Autocatalytic enhancement

Fig. 1. Growth of perturbations into sinusoidal patterns (vertical - concentration, horizontal - distance), a) An initial small positive perturbation of X from its uniform steady-state value of Xo- b) If X is autocatalytic (enhances its own production) and cross-catalyzes Y, the initial X perturbation grows and begins to produce a Y peak. Y diffuses faster than X. c) If Y inhibits X, it begins to create troughs in X, which d) also become troughs in Y (since X activates Y). The pattern achieves a spacing dependent on reaction and diffusion constants. Reproduced from Harrison (1993), redrawn from Maynard Smith (1968), with permission.

In order to explain the optical activity of living matter, Frank [19] has proposed a kinetic model which describes the accumulation of one enantiomer due to spontaneous symmetry breaking and autocatalytic enhancement of its concentration. Although the energy difference between D- and L-enantiomers is extremely small, Frank s model has been extended in order to discuss whether a very small systematic chiral perturbation rather than an accidental event can determine which enantiomer is formed [20]-[23]. For example, Kondepudi and Nelson [23] considered the reaction scheme... 

Illustration 9.5 indicates that one may have parallel paths leading from reactants to products and that in the case of an autocatalytic reaction, one path may be preferred over a second until the product level builds up to a point where the second becomes appreciable. In this example, the magnitudes of the rate constants are such that the vast majority of the reaction occurs by the autocatalytic path. In cases such as these it is desirable to use a CSTR or recycle reactor to enhance the reaction rate by virtue of the back-mixing of product species. 

Thus, no evidence for the direct bimolecular one-electron reduction of HOC1 could be obtained. Under conditions of relatively high pH and very low Cl- concentrations, where the three pathways shown above should be minimized, the reaction is autocatalytic, presumably because of the production of Cl- and its enhancement of reaction through reactions 7 and 8. 

Confinement—Deflagration rates of substances such as azo compounds, peroxides, and certain lead oxides may accelerate by pressure increase, especially when the governing decomposition reaction is gas-phase controlled [28]. Initiation of a deflagration at the bottom or at the center of a closed or partially closed vessel may lead to an increase of eh deflagration rate by a factor of more than 100 in comparison with top initiation. Autocatalytic decomposition by a volatile catalyst is enhanced by confinement. 

Dissolved iron(III) is (i) an intermediate of the oxidative hydrolysis of Fe(II), and (ii) results from the thermal non-reductive dissolution of iron(III)(hydr)oxides, a reaction that is catalyzed by iron(II) as discussed in Chapter 9. Hence, iron(II) formation in the photic zone may occur as an autocatalytic process (see Chapter 10.4). This is also true for the oxidation of iron(II). As has been discussed in Chapter 9.4, the oxidation of iron(II) by oxygen is greatly enhanced if the ferrous iron is adsorbed at a mineral (or biological) surface. Since mineral surfaces are formed via the oxidative hydrolysis of Fe(II), this reaction proceeds as an autocatalytic process (Sung and Morgan, 1980). Both the rate of photochemical iron(II) formation and the rate of oxidation of iron(II) are strongly pH-dependent the latter increases with... 

Prepolymer produced via the terephthalic acid (TPA) monomer route shows an increased reactivity in comparison with that produced by the dimethanol tereph-thalate (DMT) monomer process [49], This behavior is possibly caused by the enhanced CEG content, which is usually higher in products from the TPA process as a result of insufficient conversion of the acid monomer in the esterification reaction (Figure 5.23). The increased reactivity may be caused by an autocatalytic influence of the carboxylic groups which seems to be disturbed by an unbalanced content of OH groups in the case of degradation. 

By sonically inducing the polymerisation of methyl methacrylate (MMA), Price [65] has extended the work of Kruus and studied the effect of the absence and presence of the initiator azobisisobutyronitrUe (AIBN). Similar conversions to Kruus (2-3% per hour) were obtained in the absence of initiator at 25 °C. However considerable improvements in the polymerisation rate were observed when 0.1% of initiator were used (Fig. 5.40), the reaction appearing to become autocatalytic. This no doubt is due to the faster production of polymer in the initiated system (faster initiation due to enhanced initiator breakdown) which is then available for degradation to produce more free radical entities. 

We call this type of reaction autocatalytic because the initiation step generates the catalyst [CH3-], which is necessary to make the overall process proceed, and the concentration of the catalyst molecule is not changed by the propagation reaction steps. It is called autocatalytic because the reaction itself generates the catalyst that promotes the reaction. Note also that the overall rate is enhanced by large fcj and inhibited by large Jit even though and ki are much smaller than kp. 

As described in the preceding section, asymmetric amplification has been reported in the non-autocatalytic enantioselective addition of dialkylzincs. In asymmetric autocatalysis, amplification of has a more significant role, because the product of the asymmetric autocatalysis itself is capable of acting as the asymmetric autocatalyst. Once the product, i.e. the asymmetric autocatalyst with an enhanced , is formed in the asymmetric autocatalytic reaction, the product catalyzes the formation of itself with higher . From the viewpoint of the molecule, an asymmetric autocatalyst with dominant absolute configuration catalyzes... 

In the absence of any chiral factors, the probability of the formation of S- and 77-enantiomers is 1 to 1. However, the numbers of the resulting two enantiomers are not exactly the same in almost all cases. Mislow197 described the inevitability of small enantiomeric enrichment in absolute asymmetric synthesis. According to the statistics, it is expected that a fluctuation in the ratio of the S- and 77-enantiomers becomes more and more likely as the numbers in the enantiomer mixture become smaller198. Thus, if the asymmetric autocatalysis is initiated without adding any chiral substance, small fluctuations of enantiomers produced in the initial stage could be enhanced by consecutive asymmetric autocatalytic reaction of pyrimidyl alkanol with amplification of chirality. 

Nitration. All nitration reactions are potentially hazardous because of the explosive nature of the products and the strong oxidizing tendency, characteristic of the nitrating agent. The nitration reaction and the oxidation side reaction are highly exothermic. Therefore, these reactions may be extremely rapid and become uncontrollable. Close temperature control must be maintained (80,81). Sensitivity is enhanced by the presence of impurities, and rapid autocatalytic... 

We have presented various simple scenarios that we are aware of in relevance to the ee amplification of the Soai reaction a quadratic autocatalytic model in a monomer or homodimer system, and a linear autocatalytic model in an antagonistic heterodimer system. All these models can realize ee amplification such that the final value of the ee 0ool depends on, but is larger than, the initial value 0ol> as schematically shown in Fig. 8. The curve in the figure represents 0o - (poo for a given initial ratio qo/c, namely the ratio of the amount qo of the total chiral initiators R and S relative to that of the total reactants c. Amplification is more enhanced if the ratio of the chiral initiator qo/c is smaller. This plot also shows the possibility of increasing the final ee by repeating the reaction. 

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