Catalysis autocatalysis

Consequently, at intermediate values of conversion, the rate will go through a maximum. At first glance, it might seem that the reaction is auto-catalytic. In reality, it is inhibited by the reactant itself. This unusual result shows the narrow connection between catalysis, autocatalysis and negative catalysis the kinetic patterns are dictated by the reactions with the active centers, their multiplication or their destruction. 

At relatively low temperatures, the effect of added nitric acid was to catalyse the reaction strongly, and to modify it to the autocatalytic form. At higher temperatures the effect of this additive was much weaker, as was the induced autocatalysis. Under these circumstances the catalysis was second-order in the concentration of nitric acid, and the presence of 0-25 mol l i of it brought about a sixfold change in the rate. 

Chloroanisole and p-nitrophenol, the nitrations of which are susceptible to positive catalysis by nitrous acid, but from which the products are not prone to the oxidation which leads to autocatalysis, were the subjects of a more detailed investigation. With high concentrations of nitric acid and low concentrations of nitrous acid in acetic acid, jp-chloroanisole underwent nitration according to a zeroth-order rate law. The rate was repressed by the addition of a small concentration of nitrous acid according to the usual law rate = AQ(n-a[HN02]atoioh) -The nitration of p-nitrophenol under comparable conditions did not accord to a simple kinetic law, but nitrous acid was shown to anticatalyse the reaction. 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

Goi. As noted previously, an a-chlorine atom renders a ring-nitrogen atom very weakly basic. Cyanuric chloride (5) is a very weak base both because s-triazines are of low basicity and because each of the ring-nitrogen atoms is alpha to two chlorine atoms. Hence, this compound should be insensitive to acid catalysis or acid autocatalysis and this has been observed for the displacement of the first chlorine atom with alcohols in alcohol-acetone solution and with water (see, however. Section II,D,2,6). 

The azinones and their reaction characteristics are discussed in some detail in Section II, E. Because of their dual electrophilic-nucleophilic nature, the azinones may be bifunctional catalysts in their own formation (cf. discussion of autocatalysis below) or act as catalysts for the desired reaction from which they arise as byproducts. The uniquely effective catalysis of nucleophilic substitution of azines has been noted for 2-pyridone. 

The catalytic effect of protons has been noted on many occasions (cf. Section II,D,2,c) and autocatalysis frequently occurs when the nucleophile is not a strong base. Acid catalysis of reactions with water, alcohols, mercaptans, amines, or halide ions has been observed for halogeno derivatives of pyridine, pyrimidine (92), s-triazine (93), quinoline, and phthalazine as well as for many other ring systems and leaving groups. An interesting displacement is that of a 4-oxo group in the reaction of quinolines with thiophenols, which is made possible by the acid catalysis. 

Design of chiral catalysis and asymmetric autocatalysis for diphenyl-(l-methyl-pyrrolidin-2-yl) methanol-catalyzed enantioselective additions of organozinc reagents 97YGK994. 

A fascinating area is micellar autocatalysis reactions in which surfactant micelles catalyse the reaction by which the surfactant itself is synthesized. Thus synthesis of dimethyldoceylamino oxide (reaction between dimethyl dodecyl amine and H2O2) benefits from this strategy. Here an aqueous phase can be used and an organic solvent can be avoided. Synthesis of mesoporous molecular sieves benefit through micellar catalysis and silicate polymerization rates have been increased by a factor 2000 in the presence of cetyltrimethyl ammonium chloride (Rathman, 1996). 

Another attempt to explain the homochirality of biomolecules is based on autocatalysis. The great advantage of asymmetric catalysis is that the catalyst and the chiral product are identical and thus do not need to be separated (Buschmann et al., 2000). The racemic mixture must have been affected by a weak perturbation in order that autocatalysis, which acts as an amplifier of enantioselectivity, could have led to only one of the two enantiomeric forms. This perturbation could have been due to the slight energy difference of the enantiomers referred to above, or to statistical fluctuations. 

The most important clue to explain the essence of chemical reaction has probably been the discovery of catalysis, without which a chemical industry would not exist. Catalysis is not understood in detail but it clearly dictates the course of a reaction by re-inforcing the spontaneous fluctuations that occur in a reaction medium. The most dramatic effects are produced in autocatalysis where a reaction product acts as a catalyst. Such feedback... 

Autocatalysis is a special type of molecular catalysis in which one of the products of reaction acts as a catalyst for the reaction. As a consequence, the concentration of this product appears in the observed rate law with a positive exponent if a catalyst in the usual sense, or with a negative exponent if an inhibitor. A characteristic of an autocat-alytic reaction is that the rate increases initially as the concentration of catalytic product increases, but eventually goes through a maximum and decreases as reactant is used up. The initial behavior may be described as abnormal kinetics, and has important consequences for reactor selection for such reactions. 

Another practically perfect asymmetric catalysis has been observed in reactions using (2-alkynyl-5-pyrimidyl)alkanols as the catalyst. The asymmetric autocatalysis shown in Scheme 8-59 gives the corresponding product in high yield with over 99% ee.116... 

Autocatalysis (i.e., catalysis by one of the products of the reaction), should also be considered. The impact of the accumulation of the catalytic product must be evaluated, and appropriate consideration given to runaway potential and prevention. 

