Preequilibrium, fast

Solvent isotope effects are usually in the range / h20+ = 2-3. These values reflect the greater equilibrium acidity of deuterated acids (Section 4.5) and indicate that the initial protonation is a fast preequilibrium. 

A system of this type is commonly said to possess a fast preequilibrium step. Proton transfers constitute a veiy important class of fast preequilibria, as illustrated by Scheme XVII for acid-catalyzed ester hydrolysis. 

An important special case can be derived from this general result. This is the case of a fast preequilibrium, in which the A + B AB system rapidly equilibrates, the AB C step being much slower. Then the relaxation time for the first step is much shorter than that for the second, and some measure of uncoupling takes place. For such a system k,2, 21 23- 32, and we obtain Cn = k 12, a,2 = /c2, 021 = k, 2, 022 = 21- Equations (4-22) then give Tf = k, 2 + 21 and th 0. Because these are approximations, the result for ti is reasonable but that for Tn is not. To reach a reasonable result for Tn we use Eq. (4-24a),... 

In the first step the hydrated ion and ligand form a solvent-separated complex this step is believed to be relatively fast. The second, slow, step involves the readjustment of the hydration sphere about the complex. The measured rate constants can be approximately related to the constants in Scheme IX by applying the fast preequilibrium assumption the result is k = Koko and k = k Q. However, the situation can be more complicated than this. - °... 

The appearance of Cd in the denominator means that D is coupled to a reversible step prior to the rds. If k, and k- were so large that the fast preequilibrium assumption is valid, then the Cd term in the denominator would drop out, and we would have v = A 2 CaObCd, giving the composition of the rds transition state. If k2 is very much larger than it, and k i, Eq. (5-59) becomes v = A CaOb the first step is now the rds, and the rate equation gives the transition state composition. 

Al. A fast preequilibrium protonation of substrate followed by a slow ratedetermining reaction of the protonated substrate. Subsequent steps (such as attack by water) are fast. 

A2. A fast preequilibrium protonation followed by a slow rate-determining attack by nucleophile. 

Instead or in addition, the reactants A and B may associate in a fast preequilibrium, or one of them may bind to a third component. Such interactions will exert important rate effects and for that reason must be accounted for. The existence of equilibria in which the reactants participate may translate to an important effect on the chemical mechanism or to a trivial one. Either way, the issue must be addressed to arrive at a reliable mechanism. The matter can grow complicated, in that the concentration variables that affect the rate may do so because they really do enter the mechanism, or because they participate in extraneous equilibria. Of course, they may play both roles. Sorting out these matters sounds complicated, but it is not difficult if one proceeds systematically. 

The tetrachloropalladate is the resting state of the catalyst and in a fast preequilibrium the crucial intermediate is formed. Note that we only have the kinetic data to support this hypothesis. The intermediate formed by the hydroxide attack is 2-hydroxyethylpalladium. After a rearrangement of the hydroxyethyl group, acetaldehyde (ethanal) forms and palladium zero is the reduced counterpart. 

The shapes of the curves in Fig. 6 are consistent with a two-step pathway, analogous to that of a hydrolytic enzyme such as a-chymotrypsin,30 in which an initial acylation burst is followed by a slow deacylation reaction. Following a fast preequilibrium binding, the first kinetic step can be attributed to acylation by substrate of the polymer imidazole residue, accompanied by simultaneous release of nit-rophenol(ate). The succeeding kinetic step would then be ascribed to hydrolysis of the acylimidazole leading to carboxylate ion and regenerated imidazole. 

This represents the case where there is a fast preequilibrium preceding the rate-determining step. It is again not necessary to work through the steady-state approximation if this situation is known to exist. If the intermediate I is in equilibrium with the reactant A, then... 

This is actually the correct way to think about this case since the steady-state approximation requires that the concentration of I is low, which it may not be if there is a fast preequilibrium. 

Because of the complex nature of semicarbazone formation, it is difficult to say with certainty at what stage in the reaction the entropy variations originate. Recent work on the mechanism of the reaction bears on this point. Jencks and Carriuolo (1960) have shown that above pH 4 the reaction involves a fast preequilibrium between carbonyl and semicarbazide, followed by a rate-controlling, acid-catalyzed dehydration of the addition compound. 

These mechanisms have been the subject of several reviews (10-13), and there is no room to discuss them in detail. A short comparison with the silylonium intermediate pathway should, however, be included. Both of these conflicting mechanisms assume the formation of a substrate-nucleophile complex in a fast preequilibrium step, and the complex reacts with a nucleophilic reagent to give the product in the rate-limitng step. It is, therefore, appropriate to compare how easily the corresponding complexes are formed and how readily they react toward the product [Eq. (71)]. The... 

