Reaction mechanisms autocatalysis

Since the product of the alkylation reaction is the sulphonic acid, the reactions showed autocatalysis. The reaction mechanism was envisaged, therefore, as involving an intimate ion pair formed via equilibrium (185)... 

Thus the above data point to the fact that in such systems the reaction mechanism of the epoxy compounds with amines involves not only autocatalysis, but also autoinhibition of the reaction in its deep stages. However, this effect can only be observed if the relative concentration of the free hydroxyl groups decreases due to an increase in hydroxyl-amine complexing as a result of the conversion of the primary into the secondary and then to the tertiary amino group. For the real epoxy-amine compositions, the diffusion mechanism of the reaction inhibition at the deep stages is still the most typical. 

In 1953, Frank proposed a reaction mechanism without showing any chemical structure for the molecules, in which a chiral product acts as a chiral catalyst for its own production (asymmetric autocatalysis) and prohibits the formation of its antipode [411. In such a reaction, if it exists, the enantiomeric purity of the product would increase as the reaction progresses. Since then, asymmetric autocatalysis has attracted considerable conceptual interest [42], However, it was not until 1990 that the first asymmetric autocatalysis in asymmetric synthesis was reported [43]. 

Kinetic analysis of asymmetric autocatalysis was performed to study the reaction mechanism of asymmetric autocatalysis. The relationship between the reaction time and the yields of the product was investigated [67]. The i-P Zn addition to pyrimidine-5-carbaldehyde 11 was performed in the presence of enantiomerically pure autocatalyst, the reaction being monitored by HPLC using naphthalene as an internal standard. The plots shown in Fig. 4(a) constitute S-shaped curves that are characteristic of an autocatalytic reaction. The relationship between time, yield and enantiomeric excess was also measured in the asymmetric autocatalysis with amplification of ee using high to low ee of pyrimidyl alkanol as the catalyst (Fig. 4b) [68]. Portions of the reaction mixture were quenched periodically and analyzed by chiral HPLC. When pyrimidyl alkanols with high to good ee are used as the asymmetric autocatalyst, the observed values of yield and ee were well matched to our simulated... 

This book illustrates the recent aspects of amplification of chirality by asymmetric auto catalysis and by forming helical structures. The first four chapters summarize experimental asymmetric autocatalysis with amplification of enan-tiopurity, the mechanism of asymmetric autocatalysis examined by NMR and calculation, the computer simulation models of the reaction mechanism of asymmetric auto catalysis, and the theoretical models of amplification of chirality. The last chapter deals with the amplification of chirality by the formation of helical structures. However, the amplification of enantiopurity in non-auto catalytic asymmetric reaction and the amplification by enantiomer separation involving crystallization or sublimation are beyond the scope of this book. 

As the reaction develops, autocatalysis arising from (16) and (20) becomes important and consequently, (XVIII) and (XIX) no longer apply. For the KCl coated vessels in which the surface destruction of HO 2 radicals is thought to be rapid, the following reactions have to be added to the basic mechanism, viz. 

The pathway from Ce + to Br is the key to negative feedback on HBr02, which helps to remove this species after being accumulated due to autocatalysis. However, different roles are associated with the two species while Br is an inhibitor that directly removes HBr02, Ce" " " is a controlling species that provides a delay allowing the autocatalysis to advance considerably before inhibition by Br causes the concentration of HBr02 to drop hence oscillations may appear. As before, proper time scales of reaction steps are necessary for oscillations to appear. Clearly, this example of isothermal oscillations is more involved than the thermokinetic one. In particular, there are three main (or essential) types of variables rather than two this observation prompts for a classification of oscillatory reaction mechanisms—one of the main topics of this chapter. 

An experimental test for autocatalysis involves addition of the suspected autocatalytic species to the reaction mixture. If the material added is the responsible agent, one may generally expect behavior like that shown in Figure 9.15. Illustration 9.6 indicates one type of rate expression and reaction mechanism that may be associated with an autocatalytic reaction. 

