Reversible competing reactions

Example 7.5. Reversible competing reactions kinetic and thermodynamic reaction control [4]... 

As the temperature approaches the NTC zone, the reversibility of reaction 2 comes into play and the steady-state concentration of alkyl radicals rises. There is a competing irreversible reaction of oxygen with radicals containing an alpha hydrogen which produces a conjugate olefin (eq. 23). 

Reversible inhibition is characterized by an equiUbrium between enzyme and inhibitor. Many reversible inhibitors are substrate analogues, and bear a close relationship to the normal substrate. When the inhibitor and the substrate compete for the same site on the enzyme, the inhibition is called competitive inhibition. In addition to the reaction described in equation 1, the competing reaction described in equation 3 proceeds when a competitive inhibitor I is added to the reaction solution. 

Terms in the denominator represent the competing reactions of an intermediate. One of the two steps reverses the reaction by which the intermediate was formed. Imagine letting each of the denominator terms, in turn, become much larger than the others, either in one s mind or in practice by adjusting the concentration variables. In the limit where one term dominates, there is a change in rate control from one step to another. In each of these limits, the composition of the transition state for the step that is then rate-controlling can be deduced from the application of Rule 1. 

Equation (1.20) is frequently used to correlate data from complex reactions. Complex reactions can give rise to rate expressions that have the form of Equation (1.20), but with fractional or even negative exponents. Complex reactions with observed orders of 1/2 or 3/2 can be explained theoretically based on mechanisms discussed in Chapter 2. Negative orders arise when a compound retards a reaction—say, by competing for active sites in a heterogeneously catalyzed reaction—or when the reaction is reversible. Observed reaction orders above 3 are occasionally reported. An example is the reaction of styrene with nitric acid, where an overall order of 4 has been observed. The likely explanation is that the acid serves both as a catalyst and as a reactant. The reaction is far from elementary. 

Although reaction (3.61) is endothermic and its reverse step reaction (-3.61) is faster, the competing step reaction (3.63) can be faster still thus the isomerization [reaction (3.61)] step controls the overall rate of formation of ROO and subsequent chain branching. This sequence essentially negates the extent of reaction (-3.48). Thus the competition between ROO and olefin production becomes more severe and it is more likely that ROO would form at the higher temperatures. 

Nucleophilic attack is faster at unsubstituted ring positions than at similarly activated but substituted ring positions.8-15-43 5 Since the addition is in most cases reversible, the opportunity exists for competing reactions. Indeed an extremely varied spectrum of reactivities is found in these systems, depending on reactants and reaction conditions. Examples are known of competing SNAr and cine substitution,46 NAr and tele substitution,47 SnAt and cine and tele substitution (equation l),5 SsAr and VNS,16 and SnAt and Sn(ANRORC).48... 

The extension of equilibrium measurements to normally reactive carbocations in solution followed two experimental developments. One was the stoichiometric generation of cations by flash photolysis or radiolysis under conditions that their subsequent reactions could be monitored by rapid recording spectroscopic techniques.3,4,18 20 The second was the identification of nucleophiles reacting with carbocations under diffusion control, which could be used as clocks for competing reactions in analogy with similar measurements of the lifetimes of radicals.21,22 The combination of rate constants for reactions of carbocations determined by these methods with rate constants for their formation in the reverse solvolytic (or other) reactions furnished the desired equilibrium constants. 

The competing reaction is reversible binding of the quencher by the excited fluor F. ... 

In the reactions discussed and exemplified above, reactants, transient species and products are related by linear sequences of elementary reactions. The transient species can be regarded as a kinetic product and, if isolable, subject to the usual tests for stability to the reaction conditions. Multiple products, however, may also occur by a mechanism involving branching. Indeed, the case shown earlier in Fig. 9.5b, where the transient is a cul de sac species, is the one in which the branching to the thermodynamic product P and kinetic product T occurs directly from the reactant. In the absence of reversibility, the scheme becomes as that shown in Scheme 9.8a, where the stable products P and Q are formed as, for example, in the stereoselective reduction of a ketone to give diastereoisomeric alcohols. The reduction of 2-norbornanone to a mixture of exo- and cndo-2-norbornanols by sodium borohydride is a classic case. The product ratio is constant over the course of the reaction and reflects directly the ratio of rate constants for the competing reactions. The pseudo-first-order rate constant for disappearance of R is the sum of the component rate constants. 

Some rate coefficients have been estimated by means of thermodynamic relationships between the rate coefficients of direct and reverse processes, or by considering ratios of rate coefficients in competing reactions, or by analogies between similar processes. 

Again this reaction is reversible on addition of acid hypochlorite and hypoiodite behave similarly. Hydrolysis of the halogens takes place only to a limited extent in neutral solutions and with chlorine a competing reaction, the oxidation of water to oxygen, becomes important. In the case of fluorine, the latter reaction heavily predominates. 

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