Acidity parameter, Lewis

Several alternative attempts were made to quantify Lewis-type interactions235,236. Following the HSAB principle, the applicability of a one-parameter Lewis acidity scale will inevitably be restricted to a narrow range of structurally related Lewis bases addition of parameters results in more general relationships237-239. The quantitative prediction of the gas-phase stabilities of Lewis acid-Lewis base complexes is still difficult. Hence the interpretation, not to mention the prediction, of solvent effects on Lewis acid-Lewis base interactions is often speculative. 

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

Ca, Cb Lewis acidity and Lewis basicity parameter (Drago) ... 

The data collected by Gritzner [79] and his analysis led him to the conclusion that different interactions of hard and soft cations with donors cannot be accounted for by using only one parameter. Gritzner [79] also tried other correlations. No correlation was found with the acceptor number and the other Lewis acidity parameter, x, introduced by Dimroth and Reichardt [19, 83]. Only those parameters which represent donor properties of solvents are correlated with the change of E /2 potentials for the electroreduction of cations. 

The change in the Lewis acidity parameter, Ej with the mixed solvent composition is either monotonie or exhibits a maximum with an value higher than those found for the pure solvents [266]. 

One difficulty arises as a result of the fact that values for the parameters characteristic of the acid and base must be determined. However, the efforts of Drago (1973) was initially directed at correlations involving heats of formation for adducts of Lewis acids and Lewis bases when both species... 

The most studied ionic liquids (ILs) in the literature are chloroaluminate-based ionic liquids. Chemical properties such as Lewis acidity depend on the nature of the anion, namely AI2CI7. Considering the acidity parameter as the key point to design a liquid-phase fluorination catalyst, a new range of acidic ILs has been set up [9]. These ILs have been synthesized starting from halogenated metallic acids MX, (with M = Ti, Nb, Ta, Sn, Sb and X = Cl and/or F), such as Ta(V)Cl Fj, Nb(V)Cl Fj, Ti(IV)Cl Fj, Sn(IV)Cl Fj, Sb(V)Cl Fj,. Among these compounds, anions derived from SbCl F, (x -i-y = 5,0[Pg.536]

The original contribution of van Oss et al. (11) to this held is the determination of the acid-base component of the surface free energy in terms of two parameters Y (Lewis) acid parameter of surface free energy Y (Lewis) base parameter of surface free energy. According to these authors, one can write ... 

Lewis acid parameter of surface free energy... 

When using the three-equation method during the calculations, the Lewis acid parameters of surface tension, 7+, for Vectra A950, Vectra B950, and Xydar are 0.08,0.05, and 0.09 mJ/m, respectively. Lewis base parameters of surface tension, 7 , are 5.61, 6.44, and 6.95 mJ/m and 7 are 40.27,... 

The Lewis acid parameters (7+) are very small, varying with reaction time from zero to 0.2 mJ/m, resulting in quite small acid-base parameters = 2 7+7 ). Moreover, the Lifshitz-van der Waals parameters (7 ) do not vary much with the reaction time. However, the Lewis base parameters (7 ) are absolutely and monotonously decreasing with an increase in the reaction time for all copolymers. 7 decreases dramatically from 19.71 mJ/m at 4... 

This approach introduces the concept of a Lewis acid parameter of surface energy, y+ and a Lewis base parameter of surface energy, y - This recognizes that a given solid or liquid (e.g. chloroform) may only exert acid properties, that is be monofunctional. In this case, y is zero. For a mono-functional base (e.g. triethylamine), y is zero. Some substances, such as water, show both electron donor and electron acceptor properties, so both y+ and y are finite. These are termed bifunctional . An inert molecule shows... 

Commonly the distinction between hydrophobic and hydrophilic substances is based on the analysis of interactions between their molecules and water as a solvent. A more precise classification of liquid and solid substances as hydrophobic and hydrophilic may be constructed basing on the apolar (LW) and polar (AB) components of their surface tensions. This three-parameter approach is of great importance for the understanding of surface behaviour [22,32,39,40]. A non-metallic substance is hydrophobic if it interacts with water by exhibiting only LW character. It has very little (or none at all) Lewis acid or Lewis base character. Typical substances at the hydrophobic end have low y surface parameters and their and components are equal to zero. Hydrophilic substances have non-zero y components of the surface tension and at least one of their y and y parameters is significant. 

Desimoni et al. initially advocated the Acceptor Number (AN) as the dominant solvent parameter" The AN describes the ease with which a solvent can act as an electron pair acceptor (Lewis acid) and... 

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. 

So far the four metal ions have been compared with respect to their effect on (1) the equilibrium constant for complexation to 2.4c, (2) the rate constant of the Diels-Alder reaction of the complexes with 2.5 and (3) the substituent effect on processes (1) and (2). We have tried to correlate these data with some physical parameters of the respective metal-ions. The second ionisation potential of the metal should, in principle, reflect its Lewis acidity. Furthermore the values for Iq i might be strongly influenced by the Lewis-acidity of the metal. A quantitative correlation between these two parameters... 

In summary, the effects of a number of important parameters on the catalysed reaction between 2.4 and 2.5 have been examined, representing the first detailed study of Lewis-acid catalysis of a Diels-Alder reaction in water. Crucial for the success of Lewis-acid catalysis of this reaction is the bidentate character of 2.4. In Chapter 4 attempts to extend the scope of Lewis-acid catalysis of Diels-Alder reactions in water beyond the restriction to bidentate substrates will be presented. 

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