Rate-determining/limiting step

The El elimination has the same rate determining first step, Dn, as the SnI substitution and is subject to the exact same limitations a carbocation stability usually... 

Carbanion protonation in water is a two-step reaction (i) movement of a Bronsted acid into a reactive position, and (ii) proton transfer to carbon. The overall rate constant for carbanion protonation may be limited by either the rate constant for formation of the reactive complex, in which case the overall rate constant for proton transfer can be estimated by using a representative rate constant for the rate-determining transport step, or by the rate constant for proton transfer to carbon. 

Typical Tafel lines on Ni in alkaline solution are seen also in Figure g (53,163,164) anodic limiting current behavior which is observed would be attributable to the rate-determining Tafel step. In support of this, the rate of anodic ionization of H2 on Ni in 1 Af KOH or 0.1 Af KOH was dependent on the first power of On the other hand, an opposite view was... 

Simplification of the rate expression is possible if the rate constants corresponding to one of the elementary steps in the reaction mechanism can be identified as being small compared to others. This is called the slow step or the rate-controlling/rate-limiting/rate-determining step in the overall reaction mechanism. In the limiting case, all elementary reaction steps of the mechanism are essentially at equilibrium except the rate-determining slow step therefore, the net steady-state rate can be expressed in terms of the slow step, and equilibrium statements can directly be written for all other steps in the mechanism. 

Electrode kinetics lend themselves to treatment usiag the absolute reaction rate theory or the transition state theory (36,37). In these treatments, the path followed by the reaction proceeds by a route involving an activated complex where the element determining the reaction rate, ie, the rate limiting step, is the dissociation of the activated complex. The general electrode reaction may be described as ... 

Hydrolysis of esters and amides by enzymes that form acyl enzyme intermediates is similar in mechanism but different in rate-limiting steps. Whereas formation of the acyl enzyme intermediate is a rate-limiting step for amide hydrolysis, it is the deacylation step that determines the rate of ester hydrolysis. This difference allows elimination of the undesirable amidase activity that is responsible for secondary hydrolysis without affecting the rate of synthesis. Addition of an appropriate cosolvent such as acetonitrile, DMF, or dioxane can selectively eliminate undesirable amidase activity (128). 

In the case of parallel reactions, the fastest reaction will set or control the overall change. In all rate determining cases, the relative speed of the reactions will change with the temperature. This is caused by different energies of activation among the steps in the sequence. This is just one more reason for limiting rate predictions from measurements within the studied domain to avoid extrapolation. 

These examples illustrate the relationship between kinetic results and the determination of reaction mechanism. Kinetic results can exclude from consideration all mechanisms that require a rate law different from the observed one. It is often true, however, that related mechanisms give rise to identical predicted rate expressions. In this case, the mechanisms are kinetically equivalent, and a choice between them is not possible on the basis of kinetic data. A further limitation on the information that kinetic studies provide should also be recognized. Although the data can give the composition of the activated complex for the rate-determining step and preceding steps, it provides no information about the structure of the intermediate. Sometimes the structure can be inferred from related chemical experience, but it is never established by kinetic data alone. 

Not all reactions can be fitted by the Hammett equations or the multiparameter variants. There can be several reasons for this. The most common is that the mechanism of the reaction depends on the nature of the substituent. In a multistep reaction, for example, one step may be rate-determining in the case of electron-withdrawing substituents, but a different step may become rate-limiting when the substituent is electron-releasing. The rate of semicarbazone formation of benzaldehydes, for example, shows a nonlinear Hammett... 

As with simple imines, the identity of the rate-limiting step changes with solution pH.. s the pH decreases, the rate of the addition decreases because protonation of the amino compound reduces the concentration of the nucleophilic unprotonated form. Thus, whereas the dehydration step is normalfy rate-determining in neutral and basic solution, addition becomes rate-determining in acidic solutions. 

TWo types of rate expressions have been found to describe the kinetics of most aromatic nitration reactions. With relatively unreactive substrates, second-order kinetics, first-order in the nitrating reagent and first-order in the aromatic, are observed. This second-order relationship corresponds to rate-limiting attack of the electrophile on the aromatic reactant. With more reactive aromatics, this step can be faster than formation of the active electrq)hile. When formation of the active electrophile is the rate-determining step, the concentration of the aromatic reactant no longer appears in the observed rate expression. Under these conditions, different aromatic substrates undergo nitration at the same rate, corresponding to the rate of formation of the active electrophile. 

Consider the series reaction A—>B—>C. If the first step is very much slower than the second step, the rate of formation of C is controlled by the rate of the first step, which is called the rate-determining step (rds), or rate-limiting step, of the reaction. Similarly, if the second step is the slower one, the rate of production of C is controlled by the second step. The slower of these two steps is the bottleneck in the overall reaction. This flow analogy, in which the rate constants of the separate steps are analogous to the diameters of necks in a series of funnels, is widely used in illustration of the concept of the rds. 

Diffusion-limited rate control at high basicity may set in. This is more eommonly seen in a true Br nsted plot. If the rate-determining step is a proton transfer, and if this is diffusion controlled, then variation in base strength will not affect the rate of reaction. Thus, 3 may be zero at high basicity, whereas at low basicity a dependence on pK may be seen. ° Yang and Jencks ° show an example in the nucleophilic attack of aniline on methyl formate catalyzed by oxygen bases. 

The notion of concurrent SnI and Sn2 reactions has been invoked to account for kinetic observations in the presence of an added nucleophile and for heat capacities of activation,but the hypothesis is not strongly supported. Interpretations of borderline reactions in terms of one mechanism rather than two have been more widely accepted. Winstein et al. have proposed a classification of mechanisms according to the covalent participation by the solvent in the transition state of the rate-determining step. If such covalent interaction occurs, the reaction is assigned to the nucleophilic (N) class if covalent interaction is absent, the reaction is in the limiting (Lim) class. At their extremes these categories become equivalent to Sn and Sn , respectively, but the dividing line between Sn and Sn does not coincide with that between N and Lim. For example, a mass-law effect, which is evidence of an intermediate and therefore of the SnI mechanism, can be observed for some isopropyl compounds, but these appear to be in the N class in aqueous media. 

The ratio of products 15 and 16 is dependent on the structures, base, and the solvent. The kinetics of the reaction is likewise dependant on the structures and conditions of the reaction. Thus addition or cyclization can be the rate-determining step. In a particularly noteworthy study by Zimmerman and Ahramjian, it was reported that when both diastereomers of 20 were treated individually with potassium r-butoxide only as-epoxy propionate 21 was isolated. It is postulated that the cyclization is the rate-limiting step. Thus, for these substrates, the retro-aldolization/aldolization step reversible. ... 

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