Hydrogen-ion catalysis

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. 

The work of Zemer and Bender34 on the hydrolysis of o-carboxyphthalimide (XV below) shows the presence of a two-step mechanism. The hydrolysis of o-carboxyphthalimide is considerably faster than that of phthalimide at pH 1.5-4, but of similar rate in regions where hydroxide and hydrogen ion catalysis is important. This extra reactivity of o-carboxyphthalimide has a maximum at pH 2.9, a value which is not directly related to the acidities of the reagents (3.65 for XV and 15.7 for H20). A pH-rate maximum cannot be explained by concurrent acid and base catalysis since both are additive, and since two... 

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. 

Mares et al. [52] recently concluded that hydrogen-ion catalysis during this esterification was not likely and that catalysis by undissociated acid occurred together with a reaction in which, kinetically at least, no form of catalysis was involved. On the other hand, Vansco-Szmercsanyi et al. [53] concluded that polyesterification of maleic, fumaric and succinic acids with ethylene glycol or 1,2-propylene glycol was catalysed by protons. Much work clearly remains to be done on defining the detailed mechanism of catalysis by the acidic species during esterifications. 

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. 

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. 

A development similar to that presented for hydrogen ion catalysis will yield the following relationships ... 

The second mechanism differs in that [Cr(iii)(02 )] is formed with subsequent rapid reaction with Cr ". There is no e.s.r. evidence for Cr and the three-electron redox process is similar to that proposed previously by Rocek. Hydrogen ion catalysis has also been demonstrated in the interconversion of diperoxo-vanadate(v) and monoperoxovanadate(v) species co-ordinated to organic ligands. - In this case there appears to be no evidence for any redox process. 

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... 

Although the dehydration of (4) to (7) is rapid, the initial ultraviolet absorption could be obtained accurately by extrapolation and the ionization constant determined. This is because the pKa value (7.77) is in a pH region where hydrogen ion catalysis is small. In contrast the pK value for the equilibrium is low and in a region where the process (8) (3) is strongly catalyzed by hydrogen ions—... 

We next measure k for a reaction thought to be capable of being catalysed by the macromolecular acid, in the presence of the same resin and at the same value of a. If it is assumed that also for this reaction the same rate constant applies for hydrogen ion catalysis inside the resin and in homogeneous solution, it is then possible to evaluate ha equation 14. 

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... 

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