Substance 67 autocatalytic reaction

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. 

This method was validated with over 100 substances and compared with the results of the classical study by isothermal experiments. For apparent activation energies above a level of 220 kj mol"1, 100% of the samples showed an autocatalytic character in isothermal experiments. This method can be used as a screen to distinguish clearly autocatalytic reactions from others that should be studied by isothermal experiments. This reduces the number of isothermal experiments required. 

To obtain a more realistic estimation of the behavior of an autocatalytic reaction under adiabatic conditions, it is possible to identify the kinetic parameters of the Benito-Perez model from a set of isothermal DSC measurements. In the example shown in Figure 12.11, the effect of neglecting the induction time assumes a zero-order reaction leading to a factor of over 15 during the time to explosion. Since this factor strongly depends on the initial conversion or concentration of catalyst initially present in the reaction mass, this method must be applied with extreme care. The sample must be truly representative of the substance used at industrial scale. For this reason, the method should be only be applied by specialists. 

As stated in Preface, each individual self-heating chemical, including every gas-permeable oxidatively-heating substance, is classified into either of the two large groups, i.e., the thermal decomposition or TD type and the autocatalytic reaction or AC type. 

In Chapter 3, a classification of self-heating chemicals, except gas-permeable oxidatively-heating substances, is introduced. Treatments of gas-permeable oxidatively-heating substances are made in Chapters 7 and 8. Self-heating chemicals are divided into two large groups, i.e., the thermal decomposition or TD type and the autocatalytic reaction or AC type. The TD type is subdivided into liquid chemicals, for each of which the Semenov equation is applied to calculate the Tc, and, solid (powdery, in reality) chemicals, for each of which the F-K equation is applied to calculate the Tc. On the other hand, the AC type is subdivided into high explosives of the true AC type and powdery chemicals of the quasi-AC type. 

In addition to strictly chemical catalysis processes, Eq. (3.18) may represent other situations such as for example combustion (the combustion of B requires of heat A, but the reaction is exothermic, so that more heat A is produced), or secondary crystallization from a solution (B is the substance in solution which crystalizes in form A if some nuclei of A are already present). If B is kept at constant concentration we have the simplest autocatalytic reaction ... 

Note that in an autocatalytic reaction, the substance produced stimulates the reaction thus, the reaction rate is proportional to both Cb(0 and Cao CufO. that is... 

Paraffins degrade relatively slowly by oxidation. This is an autocatalytic reaction that is accelerated by heat and light or by substances that function as oxidation catalysts. Tertiary carbon atoms are the most sensitive to oxidation consequently, heat stability of the following polymers decreases as shown ... 

Systems for consideration under this heading are conveniently classified into two groups, distinguished by the relationship existing between the reactant and the additive considered, which may be (i) a product of the decomposition process or (ii) a substance chemically different from all the participating phases. In certain important respects, reactions of type (i) may be regarded as specific instances of the autocatalytic behaviour characteristic of the reactant-derived product in many nucleation and... 

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. 

The same reaction also occurs at a lower temperature. 0.665 % of the substance decomposed at 20°C to form benzotrifuroxane in 3 years 2.43% at 35°C in one year 0.65% at 50°C in 10 days and at 100°C the substance underwent complete change in 14 hr. This decomposition is not, however, autocatalytic. This reaction — the formation of furoxane derivatives from aromatic azides with nitro group in the or/Ao-position — is of a general character (Boyer al. [160]). Despite the ease with which it decomposes trinitrotriazidobenzene has not been rejected for use as an initiator. In some countries large scale experiments are in progress to examine the possibilities of developing its practical application. 

At 250° to 400°C. There is a serious gap in our information about alkane or alkyl radical oxidations between 150° and 250°C. Above about 250°C. the oxidations of organic substances and alkanes in particular become autocatalytic (42). The autocatalysis must arise from reactions such as 12, 12, and 13, and below 350°C. Reaction 12 seems by far the most likely. Around 400°C. Reaction 13 may become important and at... 

