Examples autocatalysis

Sometimes, even spatially homogeneous chemical systems can cause bistability and show complex behavior in time. For example, autocatalysis may occur due to the particular molecular structure and reactivity of certain constituents, and reactions may evolve to new states by amplifying or repressing the effect of a slight concentration perturbation. 

For example, autocatalysis rarely, if ever, arises from a single elementary step. Far more likely than the simple reaction A- - X 2X is a sequence like... 

Under isothermal conditions, nonlinear models that lead to dynamic behavior typically incorporate some sort of a feedback, for example, autocatalysis, when a certain step is accelerated by a product of this step. 

There are many reactions in which the products formed often act as catalysts for the reaction. The reaction rate accelerates as the reaction continues, and this process is referred to as autocatalysis. The reaction rate is proportional to a product concentration raised to a positive exponent for an autocatalytic reaction. Examples of this type of reaction are the hydrolysis of several esters. This is because the acids formed by the reaction give rise to hydrogen ions that act as catalysts for subsequent reactions. The fermentation reaction that involves the action of a micro-organism on an organic feedstock is a significant autocatalytic reaction. 

Autocatalysis may arise when the nucleophilic atom of the reagent is bound to a hydrogen atom which is eventually eliminated during the reaction. This occurs with neutral reagents such as primary or secondary amines, thiols, and alcohols. If the displaced group (usually an anion) is a sufficiently weak base, the proton is effectively transferred to any basic reactant. Hence, the best known examples of autocatalysis involve chloro-A-heteroaromatic compounds as the substrates. 

A bifunctional autocatalytic effect of azinones in general is possible in certain nucleophilic reactions such as amination. Zollinger has found that 2-pyridone is the best catalyst for anilino-dechlorination of various chloroazines. It seems likely that examples of autocatalysis will be found when the substrate contains an azinone moiety. The azinone hy-products of displacement reactions may also function in this way as catalysts for the main reaction. 

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 ... 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

The nucleophilic displacement reactions of organolithium compounds with alkyl halides are second order insofar as the rates have been measured, but there are unexplained examples of autocatalysis and non-reproducable rate constants. The product of the reaction in the case of the methylallyl chlorides is the same mixture regardless of... 

Autocatalysis happens when a reaction product, formed during reaction, acts as a catalyst which accelerates the progress of the reaction even at constant temperature. An example is the acid-catalysed saponification of various esters and related compounds. Autocatalytic reactions can be easily experimentally identified by means of differential thermal analysis methods. 

When one of the product itself acts as a catalyst for that reaction, the phenomena is called autocatalysis. For example,... 

In 1969 Calvin [64] proposed a scheme for autocatalytic symmetry breaking, which he called stereospecific autocatalysis . Calvin s mechanism has been validated experimentally in the context of the total spontaneous resolution during the crystallization of racemic mixtures. During crystallization, crystals of one enantiomer may spontaneously separate, leaving the other enantiomer in solution. If the possibility of the equilibration of the enantiomers in solution exists and if the enantiomer in solution can convert rapidly to the enantiomer that is crystallizing before crystallization is complete, then the entire racemate may deposit as a single enantiomer. At least half a dozen examples of Calvin s stereospecific autocatalysis involving such... 

In the 45 years since its proposal, Frank s autocatalytic mechanism (Section 11.3, above) has spawned numerous theoretical refinements including consideration of such factors as reversibility, racemization, environmental noise, and parity-violating energy differences. [100,101] In contrast to the above examples of stereospecific autocatalysis by the SRURC, however, none of these theoretical refinements is supported by experimental evidence. While earlier attempts to validate the Frank mechanism for the autocatalytic amplification of small e.e.s in other experimental systems have generally been unsuccessful, several recent attempts have shown more promising results. [102,104]... 

Asymmetric autocatalysis using (5)-pyrimidyl alkanol 2a with only 2% ee afforded (5)-2a with an increased ee of 10%, [Eq. (9.4)]. The (5)-2a obtained with 10% ee was then used as an asymmetric autocatalyst for the following asymmetric autocatalysis. (5)-Pyrimidyl alkanol 2a with an increased ee of 57% was obtained. The subsequent consecutive asymmetric autocatalysis and the use of that product as an asymmetric autocatalyst for the following round gave (5)-pyrimidyl alkanol 2a with 81 % and 88% ee, respectively. Thus, the overall process was the asymmetric autocatalysis of (5)-2a starting from a low ee of 2% with significant amplification of chirality to 88% ee, with the increase in the amount without need for other chiral auxiliary. ° This stands as the first example of an asymmetric autocatalysis with amplification of ee. In addition, one-pot asymmetric autocatalysis of pyrimidyl alkanol 2b also significantly increased the chirality from 0.28 to 87% ee. ... 

