Lewis acid—base adduct cations containing N—Xe—F linkages with nitrogen bases that are resistant to oxidation by the strongly oxidizing XeF+ cation having an estimated electron affinity of the XeF+ cation of 10.9 eV (12). The thermally unstable colorless salt,... 

Nucleophilicity roughly parallels basicity when comparing nucleophiles that have the same reacting atom. For example, OH- is both more basic and more nucleophilic than acetate ion, CH3CO2-, which in turn is more basic and more nucleophilic than H20. Since "nucleophilicity" is usually taken as the affinity of a Lewis base for a carbon atom in the Sfj2 reaction and "basicity" is the affinity of a base for a proton, it s easy to see why there might be a correlation between the two kinds of behavior. 

Kitatani et al. found that tungsten(VI) chloride would catalyse the formation of a range of oxazoles from benzoyl(phenyl)diazomethane and nitriles (Scheme 17).<74TL1531, 77BCJ1647> The reaction with acetonitrile was studied with a range of other metal chlorides, but all proved less satisfactory than WCle. They attributed the catalytic nature of tungsten(Vl) chloride to both its Lewis acidity and the affinity of tungsten for carbenes. 

Lewis DFV, Eddershaw PJ, Dickins M, Tarbit MH, Goldfarb PS. Structural determinants of cytochrome P450 substrate specificity, binding affinity and catalytic rate. Chem Bio Interact 1998 115 175-99. 

Many years ago, geochemists recognized that whereas some metallic elements are found as sulfides in the Earth s crust, others are usually encountered as oxides, chlorides, or carbonates. Copper, lead, and mercury are most often found as sulfide ores Na and K are found as their chloride salts Mg and Ca exist as carbonates and Al, Ti, and Fe are all found as oxides. Today chemists understand the causes of this differentiation among metal compounds. The underlying principle is how tightly an atom binds its valence electrons. The strength with which an atom holds its valence electrons also determines the ability of that atom to act as a Lewis base, so we can use the Lewis acid-base model to describe many affinities that exist among elements. This notion not only explains the natural distribution of minerals, but also can be used to predict patterns of chemical reactivity. 

Metals that are soft Lewis acids, for example cadmium, mercury, and lead, are extremely hazardous to living organisms. Tin, in contrast, is not. One reason is that tin oxide is highly insoluble, so tin seldom is found at measurable levels in aqueous solution. Perhaps more important, the toxic metals generally act by binding to sulfur in essential enz Tnes. Tin is a harder Lewis acid than the other heavy metals, so it has a lower affinity for sulfur, a relatively soft Lewis base. 

Although all sources of reactive free radicals which have been tried initiate the polymerization of unsaturated monomers, the converse of this statement, namely, that all initiators are free-radical-producing substances, is not true. Thus, strong acids (in the Lewis sense) such as AICI3, BF3, and SnCL, which are characterized by a strong affinity for a pair of electrons, bring about rapid polymerization of certain monomers. These polymerizations also proceed by chain mechanisms. The propagating center is, in this case, a positively... 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

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