In both these cases there is an autocatalytic element, i.e. one which is both the product of the reaction and which tends to increase its rate. This is the substance B in the first case and heat in the second. It is this element of feedback that is the source of the interesting behaviour. The first two terms in both of these equations represent the access to the site of reaction, in this case the stirred tank, in accordance with the criterion of actuality. The feasibility of these simple reaction schemes can be established by showing that they can be embedded in a fully reversible mechanism and the simple system recovered by limiting processes that do not violate the laws of thermodynamics or kinetics (for example, the Wegscheider condition). Yablonskii and his coworkers (Bykov et al. 1978,1979a, b, c Yablonskii Bykov 1979 Gol dshtein et al. 1986) have considered a number of simple models from which it is clear that the autocatalytic feature is essential. In the bimolecular surface reaction the autocatalytic role is played by the vacant sites which are indeed the product of the main reaction which releases those previously held by adsorbate and, at the same time, are a positive influence on the rate of reaction. 

Since a reaction product catalyses the reaction, the initial concentration of product also has a strong effect on the TMRad. In the case illustrated in (Figure 12.6), an initial conversion of 10% leads to a reduction of the TMRad by a factor of 2. This also has direct implications for process safety the thermal history of the substance, that is, its exposure to temperature for a certain time increases initial product concentration, leading to effects comparable to those illustrated in Figure 12.5. Hence it becomes obvious that substances showing an autocatalytic decomposition are very sensitive to external effects, such as contaminations and previous thermal treatments. This is important for industrial applications as well as during the experimental characterization of such decompositions the sample chosen must be representative of the industrial situation, or several samples must be analysed. 

Frank proposed a mechanism for the autocatalytic self-replicating process in which a chemical substance catalyzes its own production and acts as an anticatalyst for the production of the enantiomer without mentioning any actual compound or actual reaction [17]. hi this kinetic model, it is possible to obtain an enantiomerically enriched compound from an ex-... 

How specific must the reactions of an autocatalytic cycle be for it to grow Imagine a cycle with n constituents, and m other active substances in the medium (which also include the reagents). Considering all possible reactions between the constituents and the other substances, King found that the cycle grows exponentially only if... 

Other typical chain reactions include those of hydrogen with halogen in the gas phase and oxidation of organic substances at moderate temperatures (autoxidation). A special facet of the latter reactions is that the product or an intermediate can act as initiator, and the reaction then is autocatalytic. 

The chain reaction mechanism is frequently referred to as autocatalytic because it starts slowly, but the rate becomes faster as the reaction proceeds. Not many examples of drug substances that decompose by a free radical chain mechanism are known because the process requires participation of a very reactive (i.e., unstable) compound. This usually means a compound susceptible to oxidation and is illustrated by the photo-oxidation of benzaldehyde, as shown in Scheme 2.2 (Moore, 1976) ... 

Thus, one theory for delayed action is the quenching of free radical crosslink precursors by monomeric polysulfides. It has been found that, if bisalkylpolysullides are mixed with uncured rubber stocks, more delay results. It is also been shown that the early reaction products formed by the interaction between accelerator and sulfur (Ac-S -Ac) are inhibitors of crosslink formation. The very substances that give rise to the formation of the crosslink precursor (rubber-Sjc-Ac) inhibit the formation of the crosslinks. We note that other mechanisms for delayed action have been proposed. In the case of acceleration by benzothiazolesulfenamides, the accelerator is depleted in an autocatalytic fashion with the formation of 2-mercaptobenzothiazole (MET). The rate of this depletion is about proportional to the amount of MET present. There is strong evidence, which indicates that the following reactions occur in sulfenamide-accelerated systems ... 

Even complex chemical reaction mechanisms can be separated into several definite elementary reactions, i. e. the direct electronic interaction process between molecules and/or atoms when colliding. To understand the total process B-fot example the oxidation of sulfur dioxide to sulfate - it is often adequate to model and budget calculations in the climate system to describe the overall reaction, sometimes called the gross reaction, independent of whether the process A Bis going via a reaction chain A C D E. .. Z B. The complexity of mechanisms (and thereby the rate law) is significantly increased when parallel reactions occur A X beside A- C,E- X beside E F. Many air chemical processes are complex. If only one reactant (sometime called an educt) is involved in the reaction, we call it a unimolecular reaction, that is the reaction rate is proportional to the concentration of only one substance (first-order reaction). Examples are all radioactive decays, rare thermal decays (almost autocatalytic) such as PAN decomposition and all photolysis reactions, which are very important in air. The most frequent are... 

This chain of reactions satisfies onr feedback postulates. In step 1, the concentration of X increases in step 2, Y is produced at the expense of X in step 3, substance Y enhances the production of X (at flie expense of itself-this is an autocatalytic step) then again X transforms to Y (step 2). etc. 

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