The commonly accepted kinetic scheme for epwxy-amine reactions considers two reaction paths a non-catalyzed and an autocatalyzed p>ath. The autocatalysis is attributed to the formation of complex between generated or initially present hydroxyl groups, amino group and epoxy groups. A simple reaction mechanism is presented in Figure 3 although it can be improved considering some equilibrium reactions for the complexes formation (Ehlers et al., 2007). With appropriate mass balances it is possible to set out rate equations that can be fitted to experimental data to extract the relevant kinetic p>arameters... 

Autocatalysis occurs as in the Lotka-Volterra mechanism and the oregonator. If the concentrations of A and B are maintained constant, the concentrations of X and Y oscillate with time. A graph of the concentration of X against that of Y is a closed loop (the limit cycle of the reaction). The reaction settles down to this limit cycle whatever the initial concentrations of X and Y, i.e. the limit cycle is an attractor for the system. The reaction mechanism Is named after the city of Brussels, where the research group that discovered It Is based. 

All the component reactions investigated are found to be exothermic. Initial temperature rise of bromide + bromate reaction was found to be the highest (0.55°C/min) while that of cerous + bromate + malonic acid was to be found to be quite low (0.085°C/min). Thus in the first stage, when the reaction was mixed, the latter reaction involving autocatalysis predominates and the temperature rise is very slow. On the other hand, when Br + BrOj reaction involving inhibition reaction becomes dominant, there is a sharp rise in temperature. The thermochemical behaviour is thus in conformity with the FKN mechanism (Br -control mechanism). 

Autocatalysis, carbon formation and surface effects The third or autocatalytic stage In the methane pyrolysis. In which the yield of ethane begins to rise sharply again after the steady-state plateau (fig. 1) Is not predicted or explained by the reaction mechanism postulated above. The autocatalysis Is most evident In the yield of ethane, but almost certainly affects the other products as well, although It Is less obvious because their yields are already rising sharply. Autocatalysis has frequently been reported in the decomposition of methane, and under various conditions of pressure, temperature, conversion or surface, may have a variety of causes. It is most commonly associated with the formation of carbon, and attributed to reactions occuring at a carbon surface. 

In the given example the autocatalytic course of the reaction is consistent with its mechanism. However, in the overwhelming majority of cases the reaction mechanism and the kinetic equation have nothing in common. It has been shown that autocatalysis of the kind... 

Oxidation of aliphatic aldehydes by benzyltrimethylammonium chlorobromate to the corresponding carboxylic acid proceeds via the transfer of a hydride ion from the aldehyde hydrate to the oxidant. The oxidation of aUyl alcohol with potassium bromate in the presence of osmium(Vin) catalyst in aqueous acidic medium is first order in bromate, Os(Vni) and substrate, but inverse fractional order in H+ the stoichiometry of the reaction is 2 3 (oxidantsubstrate). The active species of oxidant and catalyst in the reaction were understood to be BrOs and H2OSO5, respectively, which form a complex. Autocatalysis by Br, one of the products, was observed, and attributed to complex formation between Br and osmium(VIII). First-order kinetics each in BrOs, Ru(VI), and substrate were observed for the ruthenium(VI)-catalyzed oxidation of cyclopentanol by alkaline KBrOs containing Hg(OAc)2. A zero-order dependence on HO concentration was observed and a suitable mechanism was postulated. The oxidation reaction of aniUne blue (AB+) with bromate at low pH exhibits interesting non-linear phenomena. The depletion of AB+ in the presence of excess of bromate and acid occurs at a distinctly slow rate, followed by a very rapid reaction. A 12-step reaction mechanism, consistent with the reaction dynamics, has been proposed. The novel cyclohexane-l,4-dione-bromate-acid system has been shown to exhibit a rapid oscillatory redox reaction superimposed on a slower... 

In contrast to its effect upon the general mechanism of nitration by the nitronium ion, nitrous acid catalyses the nitration of phenol, aniline, and related compounds. Some of these compounds are oxidised under the conditions of reaction and the consequent formation of more nitrous acids leads to autocatalysis. 

The kinetics of nitration of anisole in solutions of nitric acid in acetic acid were complicated, for both autocatalysis and autoretardation could be observed under suitable conditions. However, it was concluded from these results that two mechanisms of nitration were operating, namely the general mechanism involving the nitronium ion and the reaction catalysed by nitrous acid. It was not possible to isolate these mechanisms completely, although by varying the conditions either could be made dominant. 

