Autocatalytic processes

Moreover, we observed for iron deposition from [Fe(CO)5] in the presence of water vapor that autocatalysis should be responsible for the fact that the more quantities of iron are deposited, the more the loading is close to the expected value. 

at 5% theoretical loading the measured to theoretical iron loading ratio is 0.82, whereas at 25% this ratio increases to 0.96 [77]. 

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Bromide ion acts as an inliibitor through step (9) which competes for HBr02 with the rate detennining step for the autocatalytic process described previously, step (4) and step (5). Step (8) and Step (9) constitute a pseudo-first-order removal of Br with HBr02 maintained in a low steady-state concentration. Only once [Br ] < [Br ] = /fo[Br07]//r2 does step (3) become effective, initiating the autocatalytic growth and oxidation. 

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. 

Surfaces. Essentially any electrically conductive surface can be electroplated, although special techniques may be required to make the surface electrically conductive. Many techniques ate used to metalline nonconductive surfaces. These are weU-covered ia the Hterature (3) and can range from coating with metallic-loaded paints or reduced-silver spray, to autocatalytic processes on tin—palladium activated surfaces or vapor-deposited metals. Preparation steps must be optimized and closely controlled for each substrate being electroplated. 

Pitting on stainless steels is almost always associated with acid conditions that develop within pits by autocatalytic processes. Solution pH outside the pit may be only mildly acidic or not acidic at all (as in the case of sea water, for example). 

Degenerate Explosion it was a free radical autocatalytic process and control was difficult, but manageable. The main disadvantage was that it produced as much or more acrolein as propylene oxide. Because no market existed for acrolein at that time, the project was abandoned. Within two years, the acrylic market developed and a new project was initiated to make acrolein and acrylic acid by vapor-phase catalytic oxidation of propylene. 

Owing to the hydrolysis reaction, pit development is an autocatalytic process and often there is an induction period before pit growth attains... 

Recent work has shown that tin may be deposited by an autocatalytic process using transition-metal ion reducing agents. Very thick coatings may also be economically applied to a variety of substrates by the process of roll bonding . 

Torkar et al. [702,706—708] identified nucleation as an autocatalytic process at the (hk0) planes of hexagonal platelets of NaN3. The decelera-tory reaction fitted the first-order equation [eqn. (15)]. Values of E tended to be irreproducible for the pure salt E was about 180 kJ mole 1 but this was reduced to about half by doping. This influence of an additive and the observed similarities in magnitudes of E for decomposition and for diffusion were interpreted as indicating that growth of nuclei was controlled by a diffusion process. 

This behavior is consistent with an autocatalytic process, whereby the liberated carboxylic end groups catalyze the cleavage of additional ester groups (Eq. 3) ... 

Statistical evaluation shows that the experimental data are better correlated with Eqs. 7 and 8, i. e., an autocatalytic process, than with Eq. 9 although it is not possible to rule out the superposition of both mechanisms. 

It is noteworthy that, as early as 1929, Shibata and Tsuchida reported a kinetic resolution of rac-3,4-dihydroxyphenylalanine by selective oxidation of one enantiomer using a chiral cobalt complex [Co(en)3NH3Cl]Br2 as a catalyst [46,47]. Figure 12 shows a highly enantioselective addition of diisopropy-Izinc to 2-(ferf-butylethynyl)pyrimidine-5-carbaldehyde via an autocatalytic process in the presence of a chiral octahedral cobalt complex with ethylenedi-... 

Explosions can also be seen as the result of an autocatalytic process ... 

No published work exists on the kinetics of acid permanganate oxidation of hydroxy-acids although Pink and Stewart have noted that benzilic acid is oxidised by acid permanganate to benzophenone in an autocatalytic process with knjo/ Dio of unity. 

Lipid hydroperoxides are either formed in an autocatalytic process initiated by hydroxyl radicals or they are formed photochemically. Lipid hydroperoxides, known as the primary lipid oxidation products, are tasteless and odourless, but may be cleaved into the so-called secondary lipid oxidation products by heat or by metal ion catalysis. This transformation of hydroperoxides to secondary lipid oxidation products can thus be seen during chill storage of pork (Nielsen et al, 1997). The secondary lipid oxidation products, like hexanal from linoleic acid, are volatile and provide precooked meats, dried milk products and used frying oil with characteristic off-flavours (Shahidi and Pegg, 1994). They may further react with proteins forming fluorescent protein derivatives derived from initially formed Schiff bases (Tappel, 1956). 

The various types of heterogeneous reactions are shown in Table 3.3. They are broadly grouped as solid-gas, solid-liquid, solid-solid, liquid-gas, and liquid-liquid reactions. The different types included in each group are also shown in the compilation. Some representative processes have been indicated as examples. It may be pointed out that in the group of solid-liquid reactions a specific mention of what is known as autocatalytic reactions has not been made. The autocatalytic processes occur when the liquid product reacts further with the solid undergoing reaction. The dissolution of copper in dilute sulfuric acid (or aqueous ammonia) in the presence of oxygen may be cited as an example ... 

