Hydrolysis kinetic phenomenon

With hydrolysis constants of = 10 and 10 one obtains for the saturation eoneentrations 4.8 and 4.2 M, pH values of 4.3 and 2.4 for saturated NiCl2 and FeCl2 solutions, respeetively. These caleulations show that the precipitation of hydroxide within a eorrosion pit may be prevented for these metals by acidification. However, as diseussed before, passivation should nevertheless be possible for many teehnically important metals. For Fe, Ni, and other metals, especially in acidic media, passivity eannot be explained on the basis of thermodynamic equilibria and Pourbaix diagrams. In these eases passivity is a kinetic phenomenon. Otherwise these metals should not show passivity in a strongly acidic environment. Thus, different explanations are required for the stability of a corrosion pit. 

The kinetics of vinyl acetate emulsion polymeriza tion in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly(vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stabihty, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

A reaction with a rate constant that conforms to Eq. (10-21)—particularly to the feature that the catalysts are H+ and OH-, and not weak acids and bases—is said to show specific acid-base catalysis. This phenomenon is illustrated by the kinetic data for the hydrolysis of methyl o-carboxyphenyl acetate16 (the methyl ester of aspirin— compare with Section 6.6) ... 

Kinetic investigations of amide hydrolysis showed that the rate of hydrolysis in basic media is proportional to the concentration of amide and hydroxide ion. Similarly, early work180-183 on the acid-catalysed hydrolysis of amides showed that the rate of acidic hydrolysis is, in general, proportional to the concentration of amide and hydroxonium ion. In several acidic hydrolyses, however, a maximum is observed in the pH-rate profile at 3-6 pH units, a phenomenon first reported by Benrath184 and since supported by other workers185"190. This behaviour of amides is in contrast to the hydrolysis of nitriles whose rate constant increases continuously with the hydrogen ion concentration191. 

The substrate concentration dependence of the reaction rates was investigated kinetically to analyze the substrate binding effect. Figure 4 shows the relationships between the hydrolysis rate of amylose in the presence of the random copolymer catalyst and the concentration of the substrate at some reaction temperatures. The reaction rate clearly showed the saturation phenomenon at each reaction temperature. If the reaction proceeds via complex formation between catalyst and substrate, the elementary reaction could be described in the most simplified form as... 

Similar investigation was made for sucrose as a substrate. The reaction rate, however, did not show saturation phenomenon, and as shown in Figure 6, the first-order plots of the reactions gave fairly good straight lines parallel with each other. This result suggests that the hydrolysis of low-molecular-weight substrate follows ordinary second-order kinetics. 

The examples presented in Section 8.3 demonstrate this synergy in an approach using calorimetry and IR-ATR spectroscopy. For the hydrolysis of acetic anhydride, the combination of the two analytical techniques enabled a differentiation between the heat effect due to the chemical reaction and that due to a physical phenomenon - in this case, mixing. Due to this separation of the physical heat effect, a more reliable value for the chemical heat effect was obtained. For the sequential epoxidation of 2,5-di-fert-butyl-l,4-benzoquinone, the importance of selection of an appropriate kinetic model has been demonstrated. For complex reaction systems, several models can be postulated. The appropriateness of these models can then be tested on the basis of experimental data. Combined analytical techniques provide an enriched data set for this purpose as has been demonstrated for this example. After the selection of the most appropriate model, the corresponding parameters can be used... 

The results of this investigation show that CaCC>3 dissolution is controlled by mass transfer and not surface reaction kinetics. Buffer additives such as adipic acid enhance mass transfer by increasing acidity transport to the limestone surface. Dissolution is enhanced at low sulfite concentration but inhibited at high sulfite concentration, indicating some kind of surface adsorption or crystallization phenomenon. The rate of dissolution is a strong function of pH and temperature as predicted by mass transfer. At high CO2 partial pressure, the rate of dissolution is enhanced due to the CO2 hydrolysis reaction. 

