Kinetics of Rapid Reactions

Zhdanov VP, Kasemo B. Kinetics of rapid reactions on nanometer catalyst particles. Physical Review B 1997 55(7) 4105-8. 

An important point about kinetics of cyclic reactions is tliat if an overall reaction proceeds via a sequence of elementary steps in a cycle (e.g., figure C2.7.2), some of tliese steps may be equilibrium limited so tliat tliey can proceed at most to only minute conversions. Nevertlieless, if a step subsequent to one tliat is so limited is characterized by a large enough rate constant, tlien tire equilibrium-limited step may still be fast enough for tire overall cycle to proceed rapidly. Thus, tire step following an equilibrium-limited step in tire cycle pulls tire cycle along—it drains tire intennediate tliat can fonn in only a low concentration because of an equilibrium limitation and allows tire overall reaction (tire cycle) to proceed rapidly. A good catalyst accelerates tire steps tliat most need a boost. 

Thiopyrones and selenopyrones can be alkylated more readily than pyrones. Thus 2,6-dimethyl-4/f-pyran-4-thionc (4,6-dimethyl-4-thiopyrone) (23, Y = S) reacts rapidly with methyl iodide yielding a 4-methylmercaptopyrylium iodide (24, Y = S, R = Me, X = I). Many alkylating agents were investigated by King et al. The kinetics of the reaction between 2,6-dimethyl-4-thiopyrone and substituted phenacyl bromides was found to be described by the Hammett... 

Meluch et al.10 reported that high-pressure steam hydrolyzes flexible polyurethane foams rapidly at temperatures of 232-316°C. The diamines are distilled and extracted from the steam and the polyols are isolated from the hydrolysis residue. Good results were obtained by using reclaimed polyol in flexible-foam recipes at file 5% level. Mahoney et al.53 reported the reaction of polyurethane foams with superheated water at 200°C for 15 min to form toluene diamines and polypropylene oxide. Gerlock et al.54 studied the mechanism and kinetics of the reaction... 

The kinetics of enzyme reactions were first studied by the German chemists Leonor Michaelis and Maud Menten in the early part of the twentieth century. They found that, when the concentration of substrate is low, the rate of an enzyme-catalyzed reaction increases with the concentration of the substrate, as shown in the plot in Fig. 13.41. However, when the concentration of substrate is high, the reaction rate depends only on the concentration of the enzyme. In the Michaelis-Menten mechanism of enzyme reaction, the enzyme, E, and substrate, S, reach a rapid preequilibrium with the bound enzyme-substrate complex, ES ... 

Kennedy and co-workers10 studied the kinetics of the reaction between Me3Al and t-butyl halides using methyl halide solvents as a model for initiation and termination in cationic polymerization. Neopentane was generated rapidly, without side reactions and rates were determined by NMR spectroscopy. The major conclusions were ... 

Kinetics of the reactions with all the oxidants have been reported (Table 16). The usual product is formic acid, which is the first molecule formed resistant to very rapid secondary oxidation. Six equivalents of Ce(IV) are destroyed in oxidising one molecule of substrate to one of HC02H °, viz. 

The formation of nitrosamines in aprotic solvents has applicability to many practical lipophilic systems including foods (particularly bacon), cigarette smoke, cosmetics, and some drugs. The very rapid kinetics of nitrosation reactions in lipid solution indicates that the lipid phase of emulsions or analogous multiphase systems can act as "catalyst" to facilitate nitrosation reactions that may be far slower in purely aqueous media (41, 53, 54). This is apparently true in some cosmetic emulsion systems and may have important applicability to nitrosation reactions in vivo, particularly in the GI tract. In these multiphase systems, the pH of the aqueous phase may be poor for nitrosation in aqueous media (e.g., neutral or alkaline pH) because of the very small concentration of HONO or that can exist at these pH ranges. 

The kinetics of the reaction is first order in both nitrile complex and azide.909 NaN3 reacts with [Co(tetren)(NCMe)]3+ at pH 5.7 to give the 5-methyltetrazolato complex [Co(tetren)(N4CMe)]2+. The reaction is biphasic, involving the initial rapid formation of the AM-bonded tetrazole followed by the slow linkage isomerization to the N2-bonded complex.907... 

The exchange current density, /0- From equations (1.28) and (1,29) it is clear that /Q is a direct measure of the kinetics of the reaction at the equilibrium potential Er. A high exchange current density indicates a facile electrochemical reaction that will rapidly become limited by transport at potentials away from E°. 

The first class, discussed in detail in Chapter 6, was reaction between a fluid and the minerals it contacts. The kinetics of the reactions by which minerals dissolve and precipitate was the subject of the preceding chapter (Chapter 16). The second class of reactions commonly observed to be in disequilibrium in natural waters, as discussed in Chapter 7, is redox reactions. The subject of this chapter is modeling the rates at which redox reactions proceed within the aqueous solution, or when catalyzed on a mineral surface or by the action of an enzyme. In the following chapter (Chapter 18), we consider the related question of how rapidly redox reactions proceed when catalyzed in the geosphere by the action of microbial life. 

