Method of competitive reactions

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

Determination of Relative Reactivity by the Method of Competitive Reactions... 

The above mentioned studies were in most cases performed with the aim of obtaining relative reactivities or relative adsorption coefficients from competitive data, sometimes also from the combination of these with the data obtained for single reactions. In our investigation of reesterification (97,98), however, a separate analysis of rate data on several reactions provided us with absolute values of rate constants and adsorption coefficients (Table VI). This enabled us to compare the relative reactivities evaluated by means of separately obtained constants with the relative reactivities measured by the method of competitive reactions. The latter were obtained both from integral data by means of the known relation... 

We have further attempted to suggest a procedure which would make use of the advantages of the method of competitive reactions, i.e. its simplicity and little time demand, and at the same time would yield separately the absolute values of rate constants and adsorption coefficients also for reactions with a more complicated kinetics. Using the values of relative reactivities S from the method of competitive reactions, the adsorption coefficients, for example, of the alcohols (Kb) in the reesterification reaction described by Eq. (26) can be evaluated from the relation... 

The relative reactivities obtained by the method of competitive reactions corresponded to the values of the separately obtained rate and adsorption constants. The reactivities obtained by the competitive method differ, of course, from the ratio of the rates of the separately studied single reactions this difference increases with the difference in the values of the adsorption coefficients of competing substances. 

The method of competitive reactions was used (38) to measure the relative rates of addition of Et3 ClnSiH (n = 0-3) to 1-heptene with Co2(CO)8 and RhCl(PPh3)3 as catalysts. With the rhodium catalyst at 80°C no hydrosilation took place at 120°C only that silane with a greater number of Si—Cl bonds yielded products. 

If the relative reactivity value is obtained using the method of competitive reactions, the reactivity is given by the ratio of the rate equations of type (3) for both compounds (74) the denominators of the fractions cancel out ... 

If at a 99% conversion of B the conversion of A reaches 1% (the approximate level of a usual chromatographic analysis), the selectivity value at comparable rate constants fc H and /cbh gives the relative adsorption coefficient Xa,b = 2.18 X 10" . If differences in the adsorptivity of A and B are still higher, the method of competitive reactions becomes unsuitable for the determination of the relative adsorption coefficient. An ideal case is represented by the state where both reactions proceed at comparable rates. 

The, in Eqs. (1) and (2) denotes equilibrium or rate constant relative to a reference compound. In most homogeneous reactions, the determination of fcjei for series of related compounds is connected without any special difficulty. Using the method of competitive reactions one can easily find out the values of relative rate constants without knowing the over-all kinetics. 

The comparative reactivities in electrophilic substitution of selenolo[3,2-Z ]thio-phene (143) and -selenophene (144) with respect to thieno[3,2-Z ]thiophene (142) were studied by the method of competitive reactions (80CS206). The reactivity was found to change in the following order 144 >143 >142. This series agrees well with a higher substitution rate in selenophene compared to thiophene in acetylation, formylation and chlorination reactions. 

Alkyl radicals rapidly detach H from R3SnH. For the reaction of Ph3CCH2Cl with Ph3SnH the products of the reaction of R3SnH with both radicals were found, namely, Ph3CCH3 and Ph2CHCH2Ph. The isomerization rate constant was estimated by the method of competitive reactions to be 5-10 s (373 K). Substituents affect isomerization of this type, which is seen from the yield of the isomerization products... 

Recently, this work has been extended and further developed by Brown-Wensley into a preparative method for the synthesis of disilanes. The results of competitive reactions with several silanes allow insight into the reaction kinetics, in particular the relative rates of disilane formation versus hydrosilation (Table 5a, b) [61]. 

