Acid-base catalysis Bronsted equation

Bronsted acid/base catalysis (equation, a, p) 113, 210, 355ff.,360ff., 392 Buckminsterfullerene, reaction with ArN 188... 

The view of generalized acid-base catalysis as a prototropic shift assisted by acids and bases raises, quite naturally, the question of the relationship between the catalytic power of the acid or base and its own ionization constant. It had early been recognized that there is a correlation between the two constants. Taylor proposed the first quantitative relation, that the acid-catalytic constant of an acid knA was proportional to XjaA, the square root of its ionization constant. For generalized acid-base catalysis, the Bronsted equation, proposed later,"" has gained wide empirical use ... 

Equations (7) and (8) are special cases for aqueous solutions of the equation for generalized acid-base catalysis. As shown by Lowry 54a), the muta-rotations of sugars are reactions involving simultaneous catalysis by both acids and bases, in the generalized concept of acids and bases proposed by Lowry and by Bronsted. Water functions as a complete catalyst because of its amphoteric dissociation into ions H20<- H+ + OH. Acids or bases alone are not effective catalysts but in mixture are complete catalysts. 

Belke et al. (1971) reported general base and general acid catalysis in cyclization of 2-hydroxymethylbenzamide [equation (18)]. However, with 2-hydroxymethyl-6-aminobenzamide strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data). In contrast with the unsubstituted amide, the Bronsted plot is nicely linear. An amino-group in the 6-position might assist decomposition of a tetrahedral intermediate as in [37a, b] or a kinetic equivalent. The pH-rate constant profile for spontaneous cyclization at zero buffer concentra-... 

Lowry and co workers141,222 studied the mutarotation of tetra-O-methyl-a-D-glucopyranose, and found that the rate of reaction is low in dry pyridine or in dry cresol, but high in a mixture of the two solvents or in either solvent when moist. Lowry and Smith57 concluded that the mutarotation requires an acid catalyst and a base catalyst, and that amphoteric solvents are complete catalysts for the process, whereas aprotic solvents are not. They also showed that molecules of undissociated acids, cations of weak bases, and anions of weak acids have catalytic properties. Much the same concept was developed independently by Bronsted and Guggenheim,189,223 and came to be known as generalized acid and base catalysis. It was found that the rate of mutarotation of a sugar in the presence of a mixture of several catalysts may be represented by an equation of the type ... 

A somewhat similar mechanism can be envisaged for catalysis by dimeric water molecules. A bifunctional mechanism for the reaction with water seems relatively unimportant, however, because the catalytic coefficient for water molecules is in approximate agreement with the base catalysis predicted by the Bronsted equations, as described earlier (see p. 24). In contrast, the catalytic effect of 2-pyridinol is far greater than would be expected from its acid and base dissociation constants. 

Two main topics are usually distinguished the influence of the catalyst in proton-transfer reactions showing general acid or general base catalysis (the Bronsted equation) and the nucleophilicity of reagents in bimolecular reactions. These topics, however, are not totally unconnected. 

The Bronsted catalysis law states that the individual catalytic constants, kn, should be related to the equilibrium acidities by Equation 8.12, or, for a base-catalyzed process, by Equation 8.13. (See Section 3.3, p. 141, for further dis-... 

Lowry s theory that the substrate is attacked simultaneously by acids and bases leads to a variety of possibilities of hydroxide ion and hydrogen ion catalysis in aqueous solution according to the values of Xa and Xb in Bronsted s equations. With Xb large and Xa very small, the reaction is catalyzed by base but not by acid with Xa large and Xb very small, the reaction is catalyzed by acid but not by base with Xa and Xb of intermediate magnitudes, the reaction is catalyzed by both acids and bases and with both Xa and Xb either very large or very small, the reaction is not apparently faster in the presence of either acids or bases. 

Bronsted and Pedersen showed that Equation (86) is also obeyed in aniline buffers, and shortly afterwards the same was found to be the case for catalysis by seven substituted anilines. A little later the same behaviour was observed for seven cation bases, typified by [Co(NH3)50H] which can accept a proton to give [Co(NH3)5H20]. The parallel catalytic effect exerted by these bases of very different charges and chemical nature provided a firm foundation for the theory of general catalysis by acids and bases. 

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