Reactions, competing, definition

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). 

These equations do not provide complete definition of the reactions that may be of significance in particular solvent extraction systems. For example, HTTA can exist as a keto, an enol, and a keto-hydrate species. The metal combines with the enol form, which usually is the dominant one in organic solvents (e.g., K = [HTTA]en i/[HTTA]]jet = 6 in wet benzene). The kinetics of the keto -> enol reaction are not fast although it seems to be catalyzed by the presence of a reagent such as TBP or TOPO. Such reagents react with the enol form in drier solvents but cannot compete with water in wetter ones. HTTA TBP and TBP H2O species also are present in these synergistic systems. However, if extraction into only one solvent (e.g., benzene) is considered, these effects are constant and need not be considered in a simple analysis. 

In the hydroformylation reaction, V, the situation is even worse. Here there is no definite stereochemistry between the phosphine ligand and the metal. One of the reactants, carbon monoxide, competes so well with the phosphine for sites on the metal that it is difficult to insure that the chiral agent is present when the new asymmetric center is formed. 

Although the term selectivity is often used, there is no widespread agreement yet on an exact definition. If two competing reactions are unselective , a 50 50 product ratio would be obtained from an unbiased competition between the two. It might then be considered to have no selectivity. To convert this product ratio of unity into a numerical value of selectivity equal to zero, we take the logarithm of unity [40]. There are various other definitions of selectivity, and caution is needed when obtaining data from the published literature often, a logarithmic term is not used [41 ]. 

The results in Table 2.4 are also relevant to preparative scale reactions. Adapting the definition of selectivities (Equation 2.10) for the reaction shown in Scheme 2.21 by rearranging Equation 2.14 gives a definition of S for competing methanolysis and aminolysis (Equation 2.17). Although the molar concentration of methanol in almost pure solvent is high (24.7 M), the major product is amide even when the concentration of m-nitroaniline is only 10-2 M (Table 2.4), and S is calculated from Equation 2.17 to be over 8000 very high yields are predicted for reactions in more concentrated solutions of m-nitroaniline. In contrast, under the same conditions, the less basic amine o-nitroaniline has an S value of only 6 [44] ... 

In conclusion, the hypothesis that the Ugi reaction proceeds, at least in polar solvents, through the competing mechanisms B and C seems reasonable, and may explain some unexpected experimental results. The intervention of mechanism A, especially in non-polar solvent, may not, however, be definitely ruled out. 

Since the moment of inertia ratio will be nearly unity, the major source of the isotope effect at a given energy is the activated complex ratio. A pure intramolecular statistical secondary effect can scarcely arise, since by definition no net differential effect on the competing reaction coordinates arises due to isotopic substitution. However, in practice, small mechanistic effects mentioned earlier can exist for isotopic substitution which preferentially affects one of the competing bond-rupture sites. 

The equilibrium constant approach works well when single simple reactions occur, but not when there are competing reactions. The formal definition of chemical equilibrium is that the total Gibbs free energy is at a minimum ... 

Griller, D. Ingold, K. U. Acc. Chem. Res. 1976, 9, 13. In the context of this review, radicals are called persistent if their lifetimes in liquid solution exceed those of reactive radical species by many orders of magnitude. They may self-terminate slowly or disappear by other reactions, but these processes do not compete with the cross coupling with usual transient radicals. Stable radicals can be isolated in pure form. They are included in our definition of persistence. 

Elimination reacions are in many ways similar to Nucleophilic substitution reactions and if you haven t read that section yet, you should definitely read it as well. Nucleophilic substitution reactions and Elimination reactions share a lot of common characteristics, on top of which, the El and SnI reactions can sometimes compete and, since their products are different, it s important to understand them both. Without understanding both kinds of mechanisms, it would be difficult to get the product you desire from a reaction. 

Kuo and Rose showed that the proton that is removed is retained by the enzyme (67). Stubbe and Abeles prepared an alternative substrate in which fluoride elimination competes with carboxylation 68, 69). Neither result defines the mechanism, but they do show that it is likely that the carbanion derived from the substrate is generated as an intermediate and therefore the reaction is not concerted. Definitive results come from double-isotope fraction studies by O Keefe and Knowles (70) and by Cleland and co-woricers (71). As described for Claisen enzymes, this methodology tests whether processes occur in one or two steps. Labeling of the carboxyl to be transferred with carbon-13 and the proton to be transferred as deuterium provided the means to do this test. The results indicate clearly that proton removal from the substrate to generate the carbanion and transfer of the carboxyl occurs in distinct steps. The resulting attack of the carb-... 

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