Quite complex kinetic behavior has been identified on some surfaces. For instance, on Ir(100), the TPD data from NO-saturated surfaces display two N2 desorption peaks, one at 346 K from the decomposition of bridge-bonded NO, and a second at 475 K from the decomposition of atop-bonded NO molecules [13], Interestingly, the first feature is quite narrow, indicating an autocatalytic process for which the parallel formation of N20 appears to be the crucial step. An additional complication arises from the fact that this Ir(100) surface undergoes a (1x5) reconstruction, and that NO adsorbed on the metastable unreconstructed (lxl) phase leads to N2 desorption at lower temperatures. In another example, on the reconstructed hexagonal Pt(100) surface, when a mixed NO + CO adsorbed layer is heated, a so-called surface explosion is observed where the reaction products (N2, C02 and N20) desorb simultaneously in the form of sharp peaks with half-widths of only 7 to 20 K. The shape of the TPD spectra suggests again an autocatalytic mechanism [14],... 

Earlier studies showed that reactions of sugars with ammonia lead to small molecules such as amines or organic acids. A. L. Weber has reported important autocatalytic processes occurring when trioses are allowed to react with ammonia under anaerobic conditions, such reactions provide products which are autocatalyt-ically active. Their autocatalytic activity was determined directly by investigating their effect on an identical triose-ammonia reaction. Both an increase in the triose degradation rate and an increased rate of synthesis of pyruvate, the dehydration product of the triose, were observed. Such processes may have been of importance for prebiotic chemistry occurring on the primeval Earth (Weber, 2007). 

In accordance with the autocatalytic process, matrices are again formed. It is surprising that the autocatalysis decreases when only 1 of the 15 building blocks of the peptide has the opposite handedness, e.g., when the N-peptide fragment contains one D-amino acid as well as the 14 L-amino acids. These experimental results show that such a system is able to form homochiral products via self-replication. It can be assumed that similar mechanisms influenced the origin of homochirality on Earth (Saghatelian et al., 2001 Siegel, 2001). 

Kondepudi (2000) from Wake Forest University in Winston-Salem obtained some results which appeared sensational when crystallising a mixture of dissolved NaClC>3 and NaBrC>3 with stirring, an excess of one enantiomeric form (up to 80%) was formed. Prediction of which enantiomer will be in excess is not possible it is a matter of chance. The process is probably due to chiral autocatalytic processes similar phenomena were observed in melts. 

Poly(A) synthesis also occurred in the second system, but the product remained within the vesicles. Walde also determined the increase of the vesicle concentration, which corresponds to that expected for an autocatalytic process. In this experiment, the enzyme PNPase is first captured by the vesicle envelope, and in the second step, ADP and oleic anhydride are added the anhydride is hydrolysed to the acid. ADP passes through the vesicle double layer and is polymerized in the interior of the vesicle by PNPase to give poly(A). Hydrolysis of the anhydride causes a constant additional delivery of vesicle-forming material, so that the amount of vesicle present increases during the poly(A) synthesis. These experiments demonstrated a model for a minimal cell. Autocatalytically synthesised giant vesicles could be prepared under similar conditions and observed under a microscope (Wik et al., 1995). 

The oxidation of sulfides is a complex process involving a number of conversions [32,46], Disulfides are oxidized by hydroperoxide via the intermediate thiosulfinate RSSOR, which is very reactive to ROOH [32,52-54], The interaction of ROOH with phenolsulfoxides also gives rise to intermediate catalytic compounds, as a result of which the reaction proceeds as an autocatalytic process [46,55], The rate of the catalytic decomposition of R OOH is described by one of the following equations ... 

The first indication that A-acyloxy-A-alkoxyamidcs reacted by an acid-catalysed process came from preliminary H NMR investigations in a homogeneous D20/ CD3CN mixture, which indicated that A-acetoxy-A-butoxybenzamide 25c reacted slowly in aqueous acetonitrile by an autocatalytic process according to Scheme 4 (.k is the unimolecular or pseudo unimolecular rate constant, K the dissociation constant of acetic acid and K the pre-equilibrium constant for protonation of 25c).38... 

Dissolved iron(III) is (i) an intermediate of the oxidative hydrolysis of Fe(II), and (ii) results from the thermal non-reductive dissolution of iron(III)(hydr)oxides, a reaction that is catalyzed by iron(II) as discussed in Chapter 9. Hence, iron(II) formation in the photic zone may occur as an autocatalytic process (see Chapter 10.4). This is also true for the oxidation of iron(II). As has been discussed in Chapter 9.4, the oxidation of iron(II) by oxygen is greatly enhanced if the ferrous iron is adsorbed at a mineral (or biological) surface. Since mineral surfaces are formed via the oxidative hydrolysis of Fe(II), this reaction proceeds as an autocatalytic process (Sung and Morgan, 1980). Both the rate of photochemical iron(II) formation and the rate of oxidation of iron(II) are strongly pH-dependent the latter increases with... 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

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