Autocatalysis can cause sustained oscillations in batch systems. This idea originally met with skepticism. Some chemists believed that sustained oscillations would violate the second law of thermodynamics, but this is not true. Oscillating batch systems certainly exist, although they must have some external energy source or else the oscillations will eventually subside. An important example of an oscillating system is the circadian rhythm in animals. A simple model of a chemical oscillator, called the Lotka-Volterra reaction, has the assumed mechanism ... 

Prior sequestration of the prebiotic reactions within the micropores of weathered feldspars or other porous rock matrices also avoids many of the other problems of catalysis and dilution encountered by models of chemical biogenesis. That is, this mechanism attains viable evolutionary chemical selection among spatially discrete systems without the need to assume an unlikely capture-and-enclosure event involving a pre-existing lipid membrane. [192] Thus autocatalysis of chiral molecules could evolve before the actual appearance of free-floating lipid vesicles. 

The mechanism of the asymmetric autocatalysis with amplification of has been examined experimentally by us171 and other groups172. It is basically understood that the aggregation of the isopropylzinc alkoxide of 5-pyrimidyl alkanol is involved in the reaction. Kinetic analysis of the reaction shows that the reaction is second order in the isopropylzinc alkoxide of 5-pyrimidyl alkanol171. 

Figure 6.6(b) is better approximated by a cubic form, rate ocy2(l — y). Cubic autocatalysis has already provided us with behaviour of interest in chapter 2. In the remainder of this chapter we consider the stationary-state responses of schemes with this feedback mechanism in flow reactors. We will consider three models, with increasingly varied possible behaviour first an autocatalytic step on its own next we allow the autocatalytic species to undergo a subsequent reaction finally we add an uncatalysed reaction in competition with the autocatalysis. The local stability of such systems is... 

In general the amine-epoxy resin curing reactions show complex kinetics typified by an initial acceleration due to autocatalysis, while the later post-gelation stages may exhibit retardation as the mechanism becomes diffusion-controlled. However some workers 72 80) have found that over a limited range of conversion the kinetic data may be described by the simple models of Eq. (2-6) or (2-9). 

The mechanism of the reaction has been studied extensively, and has been shown to vary with the reaction conditions (64AHC(3)285). Cyanuric chloride is insensitive to both acid catalysis and autocatalysis, but the 2,4-dichloro (55) and 2-chloro derivatives (56) exhibit both acid catalysis and autocatalysis on solvolysis in ethanol-acetone solutions. 

Section we show that presence of two such intermediate stages is more than enough for the self-organization manifestation. Lotka [22] was the first to demonstrate theoretically that the concentration oscillations could be in principle described in terms of a simplest kinetic scheme based on the law of mass action [4], Its scheme given by (2.1.21) is similar to that of the Lotka-Volterra model, equation (2.1.27). The only difference is the mechanism of creation of particles A unlike the reproduction by division, E + A - 2A, due to the autocatalysis, a simpler reproduction law E —> A with a constant birth rate of A s holds here. Note that analogous mechanism was studied by us above for the A + B — B and A + B — 0 reactions (Chapter 7). 

Reaction 4 shows that the ruthenium center with three coordinated carbonyls can transfer one such ligand to the piperidine (presumably coordinated). The mechanism suggested for the acetate complex includes exactly analogous steps (Reactions 6 and 7). The kinetics for the hydride-catalyzed system, however, are quite different and show a first-order dependence in Ru and a more complex dependence on CO (Figure 4). Further, no autocatalysis is evident. 

Over the last 10-15 years, interest has grown significantly in the kinetics of combustion and explosion reactions, which are characterized by the presence of some mechanism of acceleration of the reaction. This acceleration, which leads to ignition, may be related either to the accumulation of active products which catalyse the reaction, the chain carriers (autocatalysis, chain explosion), or to an increase in the temperature of the mixture due to heat release in an exothermic chemical reaction (thermal explosion). 

In fact, in most mechanisms, the reaction rate is proportional to the concentration of a reaction product. Thus, the term of self-accelerating reaction would be more appropriate. But for sake of simplicity, we maintain the term autocatalytic. Our use of the word autocatalysis does not imply any molecular mechanism. 

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