An autocatalytic reaction is one promoted by its own reaction products. A good example in geochemistry is the oxidation and precipitation of dissolved Mn11 by C>2(aq). The reaction is slow in solution, but is catalyzed by the precipitated surface and so proceeds increasingly rapidly as the oxidation product accumulates. Morgan (1967) studied in the laboratory the kinetics of this reaction at 25 °C and pH > 9. 

Commercially useful materials require that the rate of the combined reaction is rapid. If the darkening takes place too slowly, or if the subsequent fading of the color is too slow, the materials will not be useful. The presence of the copper halide is essential in ensuring that the kinetics of the reaction are appropriate and that the process is reversible. 

A disadvantage of 1ST measurements is that the experiments take time (days to weeks). Also, several experiments at different temperatures are necessary to get information with respect to the kinetics of the exothermic decomposition. Finally, it may take several hours to reach equilibrium after inserting a sample due to the time-lag of the system. Thus the recorded heat effect may be inaccurate. This is a particular disadvantage in the case of rapid reactions. 

The only kinetic data that permits direct comparison of the rates of Reactions (36) and (37) is that of Baur, Gibbs and Wadsworth (3). After a very brief initial period of rapid reaction, Reactions (36) and (37) are followed. Extrapolation of the data of Baur, Gibbs and Wadsworth to long times and transformation of variables indicate that after the initial rapid reaction, chalcopyrite and oxygen react in a constant ratio, i.e., the fraction of the total copper dissolved due to O2 is... 

This review focuses on the kinetics of reactions of the silicon, germanium, and tin hydrides with radicals. In the past two decades, progress in determining the absolute kinetics of radical reactions in general has been rapid. The quantitation of kinetics of radical reactions involving the Group 14 metal hydrides in condensed phase has been particularly noteworthy, progressing from a few absolute rate constants available before 1980 to a considerable body of data we summarize here. 

The important effect of increasing pressure on the kinetics of chemical reactions has been noted since the hrst chemical experiments at high pressure. The simplest expectation derives from the observation that in liquids the viscosity rapidly increases with pressure. As a result, in strongly compressed liquids, and hnally in glasses, diffusion-controlled processes can be retarded. In contrast, however, other reaction pathways can be substantially accelerated. In general, the evolution of a reaction at high pressure can be heavily controlled by kinetic aspects, and these deeply involve intermolecular effects. 

With the availability of perturbation techniques for measuring the rates of rapid reactions (Sec. 3.4), the subject of relaxation kinetics — rates of reaction near to chemical equilibrium — has become important in the study of chemical reactions.Briefly, a chemical system at equilibrium is perturbed, for example, by a change in the temperature of the solution. The rate at which the new equilibrium position is attained is a measure of the values of the rate constants linking the equilibrium (or equilibria in a multistep process) and is controlled by these values. 

Monitoring of events following perturbations can be achieved in much shorter times by photolysis. A variety of monitoring techniques have been linked to both methods (Table 3.7). It is valuable to obtain kinetic data by more than one method, when possible. The measurement of spin-change rates have, for example, been carried out by a variety of rapid-reaction techniques, including temperature-jump, ultrasonics and laser photolysis with consistent results (Sec. 7.3). 

As the above discussion indicates, assigning mechanisms to simple anation reactions of transition metal complexes is not simple. The situation becomes even more difficult for a complex enzyme system containing a metal cofactor at an active site. Methods developed to study the kinetics of enzymatic reactions according to the Michaelis-Menten model will be discussed in Section 2.2.4. Since enzyme-catalyzed reactions are usually very fast, experimentahsts have developed rapid kinetic techniques to study them. Techniques used by bioinorganic chemists to study reaction rates will be further detailed in Section 3.7.2.1 and 3.72.2. 

The reaction of [Rh(C(0)Me)(C0)2l3] with various nucleophiles has been studied by IR [17]. Acetate ion gave immediate formation of AC2O. Amines gave amides, the more nucleophilic dialkylamines reacting more rapidly than methyl anilines. The kinetics of these reactions were interpreted as consistent with two pathways, one being elimination of Acl from [Rh(C(0)Me)(CO)l3] and the other direct nucleophilic attack at the acyl (Eq. (22)). 

The electrochemical oxidation of NO around 1.0 V (cf. Table 1) has recently been used to devise NO-selective amperometric microprobe electrodes to detect its release in biological tissues [20, 21]. In aqueous solution, NO is quickly air-oxidized to NO2. Surprisingly, the kinetics of this reaction have not been investigated. In the gas phase at room temperature, NO is rapidly oxidized by oxygen (AG° = — 70.54 kJ/mol) in a third-order mechanism [8] ... 

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