Rapid preequilibrium

The kinetics of enzyme reactions were first studied by the German chemists Leonor Michaelis and Maud Menten in the early part of the twentieth century. They found that, when the concentration of substrate is low, the rate of an enzyme-catalyzed reaction increases with the concentration of the substrate, as shown in the plot in Fig. 13.41. However, when the concentration of substrate is high, the reaction rate depends only on the concentration of the enzyme. In the Michaelis-Menten mechanism of enzyme reaction, the enzyme, E, and substrate, S, reach a rapid preequilibrium with the bound enzyme-substrate complex, ES ... 

Thermal (electrophilic) and photochemical (charge-transfer) nitrations share in common the rapid, preequilibrium formation of the EDA complex [ArH, PyNO ]. Therefore let us consider how charge-transfer activation, as established by the kinetic behaviour of the reactive triad in Scheme 12, relates to a common mechanism for electrophilic nitration. Since the reactive intermediates pertinent to the thermal (electrophilic) process, unlike those in its photochemical counterpart, cannot be observed directly, we must rely initially on the unusual array of nonconventional nitration products (Hartshorn, 1974 Suzuki, 1977) and the unique isomeric distributions as follows. 

Since the propensity to form adducts in chemistry is high and these adducts undergo a variety of reactions, the rate law (1.98) is quite common. This is particularly true in enzyme kinetics. In reality, these reaction schemes give biphasic first-order plots but because the first step is usually more rapid, for example between A and B in (1.101) we do not normally, nor do we need to, examine this step in the first instance. The value of A", in (1.107) obtained kinetically can sometimes be checked directly by examining the rapid preequilibrium before reaction to produce D occurs. In the reactions of Cu(I) proteins with excited Cr and Ru polypyridine complexes, it is considered that (a) and (b) schemes may be operating concurrently. 

An activated complex containing three species (other than solvent or electrolyte), which attends a third-order reaction, is not likely to arise from a single termolecular reaction involving the three species. Third- (and higher-) order reactions invariably result from the combination of a rapid preequilibrium or preequilibria with a rds, often unidirectional. Such reactions are... 

An alternative, kinetically indistinguishable, scheme replaces k- in (6.18) by a rapid preequilibrium followed by a slow step (kj) instead of a fast step ( 4). 

It was originally thought that the high basicity of the amido complex would make the reprotonation a diffusion controlled process so that k, was always much greater than k2. This is indeed true for the majority of Co1" systems examined and for all studied cases of Cr111, RuUI, Rh111 and Ir111 complexes. Under these conditions, the expression for the rate constant reduced to ks = 2 nlk, k2 //c i or 2 K k2 where Kl is the equilibrium constant for the proton transfer process (1). With proton transfer as a rapid preequilibrium, such systems exhibit specific base catalysis. 

In certain cases it has been shown that the rate of reaction of the a-haloalkyl derivative with a second reagent, such as an olefin or diazoalkane, is dependent upon the concentrations of both reactants (Blanchard and Simmons, 1964 Bethell and Brown, 1967). Such observations are consistent either with a one-step bimolecular reaction of the organo-metallic compound as such and the other reagent or with a rapid preequilibrium forming an intermediate, followed by a slow reaction of this intermediate with the second reactant. The latter alternative is represented in equation (14), in which b[olefin]Rate-determining reaction of an intermediate carbene... 

A kinetic study of the Vilsmeier-Haak formylation of thiophene derivatives in dichloroethane solution has been recently reported.156 Reactions of thiophene and 2-methylthiophene follow third-order kinetics, first-order in substrate, dimethylformamide (DMF), and phosphorus oxychloride. These results are in agreement with a mechanism involving a rapid preequilibrium step leading to an... 

Many base catalysed isotope exchange or racemization processes involve either rate-determining proton abstraction or rapid preequilibrium formation of the anion (19) followed by ratedetermining reaction with solvent (20) (L is an isotope of hydrogen). 

The solvent-induced change in rate is, however, much larger than expected from the relatively small difference in polarity between nitromethane and hexamethylphos-phoric triamide. This, together with the correlation between rate decrease and increase in the solvent donor number DN cf. Table 2-3 in Section 2.2.6), suggests that specific solvation and stabilization of the diazonium ion by EPD solvents play a dominant role in the reaction (5-27). Very likely, formation of an EPD/EPA complex between the reactants in a rapid preequilibrium step precedes the rate-controlHng first step [504, 792],... 

Since the overall reaction proceeds under conditions where no Co(CO)4 radicals from Co2(CO)g cleavage can be detected, splitting of the dinuclear species Co2(CO)gL, which is formed in a rapid preequilibrium (equation 4), is responsible for radical formation during the induction period. The important step for the formation of the observed products is then outer-sphere electron transfer see Outer-sphere Reaction) as depicted in equation (6). This requires the Co(CO)3L radical to act as a reducing agent towards Co2(CO)g. Since, from electrochemistry and pulse radiolysis of Co2(CO)g, it can... 

The mechanism most consistent with this rate law involves a rapid preequilibrium as represented for RMn(CO)5 ... 

In the latter case it is usually assumed that a rapid preequilibrium with a carbonyl-containing species is established in solution prior to the actual insertion step ... 

Heating 29-d6 with 30 revealed no scrambling indicating that dissociation of PR3 from one metal center is necessary and sufficient for exchange to take place. The rate constants of these processes indicated that phosphine is lost in a rapid preequilibrium step followed by rate determining combination of the unsaturated intermediate so generated with a second saturated complex as shown in Scheme 7. 

A closely related methoxide complex [Pt(Me)(OMe)(dppe)] (dppe = diphenylphosphinoethane) inserts perfluoroethylene into the Pt-OMe bond (Scheme 6.65). The mechanism of the reaction is consistent with coordination of the alkene in a rapid preequilibrium, followed by rate determining insertion. It does not involve dissociation of the methoxide ligand since exchange of this ligand with deuterated methanol is too slow to account for product formation, and the reaction of perlluorocyclopentene in deuterated methanol does not afford a deuterated product (Eq. 6.36) [206]. 

In the event that the intermediate is undetectable ( 2 is comparable in magnitude to and k-i), but the rapid preequilibrium still holds, applying the steady-state approximation to [ML5X Y] affords the relationship... 

In the neutral and alkaline hydrolysis of chloroform, dEgjdT y/as found (50) to be (—46.3 R) and ( — 67.2 R), respectively. This is the only alkaline hydrolysis to exhibit an appreciable dEgjdT. The explanation is therefore likely to be different from that preferred for other solvolysis reactions (see Section V). The authors suggest a mechanism which involves the formation of CCl3 in a rapid preequilibrium. If this mechanism is correct, it may well be the cause of the nonzero value of dEgjdT (74). 

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