Bronsted base strength

The determination by NMR of Bronsted base strength of aprotic solvent resonances is not likely to be satisfactory because of the very low concentrations of hydrogen ions in these solvents, but the use of... 

This equation corresponds to today s general convention of expressing base strength also be means of pKa, where K is considered in the sense of the Bronsted acid-base theory as a protolysis constant of the following protolytic reactions for acids ... 

Base strength refers to the relative tendency to produce OH- ions in aqueous solution by (1) the dissociation of soluble metal hydroxides or (2) by ionization reactions with water using Arrhenius theory. A more general definition, applying Bronsted-Lowry theory, is that base strength is a measure of the relative tendency to accept a proton from any acid. 

The earliest LFER, advanced by Bronsted, correlates the acid dissociation constant and base strength (1/A h) of species with its effectiveness as a catalyst in general acid (At h) and base (Atgl-catalyzed reactions respectively. The relationships take the form... 

The Bronsted relationship can be strictly accurate only over a certain range of acid and base strengths. When has diffusion-controlled values, which of course cannot be exceeded, the linear plot of log k/ y vs log must level off to a zero slope, that is a = 0. As well as being reported, although rarely, in simple metal complexes, the resultant curvature in the Bronsted plot is also shown by the zinc enzyme carbonic anhydrase (Chap. 8. Zn(II)). In... 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

In 1924 Bronsted and Pedersen [67] found that base catalytic coefficients (feB) which they had measured for a large number of bases in the decomposition of nitramide were related to the base strength of the catalysts by eqn. (46)... 

Conversely, in Bronsted-Lowry terminology (Section 10-4), the order of increasing base strength of the anions of these acids is... 

High catalytic activity, comparable to soluble acids as H2SO4 or p-toluene sulfonic acid, based on a high Bronsted acid strength,... 

Equilibrium 6.1 illustrates that water can function as both a Bronsted acid and a Bronsted base. In the presence of other Bronsted acids or bases, the role of water depends on the relative strengths of the various species in solution. When HCl is bubbled into water, the gas dissolves and equilibrium 6.3 is established. 

Values of Hpt vary considerably (Table 25.2) and illustrate the dependence of the rate of substitution on the nucleophilicity of the entering group. There is no correlation between npt and the strength of the nucleophile as a Bronsted base. 

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. 

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