The kinetics of substrate hydrolysis by both AChEs and BuChEs deviates from Michaelis-Menten kinciic.s, A.s implicated in Fig. 2 for acetylthiocholinc, only at substrate concentrations lower than 1 mM docs the rise in enzyme activity follow MichaeliS -Mentcn kinetics. At higher concentrations, AChE activity decreases, inhibited by the excess. substrate, whereas BuChE activity increases, activated by the excess substrate. Hence, the terms substrate inhibition and substrate activation are respective hallmarks of catalysi,s hy AChE and BuChE, Both phenomena can be simply described as a consequence of the formation of a ternary complex between the enzyme and two substrate molecules and thus as an allosteric phenomenon. The ternary complex in AChE has reduced or no activity compared to the Michaelis-Mcntcn complex, whereas it appears more active in BuChE hydrolysis. It is imponant to emphasize that this is a substrate-specific phenomenon. Not all AChE and BuChE substrates exhibit substrate inhibition and sub-... 

Transition-metal ions cause an enormous increase in the rate of hydrolysis of penicillins and cephalosporins (Gensmantel et al., 1978, 1980 Cressman et al., 1969). For example, copper(ri) ions can enhance the rate of hydrolysis of benzylpenicillin 10 -fold, a change in the half-life from 11 weeks to 0.1 seconds at pH 7. In the presence of excess metal ions, the observed apparent first-order rate constants for the hydrolysis of the 3-lactam derivatives are first order in hydroxide ion but show a saturation phenomenon with respect to the concentration of metal ion which is indicative of the formation of an antibiotic/metal ion complex. A kinetic scheme is shown in (3), where M is... 

The base hydrolysis of D -[Co(en)2Cl(OH)] produces mainly cis product with essentially complete (> 90 %) retention of configuration. The results of the D -[Co(en)2Cl2] base hydrolysis reaction indicate that the D -[Co(en)2Cl(OH)] ion predominates over the corresponding l optical isomer at all concentrations, while the reverse is true for the dihydroxo enantiomers—under conditions which give dihydroxo species to a greater extent than predicted by rate data in the literature. Some L -[Co(en2-(0H)2] and trflns-[Co(en)2(OH)2] appear to be formed above that predicted by the individual reaction sequences deduced from kinetics measurements in dilute solution. This effect had been noted earlier by Pearson, Meeker, and Basolo, but was later discounted by Chan and Tobe, who did not observe this phenomenon when hydroxide ion is slowly added at 0°. The results (Table 1) agree with both observations. 

Hanika et al. (2003) investigated the esterification of acetic acid and butanol in a trickle bed reactor, packed with a strong acid ion- exchange resin (Purolite 151) at 343 K - 393 K. Experimental data illustrate the benefit of simultaneous esterification and partial evaporation of the reaction products in the multi-functional trickle bed reactor. In case of total wetting of the catalyst bed, contact of vaporized products (ester and water) with catalyst was naturally limited and thus, the backward reaction i.e. ester hydrolysis was suppressed. This phenomenon shifted the chemical equilibrium conversion to high values. Saletan (1952) obtained quantitative reaction rate data for the formation of ethyl acetate from ethanol and acetic acid in fixed beds of cation exchange resin catalyst. The complex interaction of diffusion and reaction kinetics within the resin, which determine over-all esterification rate, has been resolved mathematically. 

The use of the term catalytic process was proposed by Berzelius in 1836 to describe the increase in the rate of certain chemical reactions under the influence of substances apparently foreign to the process, such as acids in the hydrolysis of starch. The long-known phenomenon has given rise to many researches and is as important in homogeneous phases as in heterogeneous or enzyme kinetics. 

Our compatriot N. A. Menshutkin made a great contribution to the development of the kinetics. In 1877 he studied in detail the reaction of formation and Iqrdrolysis of esters from various acids and alcohols and was the first to formulate the problem of the dependence of the reactivity of reactants on flieir chemical structure. Five years later when he studied the hydrolysis of tert-zmy acetate, he discovered and described the autocatalysis phenomenon (acetic acid formed in ester hydrolysis accelerates the hydrolysis). In 1887-, studying the formation of quaternary ammonium salts from amines and alkyl halides, he found a strong influence of the solvoit on the rate of this reaction (Menschutkin reaction) and stated the problem of studying the medium effect on the reaction rate in a solution. In 1888 N. A. Menschutkin introduced the term chmical kinetics in his monograph Outlines of Development of Chemical Views. ... 

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