Hydrogen-ion catalysis, specific

In equation 8.2-6a, the slope of -1 with respect to pH refers to specific hydrogen-ion catalysis (type B, below) and the slope of + 1 refers to specific hydroxyl-ion catalysis (Q if k0 predominates, the slope is 0 (A). Various possible cases are represented schematically in Figure 8.5 (after Wilkinson, 1980, p. 151). In case (a), all three types are evident B at low pH, A at intermediate pH, and C at high pH an example is the mutarotation of glucose. Cases (b), (c), and (d) have corresponding interpretations involving two types in each case examples are, respectively, the hydrolysis of ethyl orthoacetate, of P -lactones, and of y-lactones. Cases (e) and (f) involve only one type each examples are, respectively, the depolymerization of diacetone alcohol, and the inversion of various sugars. 

Some disagreement exists as to whether the second step occurs as shown [52] (A2 mechanism) or whether a carbonium ion intermediate ( h2 C02 Et) is first formed in a slow step followed by rapid reaction with water (Al mechanism). In both cases the overall rate coefficient for decomposition of the diazo compound does not refer to a single proton transfer step. This mechanism which explains the observation of specific hydrogen ion catalysis is proposed [53] for diazo compounds of the type N2 CH CO R with R = OEt, Me or Ph. 

The sucrose inversion has been extensively studied from the viewpoint of electrolyte effects (Guggenheim and Wiseman, 2), the application of the Arrhenius equation to the reaction (Leininger and Kilpatrick, 3), and the catalytic effects of acid molecules (Hammett and Paul, 4). It is probable that, in aqueous solution, we are dealing with a case of specific hydrogen ion catalysis and can postulate the equilibrium (Gross, Steiner, and Suess, 5)... 

Curve a is for specific hydrogen ion catalysis, which has already been treated. 

The exponents P and a of Eqs. 9-90 and 9-91 measure the sensitivity of a reaction toward the basicity or acidity of the catalyst. It is easy to show that as P and a approach 1.0 general base or general acid catalysis is lost and that the rate becomes exactly that of specific hydroxyl ion or specific hydrogen ion catalysis. 

T. Higuchi and A. D. Marcus, The kinetics of degradation of chloramphenicol in solution III. The nature, specific hydrogen ion catalysis, and temperature dependencies of the degradative reactions, J. Am. Pharm. Assoc., Sci. Ed. 43,530-535 (1954). 

In this case the system S + A SH+ + B is in equilibrium throughout the reaction, and the rate-determining step is the further reaction of SH+. Moreover, the velocity is proportional to the hydrogen ion concentration, although the initial proton transfer takes place from the acid A, and it would be classed experimentally as an instance of specific hydrogen ion catalysis. It is easily seen that Eq. (22) is still valid if the solution contains a number of different acid catalysts, and the same conclusion holds. 

In its kinetic behaviour the diazoacetate ion thus occupies a position intermediate between ethyl diazoacetate and diphenyldiazomethane. Equation (90) predicts specific hydrogen ion catalysis [k = /c2[H ]/A[ ) at very low acidities, and general acid catalysis k = /co+ h[J ] + a[A]) at sufficiently high acidities. In practice, both terms of (90) contribute significantly in the range of acidities corresponding to convenient reaction rates, leading to the more complex behaviour described above. 

Reference was made above to involvement of the solvent in a reaction as catalyst. In a protic solvent, reaction may be catalyzed by the solvonium ion only (the hydronium ion in water). This is specific hydrogen-ion catalysis. On the other hand, the reaction may be catalyzed by any acidic species present in the solution (general acid catalysis). The solvent molecule itself may be a catalyst. Base catalysis, similarly, may be... 

Various reactions are catalyzed by substances in the same phase as the reactants. A number of reactions in aqueous solutions are catalyzed by acids or bases. In general acid catalysis the rate depends on the concentration of unionized weak acid. In specific hydrogen-ion catalysis the rate depends on the concentration of hydrogen ions. Acid... 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

It is important for acid-catalysed reactions to determine whether the reaction is specifically catalysed by hydrogen ions or whether general acid catalysis takes place. Specific acid catalysis has been conclusively demonstrated for the benzidine rearrangement by three different sorts of kinetic experiments. In the first, it has been shown41 by the standard test for general acid catalysis (by measuring the rate of reaction in a buffered solution at constant pH over a range of concentration... 

Specificity of conventional protein enzymes is provided by precise molecular fit. The mutual recognition of an enzyme and is substrate is the result of various intermolecular forces which are almost always strongly dominated by hydrophobic interaction. In contrast, specificity of catalytic RNAs is provided by base pairing (see for example the hammerhead ribozyme in Figure 1) and to a lesser extent by tertiary interactions. Both are the results of hydrogen bond specificity. Metal ions too, in particular Mg2+, are often involved in RNA structure formation and catalysis. Catalytic action of RNA on RNA is exercised in the cofolded complexes of ribozyme and substrate. Since the formation of a ribozyme s catalytic center which operates on another RNA molecule requires sequence complementarity in parts of the substrate, ribozyme specificity is thus predominantly reflected by the sequence and not by the three-dimensional structure of the isolated substrate. 

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