Competition kinetics, fast reactions

For very fast reactions, the competition between geminate recombmation of a pair of initially fomied reactants and its escape from the connnon solvent cage is an important phenomenon in condensed-phase kinetics that has received considerable attention botli theoretically and experimentally. An extremely well studied example is the... 

We start with the case where the initial electron transfer reaction is fast enough not to interfere kinetically in the electrochemical response.1 Under these conditions, the follow-up reaction is the only possible rate-limiting factor other than diffusion. The electrochemical response is a function of two parameters, the first-order (or pseudo-first-order) equilibrium constant, K, and a dimensionless kinetic parameter, 2, that measures the competition between chemical reaction and diffusion. In cyclic voltammetry,... 

How does a reaction take place when it proceeds faster than mixing What kinds of problems are encountered when extremely fast reactions are conducted by mixing two reaction components It is known that when the reaction is faster than mixing, product selectivity of the reaction is often not determined by the kinetics, but by the manner of mixing. Of course, if only one product can be produced under any conditions, there is no problem of product selectivity. However, if there is a possibility that two or more products are produced by competing reactions, we have to consider the product selectivity. There are, in principle, two cases for competing reactions, i.e. competitive parallel... 

Rate constants of fast reactions may have to be considered in kinetic models when the product distribution is important. Free radicals, for example, may be consumed by several competitive reactions in parallel the relative magnitudes of the rate constants will determine the distribution of various products formed. 

Both types of sources are useful for continuous radiolysis or even time-resolved studies on the several-minute time scale or longer, such studies are limited by the time required to move the sample in and out of the source. Competition kinetics is often used to obtain kinetic information about reactions too fast to measure directly by time-resolved methods (2). Continuous gamma radiolysis is also a convenient method of generating radiolysis products for identification and chemical analysis (instead of using pulsed sources, where the average dose rate is much lower). 

The transient character of unstable species is intrinsically because of at least one fast reaction which they undergo as soon as they are formed (for example coalescence reaction in the case of atoms and clusters). This reaction therefore induces competition with any redox reaction which could be regarded as determining the redox potential of a transient entity. In particular, the competition does not enable the establishment of a reversible equilibrium of electron transfer with another suitable system. Thus, the redox potential of short-hved species must be evaluated from kinetic methods - the pulse technique enables us to observe whether or not electron transfer involving the transient species and a series of donor/acceptor couples, used as monitors, is elfective, and thus to establish by a bracketing method the value of the imknown redox potential. Only elementary monoelectronic transfers are considered. Thus, note that one of the forms of the reference couple, reduced or oxidized, can also be a transient radical. 

Flow systems, both continuous and discrete, are used in kinetic-based determinations for monitoring fast reactions mainly. To this end (1) the dead time in the mixing system should be several orders of magnitude lower than the half-life of the reaction concerned and (2) nearly the whole kinetic curve must be recorded in order to implement reaction rate-based determinations and perform fundamental kinetic studies (e.g., the determination of reaction orders and rate constants). The advent of stopped-flow mixing and the continuous-addition-of-reagent technique has made noncatalytic reactions competitive with equilibrium methods in practical terms. 

Addition of materials capable of reacting with electrons reduces the lifetime of enabling kinetics of its fast reactions to be determined. The powerful oxidant OH (the hydroxyl radical) has only a weak absorption in the ultraviolet, and its reactivity is best measured by a competition method based on its very fast oxidation of thiocyanate ion CNS to yield the intensely absorbing (CNS) ion (Xmax 472 nm). Addition of a second substrate X will provide competition for OH, and the intensity of the absorption of (CNS)2 will be systematically reduced as [X] is increased, enabling a rate constant to be derived. 

Haight et al. ° have published a detailed account of the kinetics and stoichiometry of the oxidation of buffered bisulphite ion by chromic acid. The reaction is fast and its study requires a rapid mixing technique. The stoichiometry varies from a Cr(VI)/S(IV) molar ratio of 1 2 to 2 3 as the initial concentrations are changed in the range 0.12 < [Cr(VI)]/[S(IV)] < 1.4 and this was explained in terms of competition between two overall reactions... 

The crucial aspect is thus to determine the fate of the ( CHO), species. Possible mechanisms for its oxidative removal are schematically shown in Fig. 9. From this scheme, it appears that the desorption of the formyl species can follow different pathways through competitive reactions. This schematic illustrates the main problems and challenges in improving the kinetics of the electrooxidation of methanol. On a pure platinum surface, step (21) is spontaneously favored, since the formation of adsorbed CO is a fast process, even at low potentials. Thus, the coverage... 

The kinetics of the coupling mechanism include a number of sometimes very fast and competitive side reactions. The following steps, for instance, proceed simultaneously as a separately prepared diazonium salt solution is combined with an initially dissolved coupling component ... 

In the case of a solution follow-up reaction (47) so fast that the thickness of the reaction layer is of the same order of magnitude as the double layer, the competition between the electron transfer step (46) and the follow-up reaction (47) can be described in a similar way with a somewhat different effect of the double layer on the electron-transfer kinetics (Saveant, 1980b, 1983). 

If the electron donor is so efficient a reductant as to react with the acceptor with a rate constant equal to the diffusion limit, then not much information can be derived from the experiments, except the knowledge of the diffusion limit itself. The opposite situation, where an endergonic electron transfer is followed by a fast bond-breaking step, is of more interest. There is then competition between the follow-up reaction and the backward electron-transfer step. If the latter is faster than the former, kinetic control is by the bond-breaking step, the electron-transfer step acting as a preequilibrium. Under these conditions, there is no difficulty to conclude from the adherence to the rate law (61) that the overall reaction is stepwise rather than concerted, since, in the concerted case, the rate law would be (62). If, in... 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

There are several examples of fast decomposition reactions of the a-adducts derived from 5-membered rings. These reactions can be viewed as resulting from effective kinetic competition of reaction paths other than return to the reactants. In all ascertained cases the products of decomposition result from ring opening, which presumably occurs subsequent to a-adduct formation. Thus 2-nitrothiophene reacts with aliphatic secondary amines to yield bis-(4-dialkylamino-l-nitrobuta-l,3-dienyl) disulfides 156. This compound is suggested to be the end product of a sequence originating from 153, whose formation is not as yet established, according to Scheme 10.187... 

It is considered in a first approach that both P° and P ° radicals are equally reactive in terminations or oxidations, so that this reaction - as hydrogen abstraction - does not modify the whole kinetics. The oxygen addition to radicals is very fast, k2 10s 109 lmoH1 s-1, so that there is practically no competitive process when 02 is in sufficient concentration to scavenge all the P° radicals. 

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