Structure, acid-base strength

This problem is in principle the same as that of the structure of the substrate, but there are some differences in practice. In the first place, the acid-base strength of the catalyst is usually measurable by direct means, and a large part of the variations in reaction velocity merely parallel these variations in acid-base strength. In the second place, it is rarely practicable to vary the strength of the catalyst over a very wide range (because of the catalytic effect of the molecules or ions of the solvent). Moreover, most measurements on catalyzed reactions have been confined to a series of very similar catalysts, and little evidence is available as to how far catalysts of widely differing structure conform to a single Bronsted relation. 

The quantity K in Equation (8) was termed by Wynne-Jones the intrinsic strength (relative to the standard chosen) and corresponds to the intercepts in Figure 2. In principle it represents the relative strength divorced from external electrostatic effects, and it has been suggested that it should be used in discussing the effect of structure on acid-base strength. However, since Equation (8) applies only to water and the alcohols, it seems probably that K still contains a considerable measure of solute-solvent interaction, and it is doubtful whether it is better suited to structural interpretations than K, measured in a given solvent. 

Acid-Base Strength and Molecular Structure which becomes... 

The contents of this section should serve to show the versatility of the nitramide reaction for the study of catalysis by bases, and it will be quoted widely in the next chapter as a tool for investigating the relations between catalytic power and the structure and acid-base strength of catalysts. However, much of the work in non-aqueous media refers to solvents in which acid-base equilibria have not been adequately investigated, and it would certainly be of interest to extend kinetic measurements to solvents which have been the subject of modem equilibrium studies, for example the dipolar aprotic solvents mentioned in Chapter 4. 

The number of cases where adsorption microcalorimetry has been successfully applied to this end has increased in recent years, especially in the field of determination of the acidic function of molecular sieves, and extensive reviews of the systems investigated using this methodology have been published [2,5-18]. In particular, a review has been written [19] summarizing some of the most recently pubUshed results concerning the appUcations of microcalorimetry to the study of the acid/base sites of zeoUtes and meso-porous materials. The efficiency of thermal analysis techniques for the characterization of the acid/base strength of zeoUte materials is discussed, as weU as their ability to provide information consistent with catalytic data [19]. The reader is referred to the Atlas of Zeolite Structures [20] for nomenclature used throughout the text. 

Fox, J.P., Jencks, W.P. General acid and general base catalysis of the methoxyami-nolysis of 1-acetyl-l,2,4-triazole. J. Am. Chem. Soc. 1974, 96(5), 1436-1449. Bruice, T.C., Donzel, A., Huffman, R.W., Butler, A.R. Aminolysis of phenyl acetates in aqueous solutions. VII. Observations on the influence of salts, amine structure, and base strength. J. Am. Chem. Soc. 1967, 89(9), 2106-2121. Williams, A., Jencks, W.P. Acid and base catalysis of urea synthesis nonlinear Bronsted plots consistent with a diffusion-controlled proton-transfer mechanism and the reactions of imidazole and N-methylimidazole with cyanic acid. J. Chem. Soc. Perkin Trans. 2. 1974, 1760-1768. 

Sections 10.9 and 10.10 discuss the relationship between molecular structure and the strengths of acids. The same ideas can be applied to bases, (a) Explain the relative strengths of the bases OH, NH,, and CH. (see Table 10.3). 

The structure of a molecule can affect its acidity or basicity in a number of ways. Unfortunately, in most molecules two or more of these effects (as well as solvent effects) are operating, and it is usually very difficult or impossible to say how much each effect contributes to the acid or base strength. Small differences in acidity or basicity between similar molecules are particularly difficult to interpret. It is well to be cautious when attributing them to any particular effect. 

---