DR. KENNETH KUSTIN (Brandeis University) Some years ago, you studied reactions of the type MX + Y = MY + X and found some interesting effects, such as chain reactions and phenomena of that sort. You also found that there was autocatalysis of such ligand substitution reactions by, I believe, triglycine. Has that turned up again Are other tripeptides or polypeptides autocatalytic How does that fit in with the base catalysis concept ... 

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. 

Significant progress has been witnessed in asymmetric catalysis/ In conventional asymmetric catalysis, the asymmetric catalyst C provides the enantioenriched product P, whose structures are generally different from those of the asymmetric catalysts. In contrast, asymmetric autocatalysis is an automultiphcation of a chiral compound P, in which the chiral product P acts as a chiral catalyst P for its own production/ ... 

This suggests immediately the definition of autocatalysis. A reactive intermediate or heat can act as catalysts to promote the reaction. However, in contrast to conventional catalysis, we do not add the catalyst from outside the system, but the catalyst is generated by the reaction (autocatalysis). We may add promoters or heat to initiate the process, which then accelerates by autocatalysis. Conversely, we may add inhibitors or cool the reactor to prevent both types of autocatalysis. 

Syllabus (1957-1958) - (No discussion on "autocatalysis , but on p 140 is mentioned the "activated complex , which is an important agent in catalysis) 3) Andreev Belyaev (I960), pp 57-61 (Avtokataliticheskii Vzryv) (Autocatalytic Explosion)... 

Replacement of the organic phase with surfactants to exploit micellar phase transfer catalysis principles (Battal et al., 1997) for the alkylation of phenol and aniline. This group had previously demonstrated the synthesis of a surfactant by micellar autocatalysis, whereby the surfactant product itself catalyses the reaction (Kust and Rathman, 1995). 

The specific models we will analyse in this section are an isothermal autocatalytic scheme due to Hudson and Rossler (1984), a non-isothermal CSTR in which two exothermic reactions are taking place, and, briefly, an extension of the model of chapter 2, in which autocatalysis and temperature effects contribute together. In the first of these, chaotic behaviour has been designed in much the same way that oscillations were obtained from multiplicity with the heterogeneous catalysis model of 12.5.2. In the second, the analysis is firmly based on the critical Floquet multiplier as described above, and complex periodic and aperiodic responses are observed about a unique (and unstable) stationary state. The third scheme has coexisting multiple stationary states and higher-order periodicities. 

Co-oxidation of indene and thiophenol in benzene solution is a free-radical chain reaction involving a three-step propagation cycle. Autocatalysis is associated with decomposition of the primary hydroperoxide product, but the system exhibits extreme sensitivity to catalysis by impurities, particularly iron. The powerful catalytic activity of N,N -di-sec-butyl-p-phenylenediamine is attributed on ESR evidence to the production of radicals, probably >NO-, and replacement of the three-step propagation by a faster four-step cycle involving R-, RCV, >NO, and RS- radicals. Added iron complexes produce various effects depending on their composition. Some cause a fast initial reaction followed by a strong retardation, then re-acceleration and final decay as reactants are consumed. Kinetic schemes that demonstrate this behavior but are not entirely satisfactory in detail are discussed. 

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. 

A negative catalyst, inhibitor or stopper is a substance which decreases the rate of a reaction without causing a change in the free energy of the reaction (a topic not considered here). Autocatalysis occurs in a chemical reaction in which a product or an intermediate functions as a catalyst. Such catalysis is characterised by the existence of an induction period during the initial stages of the reaction (but, again, this is not considered here). 

Several studies have tackled the structure of the diketopiperazine 1 in the solid state by spectroscopic and computational methods [38, 41, 42]. De Vries et al. studied the conformation of the diketopiperazine 1 by NMR in a mixture of benzene and mandelonitrile, thus mimicking reaction conditions [43]. North et al. observed that the diketopiperazine 1 catalyzes the air oxidation of benzaldehyde to benzoic acid in the presence of light [44]. In the latter study oxidation catalysis was interpreted to arise via a His-aldehyde aminol intermediate, common to both hydrocyanation and oxidation catalysis. It seems that the preferred conformation of 1 in the solid state resembles that of 1 in homogeneous solution, i.e. the phenyl substituent of Phe is folded over the diketopiperazine ring (H, Scheme 6.4). Several transition state models have been proposed. To date, it seems that the proposal by Hua et al. [45], modified by North [2a] (J, Scheme 6.4) best combines all the experimentally determined features. In this model, catalysis is effected by a diketopiperazine dimer and depends on the proton-relay properties of histidine (imidazole). R -OH represents the alcohol functionality of either a product cyanohydrin molecule or other hydroxylic components/additives. The close proximity of both R1-OH and the substrate aldehyde R2-CHO accounts for the stereochemical induction exerted by RfOH, and thus effects the asymmetric autocatalysis mentioned earlier. 

An NMR kinetic study of a phosphine-catalysed aza-Baylis-Hillman reaction of but-3-enone with arylidene-tosylamides showed rate-limiting proton transfer in the absence of added protic species, but no autocatalysis.175 Brpnsted acids accelerate the elimination step. Study of the effects of BINOL-phosphinoyl catalysts sheds light not only on the potential for enantioselection with such bifunctional catalysis, but also on their scope for catalysing racemization. 

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