Specific hydroxyl 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 catalytic base B might be HO or a weaker base such as ammonia or even water. For reactions the rate is proportional only to the concentration of OH and the presence of other weaker bases has no effect.129b Such catalysis is referred to as specific hydroxyl ion catalysis.19 More commonly, the rate is found to depend both on [OH ] and on the concentration of other weaker bases. In such cases the apparent first-order rate constant (fcobs) for the process is represented by a sum of terms (Eq. 9-88). The term kH2, ) is the rate in... 

Cui ve b represents specific hydroxyl ion catalysis. In each of the above cases, the water reaction, ko, is negligible. 

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. 

Enzymes are not able to concentrate protons or hydroxyl ions to the point that specific base or acid catalysis would be effective. However, either general acid or general base catalysis can be accomplished by groups present in an enzyme in their normal states of... 

Many solution reactions are catalyzed by hydrogen or hydroxyl ions and consequently may undergo accelerated decomposition upon the addition of acids or bases. The catalysis of a reaction by hydrogen or hydroxyl ions is known as acid-base specific catalysis. In many cases, in addition to the effect of pH on reaction rate, there may be catalysis by one or more species of the buffer system. This type of catalysis is known as the acid-base general catalysis. Solutions of vitamin were found to be... 

Hydrolysis is frequently catalysed by hydrogen ions (specific acid-catalysis) or hydroxyl ions (specific base-catalysis) and also by other acidic... 

It should be mentioned that acid catalysis is not a specific property of hydrogen ions. According to Bronsted, all substances possessing an acid character will exert a similar influence, although to a much smaller degree than is shown by hydrogen ions. The same is true of basic catalysis, which is by no means restricted to hydroxyl ions. All substances with basic properties show qualitatively the same effect. The papers of Bronsted should be studied for more detailed information. 

This treatment is easily generalized and leads to the conclusion that in water or similar solvents general acid catalysis will be observable only for intermediate values of the exponent a. If a is too small, the catalytic effect of acids will be swamped by that of the solvent, while if a approaches unity, the effect of all other acids will be masked by that of the hydrogen ion. Similar conclusions apply to basic catalysis. It is possible, therefore, that those reactions in aqueous solution which appear to show specific catalysis by hydrogen or hydroxyl ions (cf. preceding sub-section) do not constitute a special class of reaction, but are actually... 

An exactly similar treatment can be applied to basic catalysis. The important general result of these considerations is that if general acid-base catalysis is observed in a reaction involving only one proton transfer, then this proton transfer is rate determining. However, it is not safe to assume that the converse is true, i.e., that the substrate and catalyst are effectively in equilibrium in reactions found experimentally to be specifically catalyzed by hydrogen or hydroxyl ions. This is because (as shown in Sec. II.4) catalysis by species other than H+ or OH- may frequently escape observation, giving a false impression of specific catalysis. 

The existence of a preequilibrium between catalyst and substrate (leading to a genuine specific catalysis) can, however, be shown unequivocally if ki in the above scheme, i.e., if the substrate has sufficiently marked basic (or acidic) properties, and the concentrations of hydrogen ions (or hydroxyl ions) are sufficiently high. Under these conditions the reaction velocity in solutions of strong acids or bases will increase less rapidly than [H+] or [OH-], corresponding to the conversion of an appreciable proportion of the substrate into its cation or anion [cf. case iib and Eq. (24) above]. The test cannot often be applied, since the... 

The results of the above analysis of reactions involving two proton transfers may be compared with those for reactions in which only a single transfer is kinetically relevant (Sec. III.3.a). For a single transfer the existence of an equilibrium between catalyst and substrate always produces the appearance of specific catalysis by hydrogen or hydroxyl ions, and the detection of general acid-base catalysis therefore excludes... 

There are many chemical reactions that are catalyzed by acids or bases, or by both. The most common acid catalyst in water solution is the hydronium ion and the most common base is hydroxyl ion. However, some reactions are catalyzed by any acid or by any base. If any acid catalyzes the reaction, the reaction is said to be subject to general acid catalysis. Similarly, general base catalysis refers to catalysis by any base. If only hydronium or hydroxyl ions are effective, the phenomenon is called specific acid or base catalysis. 

A distinction is often made between specific catalysis by hydrogen and hydroxyl ions specific hydrogen and hydroxide-ion catalysis) and catalysis by acids and bases in general general acid and base catalysis). Actually such a distinction is somewhat artificial, as we shall see. As an example of general base catalysis, consider the simple mechanism... 

Two t) es of acid-base catalysis have been observed general and specific. General acid-base catalysis refers to the case where a solution is buffered, so that the rate of a chemical reaction is not affected by the concentration of hydronium or hydroxyl ions. For these types of reactions, H+ and A oh are negligible, and therefore... 

Of greater relevance to our discussion is specific acid-base catalysis, which refers to the case where the rate of a chemical reaction is proportional only to the concentration of hydrogen and hydroxyl ions present. For these type of reactions, / ha and kp,- are negligible, and therefore... 

The reaction of ester hydrolysis and backward reaction of esterification of carboxylic acids are studied in detail. Ester hydrolysis is catalyzed by both hydrogen ions and hydroxyl ions, that is, is an example of specific acid-base catalysis. Two different methods for the cleavage of ester bonds are possible and observed experimentally acyl-oxygen and alkyl-oxygen... 

Evidence for intramolecular hydrolysis of the methyl ester (62) by metal hydroxide has been provided.329 Molecular models of the metal complex (63) indicate that when complexation with the imidazole nitrogen and the phenolic hydroxyl group occurs, it is not possible for coordination of the ester carbonyl group to occur. This point, taken in conjunction with the observed pH rate profile which shows that ionization of the M—OH2 group is associated with catalysis, eliminates metal ion activation of the carbonyl bond to intermolecular attack by OH- as a contributing factor. For base hydrolysis of (62) kOH = 2.7 x 10-2 M-1 s-1 at 25 °C. The specific rate constants for intramolecular hydrolysis by the M—OH species are 0.245 s-1 and 2 x 10-2 s-1 for the Co11 and Ni11 complexes respectively. 

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