Chemical methods for structure determination in diene pol3 mers have in large measure been superseded by infrared absorption techniques. By comparing the infrared absorption spectra of polybutadiene and of the olefins chosen as models whose ethylenic structures correspond to the respective structural units, it has been possible to show that the bands occurring at 910.5, 966.5, and 724 cm. are characteristic of the 1,2, the mns-1,4, and the m-1,4 units, respectively. Moreover, the proportion of each unit may be determined within 1 or 2 percent from measurements of the absorption intensity in each band. The extinction coefficients characteristic of each structure must, of course, be known these may be assigned from intensity measurements on model compounds. Since the proportions of the various units depend on the rates of competitive reactions, their percentages may be expected to vary with the polymerization temperature. The 1,2 unit occurs to the extent of 18 to 22 percent of the total, almost independent of the temperature, in free-radical-polymerized (emulsion or mass) poly butadiene. The ratio of trans-1,4 to cfs-1,4, however,... 

More generally, the method of competition kinetics is used to determine H-atom rate constants. The hexacyanoferrate(III) ion is a suitable solute because reaction (39) can be followed from the decrease in absorbance at 420 nm due to Fe(CN)g (8420 = 104 m mol ). When a second solute is present so that reaction (40) competes with reaction (39), G(-Fe (CN)g ) is given by Eq. (41) ... 

In this simplified scheme, a plot of [ r ]-1 vs [B]-1 should be linear. From the intercept lSc can be obtained and the slope/intercept ratio is kjsclkv.. The rate constant for the reaction Ar can be estimated by the method of competition, lie reaction is carried out in presence of a third substance which quenches the reaction physically or chemically by competing for the reaction intermediate. In this case the triplet state of the photoexcited reactant is the intermediate and if the quenching step is... 

The best way to measure the relative reactivities of different compounds toward the same reagent is by the method of competition, since this permits an exact quantitative comparison under identical reaction conditions. Equimolar amounts of two compounds to be compared are mixed together and allowed to react with a limited amount of a particular reagent. Since there is not enough reagent for both compounds, the two compete with each other. Analysis of the reaction products shows which compound has consumed more of the reagent and hence is more reactive. 

These differences in rate can be measured in a variety of ways. In some cases, the rates of the two individual reactions (1) and (2) can be measured directly and the results compared. Usually, however, it is more feasible, as well as more satisfactory, to use our familiar method of competition (Sec. 3.22) in either of two ways. 

Vyazovkin and Lesnikovich [93] proposed that the type of complex process encountered in non-isothermal experiments could be identified by analysis of the shape of the curve of the dependence of the apparent on a, foimd by isoconversional methods. Concurrent competitive reactions are characterized by an increasing dependence of the apparent value of E on a, but detailed shapes are dependent on the ratios of the contributing rates. A decreasing dependence of on a, was found for intermediate reversible processes [93]. 

The theoretical models discussed above are frequently employed in the description of the kinetics of gas-phase reactions, especially reactions of atoms and free radicals. This class of reactions is of interest in a broader scientific context, and a better understanding of their mechanism is of primary importance for the development of chemical modeling. Free atoms and radicals are very reactive species, which occur in and take part in many different reaction systems. Therefore, a radical reaction usually proceeds in competition with a few parallel or subsequent processes. The kinetic behavior of the reaction system may be very complicated and difficult for quantitative description. Theoretical investigations of the reaction kinetics provide information useful for a better understanding and correct interpretation of experimental findings. Results of ab initio calculations are employed to evaluate the rate constant in terms of the computational methods of the reaction rate theory. 

We stated in our original paper "The method of ccmpetitive reaction rate determination can be applied only if the observed relative rates are dependent on the aromatic substrate. As our observed relative rates showed only low selectivity concerning the nature of the aromatic substrate it could have been attributed to a very fast reaction taking place when the solution of nitronium salt reagent is dropped into the solution of the mixed aromatic substrate. Thus the NO2 ion would have no possibility of reaching uniform distribution in the solution before reaction occurs with the aromatics and observed relative rates may be influenced rather by statistical factors than real competition". In order to clarify this possibility and also to determine the accuracy of the method used, we carried out experiments to establish whether real competitive reaction actually occured under our experimental conditions and whether the methods employed provided results which could establish small differences between the alkylbenzenes investigated ( ). 

