Active center rate-determining

It is very useful to know whether this situation prevails, because such knowledge will gready facilitate the kinetic treatment of chain reactions. In deed, for chain reactions with long chains, the steady-state concentration of active centers is determined by the relation f< = where n represents the rate of initiation and r< the rate of termination. Termination includes the destruction of all types of active centers present in the system. Clearly, the kinetic treatment will be simplified if only one termination step needs to be taken into account because it is the one that expresses the rate of destruction of the most abundant active centers. 

Each act of proton abstraction from the a carbon converts a chiral molecule to an achi ral enol or enolate ion The sp hybridized carbon that is the chirality center m the start mg ketone becomes sp hybridized m the enol or enolate Careful kinetic studies have established that the rate of loss of optical activity of sec butyl phenyl ketone is equal to Its rate of hydrogen-deuterium exchange its rate of brommation and its rate of lodma tion In each case the rate determining step is conversion of the starting ketone to the enol or enolate anion... 

In the second method, which can be applied to compounds with an optically active center near the potentially tautomeric portion of the molecule, the effect of the isomerization on the optical activity is measured. In favorable cases both the rate of racemization and the equilibrium position can be determined. This method has been used extensively to study the isomerization of sugars and their derivatives (cf. reference 75). It has not been used much to study heteroaromatic compounds, although the very fact that certain compounds have been obtained optically active determines their tautomeric form. For example, oxazol-5-ones have thus been shown to exist in the CH form (see Volume 2, Section II,D,1, of article IV by Katritzky and Lagowski). 

It is necessary to note the limitation of the approach to the study of the polymerization mechanism, based on a formal comparison of the catalytic activity with the average oxidation degree of transition metal ions in the catalyst. The change of the activity induced by some factor (the catalyst composition, the method of catalyst treatment, etc.) was often assumed to be determined only by the change of the number of active centers. Meanwhile, the activity (A) of the heterogeneous polymerization catalyst depends not only on the surface concentration of the propagation centers (N), but also on the specific activity of one center (propagation rate constant, Kp) and on the effective catalyst surface (Sen) as well ... 

It is evident [see Eq. (5), Section II[] that for catalysts of the same or similar composition the number of active centers determined must be consistent with the catalytic activity it can be expected that only in the case of highly active supported catalysts a considerable part of the surface transition metal ions will act as propagation centers. However, the results published by different authors for chromium oxide catalysts are hardly comparable, as the polymerization parameters as a rule were very different, and the absolute polymerization rate was not reported. 

Several determinations of the number of propagation centers by the quenching technique have been carried out (98, 111). As a quenching agent methanol, labeled C14 in the alkoxyl group, proved to be suitable in this case. The number of active centers determined by this technique at relatively low polymerization rates (up to 5 X 102 g C2H4/mmole Cr hr at 75° and about 16 kg/cm2) (98, 111, 168) in catalysts on silica was about... 

Heterogeneous recombination of active particles and their interaction with molecules of the adlayer are simplest processes of this type. The rates of such reactions as functions of surface coverage by the specified reagents are fully determined by the rate of their surface diffusion towards active centers. In a number of cases, the rate of lateral diffusion is determined not only by the type of diffusing particle, but also (sometimes, predominantly) by the composition and state of the solid substrate surface. Taking into account the role played by the composi-... 

The reaction kinetics In the case of HMDS suggests a two stage mechanism for the major dichloride consuming reaction. First a slow Na surface dependent build up of long lived active centers then a propagation step In which the Na surface is not rate determining. The reaction which occurs on the surface must be fast. 

This study supports rate-determining H-OH bond breaking, which constrasts with previous reports that identified vinylidene isomerization as the key step in catalytic alkyne activation. The results indicate an enzyme-like mechanism is operative involving cooperative substrate activation by a metal center and proximal hydrogen bond donor/acceptors. In the future we will apply these principles to the activation of additional species. 

The difference in the effect of various endo-D-galacturonanases on oligomeric substrates [in particular, the difference in the rate of degradation of tri(D-galactosiduronic acid) and in the action pattern toward the tetrasaccharide] indicates, however, that it is not the substrate, but rather, the properties of the enzyme (in particular, the character of its active center) that constitute the determining factor. 

Figure 12 shows the reaction profile for the hydrosilylation process involving the most stable fi3-sily 1-ally 1 complex, 10a-anti, calculated with model B. Examination of the reaction profile suggests that the rate determining step of the catalytic cycle is the reductive elimination. More specifically, the transfer of the silyl moiety to the (J-carbon of the styrene. Since recoordination of the pyrazole ligand occurs in this step, it is possible that enhancement of this ligands ability to recombined with the Pd center may lead to improved activities. 

From the above it will be clear that the polymer chain carrying catalytic centers has to play an active role during each catalytic cycle. Therefore, we prefer to speak of macromolecular catalysis rather than of polymer catalysis, the more so, as we omitted crosslinked carriers from our studies in order to prevent that diffusion of reactants and products would become rate-determining. Consequently, the practical combination of simple separability and really macromolecular catalysis should be realized by... 

In protein-protein reactions, the donor-acceptor distance is determined by the structure of the reacting proteins, and the way(s) in which they bind and interact. For example, it is generally believed that cytochrome c binds to its reaction partners at or near the exposed heme edge, in order to minimize the reactant distance and thereby maximize the rate. The redox active centers of most proteins are sufficiently buried that the large protein imposed distances provide low intrinsic reactivity for the proteins with respect to exogenous... 

The ester of the phosphorous acid or organophos-phorsus inhibitors of the acetylcholine esterase phos-phorylate serine in the active center of the enzyme. The phosphorylated enzyme is extremely stable, resulting in an irreversible inhibition. The duration of action of this compounds is determined by the rate of enzyme synthesis de novo. 

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