Autocatalytic effect

A bifunctional autocatalytic effect of azinones in general is possible in certain nucleophilic reactions such as amination. Zollinger has found that 2-pyridone is the best catalyst for anilino-dechlorination of various chloroazines. It seems likely that examples of autocatalysis will be found when the substrate contains an azinone moiety. The azinone hy-products of displacement reactions may also function in this way as catalysts for the main reaction. 

The autocatalytic effect of HCl, the initial degradation product, on the degradation of PVC is a well-established phenomenon [104-107]. 

Tenne R, Wold A (1985) Passivation of recombination centers in n-WSe2 yields high efficiency (>14%) photoelectrochemical cell. Appl Phys Lett 47 707-709 Chaparro AM, Salvador P, Peter LM (1995) The role of surface defects in the photooxidation of iodide at n-MoSe2 evidence for a local autocatalytic effect. J Phys Chem 99 6677-6683... 

A one pot asymmetric autocatalytic effect for the above reaction has also been shown, with remarkable amplification of enantiomeric excess (Scheme 8-58).115c Thus, a trace (about 3 mg) of 2-methylpyrimidyl alcohol 141 with only a slight enantiomeric excess (0.2-0.3% ee) can be automultiplied with... 

Also, long-term isothermal storage (or low heating rate) tests are used to investigate autocatalytic effects. For example, a sample is held above 100°C for 10 hours to simulate drying operations [137]. In layer tests, the substance layer is heated by hot air passing around it with a fixed velocity. 

The autocatalytic effect of lead on the rate of reactions (3) and (4) was demonstrated146 by comparing the pyrolysis of mixtures of tetramethyl lead and hexaethyldiplumbane and of hexaethyldiplumbane and diethyl lead in the presence and absence of lead formed by reaction (4). 

In oscillatory reactions the concentration of the intermediate will increase and decrease alternately and periodically. This is due to the autocatalytic effect of one of the intermediates. 

Thus, in this section we will present the various reactive sites that can be found on the surface of the more common substrates, i.e. the surface reactive functionahties, the surface defects and, in addition, the anchoring sites on the nanoparticles which can induce autocatalytic effects. Selected examples will illustrate the major role played by surface chemistry. 

The enzymatic activity versus pH is thus generally a bell-shaped curve as in Fig. 6. An autocatalytic effect may appear when the reaction products have an acid-base effect (often the case). A simple example is the glucose oxidase reaction, shown in Fig. 7. Notably, the rate versus product (H+) curve indicates an autocatalytic effect on the alkaline branch, that is, for pH>pH (see Fig. 6). Systems presenting analogous properties have been studied by R. Caplan et al.27 and we also learn more about them from D. Thomas in this volume. 

A coating bearing one enzyme (papain) is produced on the surface of a glass pH electrode by the method previously introduced (co-crosslinking). The papain reaction decreases the pH, and the pH-activity variation gives an autocatalytic effect for pH values greater than the optimum under zero-order kinetics for the substrate (benzoyl arginine ethyl ester) the pH inside the membrane is studied as a function of the pH in the bulk solution in which the electrode is immersed. A hysteresis effect is observed and the enzyme reaction rate depends not only on the metabolite concentrations, but also on the history of the system. 

When the same kind of electrode is introduced in a solution with a high pH (i.e., pH= 10) and a lower substrate concentration (first order kinetics), an oscillation in time of the measured pH inside the membrane spontaneously occurs. This enzyme, which has been extensively studied, does not give oscillation for any conditions of pH and substrate concentration. The period of oscillation is around one-half minute, and the oscillation is abolished by introducing an enzyme inhibitor. The phenomenon can be explained by the autocatalytic effect and by a feedback action of OH- diffusion in from the outside solution. The diffusion of this ion is quicker than the diffusion of the substrate. There is a qualitative agreement between the computer simulation and the experimental results. 

In many other cases it is not at all clear that these exothermic reactions are operated in such a way that the system can remain isothermal. Self-heating and hence thermal feedback routes can be expected to have a strong autocatalytic effect on the reaction, perhaps in addition to chemical mechanisms. Recent modelling invoking cellular automata (Jaeger et al. 1985) has been to some extent successful at matching qualitatively many of the rather exotic responses which have been observed experimentally. 

At temperatures above 30°C, the rate increased slightly with temperature in a cylindrical reaction vessel but decreased slightly in a spherical vessel. A sharp increase in rate was noted as the temperature was decreased from 30°C. In the absence of water vapor or nitrogen dioxide a distinct induction period was observed, during which products were formed very slowly. The apparent rate increased slowly, and reached a maximum one-fourth to one-third of the way to reaction completion. The results quoted below were obtained near the start of the reaction but after the apparent induction period. The autocatalytic effect of nitrogen dioxide had been observed earlier in aqueous solutions. The reaction was found to obey the following rate law reasonably well. 

As a result of reaction (3-2), the concentration of hydroxyl will increase and this can produce an autocatalytic effect. In practice basic or acidic catalysts are often added to accelerate the cure, and tertiary amines are frequently used for this purpose. 

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