Autocatalytic reaction mechanism

The autocatalytic reaction mechanism apparent at low temperatures is expected to apply to catalytic hydrogen oxidation at high pressures. In addition, the above study is the first to use STM to observe the formation of dynamic surface patterns at the mesoscopic level, which had previously been observed by other imaging techniques in surface reactions with nonlinear kinetics [57]. This study illustrates the ability of in situ STM to visualize reaction intermediates and to reveal the reaction pathway with atomic resolution. 

It is further stated by Reitzner in Ref 37b, p 13f, that Arrhenius simplification does not take into account that a large number of expls follow an autocatalytic reaction mechanism, fn the case of the inorganic azides, for example, the metal is considered to be the autocatalyst. The pressure-time curves for such autocatalytic reactions are characterized by an induction period, followed by acceleratory and decay periods. 

The role of illumination consists in creating electron-hole pairs, which are necessary for the partial reactions. During the reduction of OBr" ions, Br radicals are formed as intermediates (cfr. reaction (55)), which appear to initiate an autocatalytic reaction mechanism surface states are formed, through which holes are injected into the valence band (at least at not too high OBr concentrations). These surface states, which are experimentally detected as a peak in the capacitance-potential plot [24, 81], are believed to be associated with adsorbed OBr. Furthermore, voltammetric experiments demonstrate that these surface states can be annihilated by a sufficiently large concentration of holes at the surface. The latter explains why this induced electroless etching effect is not observed at p-GaP, since in this case the holes are present at the surface in a quasi-equilibrium cloud of majority carriers, in contrast to the case of n-GaP. 

A series of studies described in the present chapter has been performed on the assumption that only high explosives are the chemicals of the true AC type. In this regard, however, it is very probable that there exist chemicals, other than high explosives, which decompose in accordance with the autocatalytic reaction mechanism. Besides, although it is quite certain that nitrate esters, such as nitroglycerine and nitrocellulose, decompose in accordance with the autocatalytic reaction mechanism, it is beyond the scope of the present document to discuss whether nucleus-substituted polynitro compounds, such as tetryl, TNT and picric acid, also decompose in accordance with the same mechanism or not. Some other possibilities are, therefore, also suggested herein for the decomposition reaction mechanisms of a few high explosives, respectively. 

It is beyond the seope of the present document to discuss whether tetryl decomposes in aecordance with the autocatalytic reaction mechanism or not. It is, nevertheless, certain that the self-heating process of tetryl confined in the closed cell and subjected to the isothermal storage test is in accord with Eq. (59). 

The plot manifests a downward curvature. The reason why such a plot has been effected is thought to be that the reagent tested was not normal or that picrie acid does not decompose in aecordance with the autocatalytic reaction mechanism. 

In any event, it is beyond the seope of the present doeument to discuss whether picric acid decomposes in accordance with the autocatalytic reaction mechanism or not. 

Under the same conditions the even more reactive compounds 1,6-dimethylnaphthalene, phenol, and wt-cresol were nitrated very rapidly by an autocatalytic process [nitrous acid being generated in the way already discussed ( 4.3.3)]. However, by adding urea to the solutions the autocatalytic reaction could be suppressed, and 1,6-dimethyl-naphthalene and phenol were found to be nitrated about 700 times faster than benzene. Again, the barrier of the encounter rate of reaction with nitronium ions was broken, and the occurrence of nitration by the special mechanism, via nitrosation, demonstrated. 

The mechanism of oxidative dyeing involves a complex system of consecutive, competing, and autocatalytic reactions in which the final color depends on the efficiency with which the various couplers compete with one another for the available diimine. In addition, hydrolysis, oxidation, or polymerization of diimine may take place. Therefore, the color of a mixture caimot readily be predicted and involves trial and error. Though oxidation dyes produce fast colors, some off-shade fading does occur, particularly the development of a red tinge by the slow transformation of the blue indamine dye to a red phenazine dye. 

A review of epoxy-novolac reaction mechanisms and kinetics is provided by Biernath et al.85 Depending on the structures of the novolac and the epoxy, reactions have been reported to proceed through an nth-order mechanism or an autocatalytic mechanism.88-92... 

As the reaction proceeds higher sulfanes and finally Ss are formed. The reaction is autocatalytic which makes any kinetic analysis difficult. The authors discussed a number of reaction mechanisms which are, however, obsolete by today s standards. Also, the reported Arrhenius activation energy of 107 17 kJ mol is questionable since it was derived from the study of the decomposition of a mixture of disulfane and higher sulfanes. Nevertheless, the observed autocatalytic behavior may be explained by the easier ho-molytic SS bond dissociation of the higher sulfanes formed as intermediate products compared to the SS bond of disulfane (see above). The free radicals formed may then attack the disulfane molecule with formation of H2S on the one hand and higher and higher sulfanes on the other hand from which eventually an Ss molecule is split off. 

The interaction of hydrogen with preadsorbed oxygen at Pt(lll) led to hexagonal and honeycomb structures to develop at 131 K, which could be associated with OH phases with also evidence for water formation. The front (bright ring) consisted mainly of OH(a) and the area behind the front of H20(a). The mechanism suggested is that H(a) reacts first with 0(a) to form OH(a) and then H20(a) the water is mobile and reacts with O(a) to form OH(a) it is therefore an autocatalytic reaction. 

Illustration 9.5 indicates one type of rate expression and reaction mechanism that may be associated with an autocatalytic reaction. 

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

Increased conversion and product purity are not the only benefits of simultaneous separation during the reaction. The chromatographic reactor was also found to be a very suitable tool for studying kinetics and mechanisms of chemical and biochemical reactions. Some recent publications describe the results on investigation of autocatalytic reactions [135], first-order reversible reactions [136], and estimation of enantioselectivity [137,138]. It is beyond the scope of this chapter to discuss the details, but the interested reader is referred to an overview published by Jeng and Langer [139]. 

We demonstrate the use of Matlab s numerical integration routines (ODE-solvers) and apply them to a representative collection of interesting mechanisms of increasing complexity, such as an autocatalytic reaction, predator-prey kinetics, oscillating reactions and chaotic systems. This section demonstrates the educational usefulness of data modelling. 

There are many mechanisms that display autocatalytic behaviour. A minimal and very basic autocatalytic reaction scheme is presented below ... 

In addition, the results of such reactions have suggested plausible models for the mechanism of abiotic generation of optical activity, including an autocatalytic feedback mechanism (261). The latter involves random development of chiral crystals from achiral starting material, and solid-state reaction leading to products in which one enantiomer is in excess and thus can bias subsequent further crystallization (262). 

Fig. 8. Concentration B in reaction mechanism for diffusion coefficient of X much less than that of Y, necessary to achieve instability in a system in which the autocatalytic mechanism occurs on a one-dimensional array of local sites, versus logarithm of site density a (solid line). Dashed and dotted lines are for the isolated and continuum site limit, respectively.

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