Catalytic constant, Br0nsted

Although the concepts of specific acid and specific base catalysis were useful in the analysis of some early kinetic data, it soon became apparent that any species that could effect a proton transfer with the substrate could exert a catalytic influence on the reaction rate. Consequently, it became desirable to employ the more general Br0nsted-Lowry definition of acids and bases and to write the reaction rate constant as... 

Figure 2.5 The Br0nsted plot for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The logarithms of the second-order constants obtained from the plot of Figure 2.4 are plotted against the pAT s of the conjugate acid of the catalytic base. The slope is the /3 value. Note that the points for amine bases ( ) fall on the same line as those for oxyanion bases (O), showing that the catalysis depends primarily on the basic strength of the base and not on its chemical nature.

The rates of proton transfer reactions cover a wide spectrum, from exasperatingly slow to diffusion controlled. Any theory which can rationalize this range has obvious merit. Such a rationalization is in fact accomplished, to a large degree, by Br nsted and Pedersen s (1923) relationship between rate (kinetic acidity) and p/sTa (thermodynamic acidity). The relationship, known as the Br0nsted equation, has the form (8) where B is the catalytic rate constant. The... 

When the source of the catalytically active hydrogen ion is a weak acid, one has to consider the weak electrolyte equilibrium involved and the change of the dissociation constant with electrolyte concentration, medium, and temperature. Br0nsted (7) termed this phenomenon secondary kinetic salt effect, but the writer would prefer to omit the word kinetic and substitute electrolyte for salt. The understanding of these... 

In order to exclude simple proton catalysis, this study also examined the catalytic activity of Br0nsted acids. It was noted that a 10 mM solution of hydrochloric acid has only a small catalytic effect (second-order rate constant Zsi = 7.62 x 10" M s compare Table 24). Another dienophile derivative also showed changes in rate (Table 25) and in the endo/exo selectivity (Table 26) "". A dramatic acceleration and an increase in the selectivity in 1,1,1-trifluoroethanol was observed in the presence of Cir"- (Table 25). 

FIGURE 3.2 Br0nsted plot for the dehydration of acetone hydrate in acetone, catalyzed by acid. The logarithm of the catalytic rate constant is plotted versus the negative of the dissociation constant of the acid. Points are shown for 32 carboxylic acids and 15 phenols. The point labeled 47 represents 2,4-dinitrophenol. Acids of other types (e.g., oximes) fell much further off the line (see Section 3.6.3). Source Bell and Higginson (1949) by permission of the Royal Society of London. 

In Fig. 1.21a, the differential heats of adsorption of CO on H—BEA zeolite and on MFI-Silicalite are reported as a function of the adsorbed amounts. Volumetric isotherms are illustrated in the figure inset. In both cases the adsorption was fully reversible upon evacuation of the CO pressure, as typical of both physical and weak, associative chemical adsorption. For H-BEA a constant heat plateau at 60kJ mol was measured. This value is typical of a specific interaction of CO with coordinative unsaturated Al(III) atoms, as it was confirmed by combining adsorption microcalorimetry and molecular modeling [73, 74, 78, 89] Note that the heat value was close to the heat of adsorption of CO at cus Al(III) sites on transition catalytic alumina, a typical Lewis acidic oxide [55, 73], Once saturated the Al(III) defects, the heat of adsorption started decreasing down to values typical of the H-bonding interaction of CO with the Br0nsted acidic sites (- 30 kJ mol , as reported by Savitz et al. [93]) and with polar defects, either confined in the zeolite nanopores or at the external surface. 

The relationship between thermodynamics and kinetics in chemical reactions is usually expressed by the Br0nsted equation (Eq. 3.70 in Section 3.4) k =gK, where k is the rate constant, fCis the equilibrium constant of the elementary stage, and and a (Polanyi parameter) are constant values for a series of reactions. These constants are determined by parameters characterizing the elementary mechanism (composition and structure ofthe activated complexes, etc.), thus allowing for the existence of an optimum catalyst, on which the rate of catalytic reaction per unit of surface has a maximum value. Equations of the type (Eq. 3.70) were used for the explanation of volcano-curves, when catalytic activity as a function of thermodynamic characteristics follows a curve with a maximum. An example for a volcano curve in methanation of CO is given in Fig. 7.6. 

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