Chemical activation reactions, consecutive

It should be borne in mind that each activated monomer and polymer should only react with nonactivated monomer (addition polymerization) in both of these chemical examples. Reaction between an activated monomer and another activated monomer or a polymer (polycondensation) should not occur. With cyclophane, the mathematical treatment of the consecutive equilibria yields different expressions to those given in Table 16-1, since the /7-cyclophane, as initial monomer unit, yields two activated monomer molecules, and each reaction with activated monomer and its successive products yields only species with uneven numbers of structural elements. In addition, the polymerization of p-cyclophane is no longer a living polymerization when the degrees of polymerization are low, since, in this case, monomolecular (that is, intramolecular) termination reactions leading to the formation of inactive rings can occur. 

Hydrogenations involving consecutive reactions are common in the organic process industry and even in the hydrogenation of fats. In the fine chemicals industry we have examples of acetylenic (triple) bonds to be selectively converted to olefinic (double) bonds. Lange et al. (1998) have shown, for the comversion of the model substance 2-hexyne into cis-2-hexene, how catalytically active microporous thin-film membranes can accomplish 100% selectivity. This unusual selectivity is attributed to avoidance of backmixing. 

In all cases an enzymic process is composed of several consecutive reaction steps. Even the simplest Michaelis-Menten type rapid equihbrium mechanism involves two steps, the binding of the substrate, S, to a specific site in the active centre, and the chemical transformation of the bound S to product P, during which the enzyme becomes free again. The Michaelis constant characterizes the affinity of the enzyme to its... 

Highly active intermediate substance, generated in the primary reaction, is always consumed for conjugated reaction product synthesis, and the higher its induction effect on the secondary reaction, the lower its consumption in the primary reaction. It follows that if products of both reactions are of great importance for consecutive biochemical reactions, in this case of chemical conjugation, in principle, the maximum effect is improbable. This gives rise to the question of whether any additional mechanism may be used to provide for effectiveness in both directions. 

The network operates through a series of enzyme-catalyzed reactions that constitute the metabolism. Each of the consecutive steps in a metabolic pathway brings about a specific chemical change, usually the removal, transfer, or addition of a particular atom or functional group. The precursor is converted into a product through a series of metabolic intermediates called metabolites. The term intermediary metabolism is often applied to the combined activities of all the metabolic pathways that interconvert precursors, metabolites, and products of low molecular weight. 

Formation of Catalytic Ensembles. Regulatory Capacity. Formation of ordered catalytic ensembles can greatly facilitate the accessibility of substrates in consecutive chemical and enzyme reactions. Capacity of catalysts to be or not to be active in proper space and proper time is of great importance especially in biological cells. A catalyst s capacity for switching activity in the appropriate space and time is very important, especially in biological cells. 

The more active a catalyst is, the more difficult it is to obtain benefits, due to an increased influence of transport phenomena on the conversion rate for fast chemical reactions. For some types of chemical reactions, such as consecutive reactions with the intermediate as the desired product, an increase of catalytic activity may lead to undesired effects if transport phenomena inside and outside the catalyst pellet play a role. 

Rate vs. temperature curves similar to that shown in Fig. 14 for the A surfaces have been obtained for the dissolution of germanium in HF + H2O2 + H2O etchants (4). These curves were attributed to two consecutive reactions taking place on the surface. Each reaction was assumed to take place on a distinct fraction of the surface. Such a mechanism is unlikely for an etching process. In addition, one of the activation energy values associated with two segments of the Ge rate curves is well below 10 kcal /mole, indicative of diffusion control, and the other is well above 10 kcal /mole, indicative of activation control. It appears thus more likely that the curve of Fig. 14 and those reported for Ge (4) represent a transition from diffusion to activation-controlled dissolution rather than two consecutive chemical reactions. 

Ascorbic acid is quoted as an example of a first-order consecutive reaction. This substance is oxidized electrochemically to an unstable electro-active intermediate, transformed by a fast chemical reaction with rate constant k into the electro-inactive dehydroascorbic acid. It is assumed that the electro-inactive form is hydrated. The halfwave potential of the anodic wave is independent of ascorbic acid... 

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