We have demonstrated the enantioselective synthesis of near-enantiopure compounds by asymmetric photodegradation of racemic pyrimidyl alkanol 2c by circularly polarized light followed by asymmetric autocatalysis. This is the first example of asymmetric autocatalysis triggered directly by a chiral physical factor CPL. 

The increase in energy content of an atom, ion, or molecular entity or the process that makes an atom, ion, or molecular entity more active or reactive. In enzymology, activation often refers to processes that result in increased enzyme activity. For example, increasing temperature often can have a positive effect on enzyme activity (See Arrhenius Equation). Other examples of enzyme activation include (1) proteolysis of zymogens (2) alterations in ionic strength (3) alterations due to pH changes (4) activation in cooperative systems (5) lipid or membrane interface activation (6) metal ion effects (7) autocatalysis and (8) covalent modification. 

The preceding situation is an example of thermal autocatalysis, and its consequence is fiequently a thermal explosion. 

We have previously encountered examples of chemical autocatalysis, where the reaction accelerates chemically such as in enzyme-promoted fermentation reactions, which we modeled as A + B 2B because the reaction generates the enzyme after we added yeast to initiate the process. The other example was the chain branching reaction such as H. -I-O2 —> OH - -0 just described in hydrogen oxidation. The enzyme reaction example was nearly isothermal, but combustion processes are both chain branching and autothermal, and therefore they combine chemical and thermal autocatalysis, a tricky combination to maintain under control and of which chemical engineers should always be wary. 

Chemical feedback may work, for example, by chain branching or autocatalysis . The mechanism by which hydrogen and oxygen react spontaneously involves a cycle of three elementary steps ... 

The evolution in time of the concentration of the species A and of the temperature rise AT, for the example data in Table 4.1, is shown in Fig. 4.1. The behaviour is in many ways similar to that of the isothermal cubic autocatalysis model of the previous chapters. The concentration of the precursor P decreases exponentially throughout the reaction. The temperature excess jumps rapidly to approximately 80 K, from which value it begins to decay approximately exponentially. At the same time, the concentration of the intermediate A rises relatively slowly to values of the order of 10"i mol dm-3. After approximately 15 s, the concentration of A and the... 

Chapter 6 considered isothermal autocatalysis in an open system here we study a classic case of thermal feedback. A rich variety of stationary-state patterns (bifurcation diagrams) are generated and considered here alongside those of the previous isothermal example. Flow diagrams are again illuminating and singularity theory provides a systematic approach. After study a reader should be able to ... 

Here F represents the functional form of the left-hand sides of the various stationary-state equations, x is the stationary-state solution such as the extent of reaction, the temperature excess, etc., and rres is the parameter we have singled out as the one which can be varied during a given experiment (the distinguished or bifurcation parameter). All the remaining parameters are represented by p, q, r, s,. For example, in eqn (7.21) the role of x could be played by the extent of reaction 1 — ass, with p = 0ad and q = tN for isothermal autocatalysis, x can again be the extent of reaction, with p = P0, q = k2, and r = jcu. 

As the simplest example, let us consider the isothermal cubic autocatalysis of 6.2, where there is catalyst inflow but no decay. In terms of the extent of conversion x = 1 — a55, the appropriate stationary-state condition is... 

As an example we can again take our cubic autocatalysis with inflow of B, so Fx and Fz are as given above, leading to... 

For every 10 °C increase in temperature, the rate of decomposition is approximately doubled, but may increase as much as 50 times if the explosive is in the molten state. The rates of decomposition depend on the condition of storage and the presence of impurities which may act as catalysts. For example, nitroglycerine and nitrocellulose decompose at an accelerated rate due to autocatalysis, whereas the decomposition rate of TNT, picric acid and tetryl can be reduced by removing the impurities which are usually less stable than the explosive itself. With many of the explosives the presence of moisture increases the rate of decomposition. 

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