The Autocatalytic Reactions

We now turn to the selection of reactions exhibiting autocatalysis. Chemical reactions with abrupt, non-linear changes of concentrations have been known for some time. They are the spectacular clock reactions, first described by Landolt in the nineteenth century41. The key to understanding the sudden and predictable (you could set your watch by their occurrence, hence the name clock ) color changes was provided over 60 years ago by Eggert and Schamow42. They analyzed the Landolt reaction (IV) (in the presence of excess iodate) 

Various reducing substrates such as arsenite, thiosulfate, ferrocyanide, etc. can substitute for sulfite, producing clock reactions in which autocatalysis can be demonstrated to 

The two systems we combined are the arsenite-iodate and the chlorite-iodide reactions. We first describe the arsenite subsystem. 

In the presence of excess iodate, the net stoichiometry of this reaction is 

Eggert and Schamow identified three major component processes making up reaction (VI)42.  

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The effect of temperature on the non-catalysed reaction was difficult to disentangle, for at lower temperatures the autocatalytic reaction intervened. However, from a limited range of results, the reaction appeared to have an experimental activation energy of c. +71 kj moh. ... 

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 stabilities of the alkaline earth peroxides, M02 2 H202 increased in the sequence Ca > Sr > Ba. Values of E for the autocatalytic reactions... 

This very situation has been encountered during the oxidation of Ta Br2 (= A) to TafiBrJj(= P) by VO, the dioxovanadium(l+) ion (= B), in aqueous solution.19 The autocatalytic reaction of Ta Br and VOJ produces Ta Br l = I), which does not accumulate in the system owing to its rapid reaction with TaeBrfr. 

Since the autocatalytic reaction is third order, a steady-state material balance gives a cubic in bout- This means there are one or three steady states. Suppose binjain = 1/15 and explore the stability of the single or middle steady state for each of the following cases ... 

At high temperatures (> 170 K), the water desorbs and so the autocatalytic reaction cannot be sustained and is an explanation for why the H2 + 02 reaction slows, the formation of OH species now being solely dependent on the H(a) + O(a) reaction, which is the slowest step in the above scheme. That the water + oxygen reaction was fast and facile was evident from the spectroscopic studies at both nickel and zinc surfaces, when the oxygen surface coverage was low and involving isolated oxygen adatoms. 

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. 

A superspiral consisting of two spirals (coiled coil), known as the leucine zip, is formed in this sequence via dimerisation. The condensation reaction, carried out in the aqueous phase, involves two peptide fragments which contain 15 and 17 amino acid residues respectively. Activation takes place via thioester formation (see Sect. 5.3.1). The ligation to a complete GCN4 matrix gives a new 32 amino acid peptide, which can itself serve as a matrix. The autocatalytic reaction exhibits a parabolic increase in the peptide concentration (caused by product inhibition see Section 6.4). 

For the autocatalytic reaction described in Example 15-10 and the data given there, calculate the volume of a combined CSTR + PFR reactor arranged as in Figure 17.7. 

In this chapter we deal with single reactions. These are reactions whose progress can be described and followed adequately by using one and only one rate expression coupled with the necessary stoichiometric and equilibrium expressions. For such reactions product distribution is fixed hence, the important factor in comparing designs is the reactor size. We consider in turn the size comparison of various single and multiple ideal reactor systems. Then we introduce the recycle reactor and develop its performance equations. Finally, we treat a rather unique type of reaction, the autocatalytic reaction, and show how to apply our findings to it. 

Consider the autocatalytic reaction A R, with = 0.001 C Cr mol/ liter-s. We wish to process 1.5 liters/s of a C o = 10 mol/liter feed to the highest conversion possible in the reactor system consisting of four 100-liter mixed flow reactors connected as you wish and any feed arrangement. Sketch your recommended design and feed arrangement and determine Af from this system. 

Figure 3-12 Plots of rand l/r versus Cao - Ca for the autocatalytic reaction A B,r = IcCa- The PFTR requires infinite residence time if no B is added to the feed.

There are many interesting problems in which complex chemistry in nonisothermal reactors interact to produce complex and important behavior. As examples, the autocatalytic reaction, A — B, r = kC/ Cg, in a nonisothermal reactor can lead to some quite complicated properties, and polymerization and combustion processes in nonisothermal reactors must be considered very carefully in designing these reactors. These are the subjects of Chapters 10 and 11. 

