Relative rates-of competing reactions

The outcome of a chemical reaction can be determined by relative rates of competing reactions and by relative stabilities of the final products. 

Scheme 39.3 Product control by affecting the relative rates of competing reaction pathways in scC02. Path A is preferred in scC02, whereas path B prevails in conventional solvents.

Thus the reaction with larger activation energy is the more temperature-sensitive of the two reactions. This finding leads to the following general rule on the influence of temperature on the relative rates of competing reactions ... 

Since product distributions depend on the relative rates of competing reactions, effects of temperature on products depend on differences in activation energies, AE. For each pair of competing reactions of the tert-Bu02 radicals, the following AE values (in kilocalories per mole) and qualitative effects of increasing temperature are estimated. Some of these values were considered under Liquid-Phase Oxidations. ... 

Two other important sol-gel parameters are temperature and solvent. Both hot and cold plates are commercially available and can be used to increase and decrease the reaction rates, respectively. Varying the temperature is most effective when it can alter the relative rates of competing reactions. Solvent can change the nature of an alkoxide through solvent exchange or affect the condensation reaction directly. It is also possible to prepare a gel without a solvent as long as another means, such as ultrasound irradiation [10] (see Section A.8.6), is used to homogenize an otherwise immiscible alkoxide/water mixture. 

A considerable degree of order can be found in this area if full and proper use is made of thermodynamic data in the form of oxidation potentials and equilibrium constants and if the relative rates of competing reactions are also considered. We shall consider first the basic thermodynamic data which are given in Table 13-5. From these, all necessary potentials and equilibrium constants can be derived by use of basic thermodynamic relationships. 

Validation of the Mechanism. The process of matching the predictions of the mechanism to experimental smog chamber data is termed validation of the mechanism. The first step in a validation procedure is to establish values for the two major classes of parameters that appear in the mechanism—the reaction rate constants and the stoichiometric coeflBcients. Base values of the rate constants can be estimated from the chemical literature. However, with the sacrifice of chemical detail present in the new, simplified mechanism is a loss in the ability to associate the rate constant values with particular reactions. Therefore, the rate constants in the simplified mechanism are more a quantitative assessment of the relative rates of competing reactions than a reflection of the exact values for particular reactions. Base values for the parameters that appear in the kinetic mechanism are thus established on the basis of published rate constants. However, we must expect that final validation values will consist of those values which produce the best fit of the mechanism to actual smog chamber data. A recent summary of rate constants for specific hydrocarbon systems was made by Johnston et al. 40) from which rate constants for the Reactions in Table I can be estimated for a number of hydrocarbons. 

Thus orientation is determined by the relative rates of competing reactions. In this case we are comparing the rate of abstraction of primary hydrogens with the rate of abstraction of secondary hydrogens. What are the factors that determine the rates of these two reactions, and in which of these factors may the two reac=. tions differ ... 

In this discussion, we have assumed that the relative rates of competing reactions depend on relative populations of the conformations of the reactants. This assumption is correct here, if, as seems likely, reaction of the free radicals with chlorine is easier and faster thar. the rotation that intexonverts conformations. 

Except for ethylene and propene, the rate data are calculated from relative rates of competing reactions. 

The comeretone of this framework has been, as always, the premise on which the science of organic chemistry rests that chemical behavior is determined by molecular structure. Chemical behavior—what happens, where in a molecule it happens, even whether it happens—comes down to a matter of relative rates of competing reactions. By and large, molecules tend to do what is easiest for them rate depends chiefly on the energy difference between the reactants and the transition state. We approach the matter of reactivity, then, by examining—mentally and, by means of models, physically—the structures involved. But what is meant by molecular structure is constantly expanding, and our interpretation of chemical behavior must reflect this. 

The sequence of chemical reactions leading to oxidative degradation and ozone formation in the atmosphere is often determined by the relative rates of competing reaction pathways at several critical points in the degradation. The protocol adopted was therefore to establish these critical points and emphasis was then placed on provision of data allowing the relative importance of these competing processes to be quantitatively defined for a range of VOCs. These critical points were found to lie for example in the reactions of RO2 and RO radicals. 

The reactions of hydrogen bromide with 1,3-butadiene serve as a striking illustration of the way that the outcome of a chemical reaction can be determined, in one instance, by relative rates of competing reactions and, in another, by the relative stabilities of the final products. At the lower temperature, the relative amounts of the products of the addition are determined by the relative rates at which the two additions occur 1,2-addition occurs faster so the 1,2-addition product is the major product. At the higher temperature, the relative amounts of the products are determined by the position of an equilibrium. The... 

Oxidation to phenols, dihydrodiols, catechols, quinones, and acids are the principal reactions of most hydrocarbons. Catechols probably develop by hydrolysis of an epoxide to a dihydrodiol followed by dehydrogenation (see Fig. 4). The hydroxylases operate independently of the glutathione S-transferaaes. Evidently the relative amount of hydroxylation versus conjugation with glutathione for a particular hydrocarbon depends on the relative rates of competing reactions. 

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