When the primary rate information consists of a ratio of unimolecular rates (derived from the study of competitive reactions), all that is required is a ratio of product yields. This is most easily done by gas chromatography as in the Stem-Volmer analysis in which both products are stable molecules. The method is universal, easily calibrated, and therefore accurate. However, it contains no information about the product energy states. Also the reaction yield must be significant in order to be detectable. 

It appears that it is more difficult, at least in this case, to separate the two partially overlapping processes using the CR method, so that there is less influence over the outcome of competitive reactions. In such situations it may be more appropriate to employ isothermal methods of preparation. However, by using the SI approach, Ae isoAermal temperatures can be determined by Ae Areshold reaction rate and a few simple experiments will reveal Ae values required to effect Ae maximum possible separation of Ae events. 

Tethering of analogues of homogeneous rhodium(l) catalysts can be achieved by either of the two routes indicated in Fig. 10 [101]. In principle, each method involves competition reactions. If the surface is prefunctionalised then the complex has both the hydroxylated surface and the pendant ligand available for reaction. (The concentration of the former may be reduced by subsequent silylation with, e.g., Me3SiCl). When the reverse sequence is adopted then the precursor complex may react with the surface either at the silicon centre or at the metal coordination sphere. The chemistry of such systems however... 

It may seem, at first sight, paradoxical that a competition reaction carried out under conditions in which the measured rate is independent of the concentration of the aromatic can tell us about the relative reactivities of two aromatics. Obviously, the measured rate has nothing to do with the rate of the product-determining step, and what is important in determining relative reactivities is the ratio of the values of ( 3.2.4) for two compounds. The criteria to be met for a correct application of the competitive method are well understood. ... 

The reaction of iodobenzene with acrylate is a good synthetic method for the cinnamate 17[7]. In the competitive reaction of acrylate with a mixture of 0-and /i-iodoanisoles (18 and 19), the o-methoxycinnamate 20 was obtained selectively owing to the molecular recognition by interlamellar montmorillonite ethylsilyldiphenylphosphine (L) as a heterogenized homogeneous catalyst used as a ligand[28]. 

The temperature is chosen to maximize reaction rate and to avoid the competitive formation of the tetrafluoride. Alternative preparation methods include the reaction between TiCl and gaseous HF,... 

In the present work, the technique of XO and MTB immobilization onto silica gel in the form of its complexes with Fe(III) and Bi(III) respectively were found. The acid - base and chemical-analytical characteristics of solid-phase reagents were examined. The optimal conditions of quantitative recovery of Pb(II) and Zn(II) from diluted solutions, such as acidity of aqueous phase, the mass of the sorbents, the volume of solutions and the time of equilibrium reaching, were found. The methods of and F" detenuination were based on a competitive reactions of Zr(IV) with immobilized MTB and or F". Optimal conditions of 0,0 and F" determination in solution using SG, modified ion associates QAS-MTB (pH = 1,5, = 5-10 mol/1). 

Structure-reactivity relationships can be probed by measurements of rates and equiUbria, as was diseussed in Chapter 4. Direct comparison of reaction rates is used relatively less often in the study of radical reactions than for heterolytic reactions. Instead, competition methods have frequently been used. The basis of competition methods lies in the rate expression for a reaction, and the results can be just as valid a comparison of relative reactivity as directly measured rates, provided the two competing processes are of the same kinetic order. Suppose that it is desired to compare the reactivity of two related compounds, B—X and B—Y, in a hypothetical sequence ... 

The study of relative rates by the competitive method can be useful. The principle was discussed in Section 3.1 in the context of parallel reactions, for which the ratio of the product concentrations is equal to the ratio of rate constants (provided the concentrations are under kinetic control). 

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