Fig. 26. STM images of the oxygen pre-covered platinum(l 1 1) surface during reaction with hydrogen. Images were recorded at a temperature of T = 111 K with a time interval of 625 K. The white ring in the upper right corner is associated with a reaction front of OH intermediates from the autocatalytic reaction. The outside is characterized by an oxygen-terminated surface, whereas water molecules from the reaction are identified inside the ring. Adapted with permission from Reference (757).

Fig. 11.6. The two possible dependences of wave velocity on the ratio of quadratic to cubic contributions to the autocatalytic reaction the straight line corresponds to eqn (11.43), the cubic result the parabola to eqn (11.44), the quadratic form. The two loci touch tangentially at q =...

The autocatalytic reaction scheme A + 2B —> 3B, B —> C was introduced in 1983s and has proved itself to be fecund of useful applications in the study of reactor stability and chemical oscillations.6 We shall depart from their notation for we wish to be able to generalize to several species, Au and it is not desirable to use the concentration of A as a reference concentration when it is going to be varied. Similarly, the several species will have different rate constants for the several autocatalytic steps and therefore the first-order rate constant of B — C is most apt for the time scale. 

By optimizing the structure of the substituent at the 2-position of the pyrimidine ring, practically perfect asymmetric autocatalysis was realized [55]. When (S)-2-methyl-l-(2-/-butylethynyl-5-pyrimidyl)-l-propanol (56c) with >99.5% ee was used as the asymmetric autocatalyst (20 mol %) in cumene, (S)-56c was obtained as the product with >99.5% ee in almost quantitative (>99%) yield (Scheme 9.28). The autocatalytic reaction was performed successively, with the products of one round serving as the asymmetric autocatalysts for the next. Even in 10 rounds, the enantiopurity and yield of the product 56c were always almost perfect (>99%, >99.5% ee) (Table 9.2). Thus, the autocatalyst 56c multiplied by a factor of ca. 107 during the 10 rounds, with no deterioration. 

Figu re 12.1 Comparison of autocatalytic (a) an nth-order (ri) reactions in an isothermal DSC experiment performed at 200°C. Both reactions present a maximum heat release rate of lOOWkg 1 at 200°C. The induction time of the autocatalytic reaction leads to a delay in the reaction course. 

This is due to the fact that under isothermal conditions, the nth-order reaction presents its maximum heat release rate at the beginning of the exposure to initial temperature, whereas the autocatalytic reaction presents no heat release rate at this time. Thus, temperature increase is delayed and only detected later after an induction period, as the reaction rate becomes sufficiently fast. Hence acceleration, due to both product concentration and temperature increase, becomes very sharp. 

In the sixth and last step, the system is still considered to be purely conductive, with heat exchange at the wall to the surroundings and the zero-order approximation of the kinetics is replaced by a more realistic kinetic model. This technique is very powerful in autocatalytic reaction, since a zero-order approximation leads to the very conservative assumption that the maximum heat release rate is realized at the beginning of the exposure and maintained at this level, respectively increasing with temperature, during the whole time period. In reality, the maximum heat release rate is delayed, and only achieved later on. Thus, heat losses may lead to a decreasing temperature during the induction time of the autocatalytic reaction. 

In conventional asymmetric catalysis, the asymmetric catalyst provides the enantioenriched product, whose structures are generally different from those of the asymmetric catalysts. In contrast, asymmetric autocatalysis is an auto-multiplication of a chiral compound P, in which the chiral product P acts as a chiral catalyst P for its own production (Scheme 2). We considered that, if the reaction product has the amino alcohol functionality as the result of alkylzinc addition, it should work as the catalyst for the next reaction, i.e., the autocatalytic reaction might proceed. 

The results of such a high enantioselectivity in the autocatalytic reaction encouraged us to investigate the enantioselective alkylation utilizing an autocatalyst with a small enantiomeric imbalance of 2% ee. In an earlier study on asymmetric autocatalysis using other compounds as the autocatalyst, the enantiomeric excess of chiral product has always been lower than that of the chiral catalyst. However, in this pyrimidine system, we found for the first time... 

The fluid-bed process being developed in the 100 B/D pilot plant has good potential for becoming an important part of our future technology. It has inherent potential advantages over the fixed-bed process as mentioned earlier. It is basically flexible for easy adaptation to petrochemicals production and better control of the autocatalytic reaction. 

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