Hydroxyl Ions 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... 

Drug molecules in solution are also catalyzed by the actual buffer species chosen and its molarity, in addition to the normal hydrogen and hydroxyl ions catalysis. The catalytic effect of various buffer species on the photodegradation rate of some drugs has been reported in the literature. 

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

Early views on the nature of catalysis regarded it as an indefinite influence of some kind. Somewhat later a rather more definite picture was formed of catalysis by the hydrogen ion (regarded as a bare proton), which was supposed to attract the reactants together in virtue of its powerful electric field. This explanation did not seem especially appropriate to hydroxyl ion catalysis and obviously would not apply to catalysis by uncharged molecules. 

Iron transferrin and apotransferrin conform to such a model when examined at discrete intervals over a pH range of 7.5 to 9.5 (Figure 26). The hatched areas represent two standard deviations about the mean but the data are not corrected for electrostatic effects which should further decrease the deviation. The difference between the two forms of transferrin is nearly constant over the pH range examined and the exchange of both decreases in a monotonic fashion, indicating their like response to hydroxyl ion catalysis. Most importantly, the exchange of the... 

Campbell, A. S., and W. S. Fyfe, 1960. Hydroxyl ion catalysis of the hydrothermal crystallization of amorphous silica a possible high-temperature pH indicator. Am. Mineral. 45 464. 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalyticaUy. Many ionic effects are known. The hydronium ion and the hydroxyl ion OH" cat-... 

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

Lipid hydroperoxides are either formed in an autocatalytic process initiated by hydroxyl radicals or they are formed photochemically. Lipid hydroperoxides, known as the primary lipid oxidation products, are tasteless and odourless, but may be cleaved into the so-called secondary lipid oxidation products by heat or by metal ion catalysis. This transformation of hydroperoxides to secondary lipid oxidation products can thus be seen during chill storage of pork (Nielsen et al, 1997). The secondary lipid oxidation products, like hexanal from linoleic acid, are volatile and provide precooked meats, dried milk products and used frying oil with characteristic off-flavours (Shahidi and Pegg, 1994). They may further react with proteins forming fluorescent protein derivatives derived from initially formed Schiff bases (Tappel, 1956). 

Chromanoxylium cation 4 preferably adds nucleophiles in 8a-position producing 8a-substituted tocopherones 6, similar in structure to those obtained by radical recombination between C-8a of chromanoxyl 2 and coreacting radicals (Fig. 6.4). Addition of a hydroxyl ion to 4, for instance, results in a 8a-hydroxy-tocopherone, which in a subsequent step gives the /zara-tocopherylquinone (7), the main (and in most cases, the only) product of two-electron oxidation of tocopherol in aqueous media. A second interesting reaction of chromanoxylium cation 4 is the loss of aproton at C-5a, producing the o-QM 3. This reaction is mostly carried out starting from tocopherones 6 or /zora-tocopherylquinone (7) under acidic catalysis, so that chromanoxylium 4 is produced in the first step, followed by proton elimination from C-5a. In the overall reaction of a tocopherone 6, a [ 1,4] -elimination has occurred. The central species in the oxidation chemistry of a-tocopherol is the o-QM 3, which is discussed in detail subsequently. 

Metal-ion catalysis of hydrogen peroxide decomposition can generate perhydroxyl and hydroxyl free radicals as in Scheme 10.26 [235]. The catalytic effects of Fe2+ and Fe3+ ions are found to be similar [235]. It is not necessary for the active catalyst to be dissolved [237], as rust particles can be a prime cause of local damage. The degradative free-radical reaction competes with the bleaching reaction, as illustrated in Scheme 10.27 [237]. Two adverse consequences arise from the presence of free radicals ... 

In aqueous solution, the rates of many reactions depend on the hydrogen-ion (H+ or h3o+) concentration and/or on the hydroxyl-ion (OH-) concentration. Such reactions are examples of acid-base catalysis. An important example of this type of reaction is esterification and its reverse, the hydrolysis of an ester. 

When present in micelles, ester quats hydrolyze faster than free unimers in the bulk phase. This is due to an increased hydroxyl ion concentration around the micelle, i.e., the local pH in the vicinity of the micelle surface is higher than in the bulk. The phenomenon is referred to as micellar catalysis and is further discussed in the Betaine esters section. 

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

Of particular interest as catalysts are the incompletely coordinated metal chelate compounds, which are sufficiently stabilized by the ligand to be stable in solution at pH values much higher than that at which the aquo metal ion would precipitate as the hydroxide and thus to become unavailable for homogeneous catalysis. Such a metal chelate would be particularly effective as a catalyst for the activation of a substrate which can coordinate to the metal ion in the chelate compound. The interaction of the substrate with the metal ion would increase its reactivity toward nucleophilic reagents such as solvent molecules or hydroxyl ions, in accordance with the following scheme ... 

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