This mechanism would involve a fast preequilibrium CIO2 + OH (02C1- OH) . The authors suggest a preequilibrium... 

Factor analysis, 445 Fast preequilibrium, 97 Fast reactions, 63, 133 Feathering technique, 73 Fick s first law, 134 Field effect, 336, 338 Final state, 3... 

This involves a fast preequilibrium step with complex (III) as an intermediate having a lower coordination number. The latter then reacts either by a simple dissociative mechanism (path A) or through a partial dissociation (path B). 

If more then one chain conformation is present will this lead to complexity in the fluorescence decay only if the rate of conformational change is comparable to the rate of excimer formation a in the dipeptides. In absence of this complication it is however necessary to interpret the obtained results taking into account the fast preequilibrium between the different conformers. 

Dissociative processes were also implicated in HCl cleavage reactions of ci5-[PtMeR(PEt3)2] (R = methyl, phenyl, or mesityl) and their subsequent isomeri-zations. The cleavage mechanism is believed to involve a fast preequilibrium between substrate and chloride, combined with a parallel protonation of both platinum species, where the proton attacks the Pt—Me bond (Scheme 3). The spontaneous isomerization of the solvated intermediates ci5-[PtR(S)(PEt3)2] was followed either by P NMR or UV/visible spectroscopy. Dissociative loss of the solvent molecule, S, was implicated because of the relative insensitivity of the reaction rate to the steric bulk of R, and because of the positive values of AS. ... 

A full mechanistic analysis was conducted of C-F bond-forming elimination from complexes 57 and 58 [77, 78]. The authors studied the activation parameters, obs as a function of the reaction medium, and obs as a function of substituents X, Y, Z, and L (see complex 62 in Eq. 46). As detailed below, all of these experiments were consistent with a Dn pathway for reductive elimination, involving fast preequilibrium dissociation of the sulfonyl or pyridine ligand (step 1) followed by rate limiting C-F coupling (step 2, Eq. 47). For clarity, the mechanistic discussion below will focus primarily on complex 57 however, in general, similar results were obtained in both cases. 

In aqueous solution the reaction proceeds via a fast preequilibrium involving an outer-sphere complex. The rate of complex formation is dominated by the rate of loss of a water molecule from the first co-ordination shell of the metal ion. 

Schindler et al. carried out a kinetic analysis for the reaction of isoprene (ip) with carbon dioxide and Ni(bpy)(cod) [103]. Ni(bpy)(cod) did not react directly with CO2 (see also Sect. 5.2), but reacted reversibly with isoprene to give Ni(bpy)(ip). The kinetic results support a mechanism where, in a fast preequilibrium step, one of the bonds between Ni and cod is cleaved and, in a following step, ip coordinates to the free coordination site (Scheme 5.23). At low temperatures Ni(bpy)(ip) can further react, through a consecutive equilibrium, with another ip molecule to give Ni(bpy)(ip)2. However, in the presence of CO2, the nickel-ip complex, Ni(bpy)(ip), reacts irreversibly with the heterocumulene to form the product complex (3-5-q )-3-methyl-3-pentenylato)nickel bipyridine (Scheme 5.23). The activation parameters for the latter step were calculated to be A// = 25 7 kJ/mol and = 184 24 J/(mol K). The large negative activation entropy indicates that the carboxylation reaction proceeds in an associative way, during which CO2... 

Palladium(ii).—Rate constants for aquation of /ra/ij-[Pd(NH8)2Cl2], /ra/w-[Pd(NH8)a(OH2)Cl]+, and /m -[Pd(NH8)a(0Ha)(N02)]+ in methanol-, acetone-, dioxan-, and dimethylformamide-water mixtures correlate linearly, though with only three points to define each line, with the function D — 1)/(2D + 1) of the dielectric constants (D) of the respective solvent mixtures for each pair of solvents. The slopes of these linear plots are very different for each organic co-solvent. An associative mechanism is assumed to operate since in all cases the activation entropies are large and negative. Aquation of [Pd(N02)4] is subject to acid catalysis but not, as would be expected, to base catalysis. The mechanism of aquation of the cis- and /ronr-isomers of [Pd(gly)2] involves a fast preequilibrium ... 

At high pressures the reaction is a first-order process. Under those conditions, the rate of energisation and de-energisation are relatively fast steps, which can be treated as a fast preequilibrium. Thus, the rate determining step is the transformation of A into products. 

In contrast, the rate constant of a mechanism of base catalysis involving a fast preequilibrium... 

Kinetic studies have demonstrated that the first step of the reaction is a fast preequilibrium K = k lk.]) and the second step ( 2) is rate limiting. 

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