Reactions, competing, definition constants

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. 

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 ... 

Zollinger [15], to interpret the product yield results. The yields of phenol product, z-ArOH, from reaction with water and halo product, z-ArX, from reaction with Br or Cl are assumed to be determined primarily by the position of equilibrium between the ion-molecule and ion-ion pairs. The dediazoniation rate constant for each pair is probably of secondary importance for these dediazoniations because dediazoniation reactions are notoriously insensitive to the polarity of the reaction medium (see later). For several decades the basic consensus on the dediazoniation mechanism has been the rate-determining loss of N2 to give a highly reactive aryl cation intermediate that is trapped extremely rapidly and competitively by available nucleophiles [15]. However, more recent ah initio calculations provide support for a bimolecular mechanism in which C—N bond cleavage is almost complete and bond formation with the nucleophile has barely begim [16]. Because both mechanisms lead to the same definition for the selectivity of the reaction toward competing nucleophiles [Eq. (1)], the bimolecular pathway for dediazoniation is not included in Scheme 1. 

---