Reaction mechanisms with initial fast step

Mechanisms with a Fast Initial Step If the rate-determining step in a mechanism is not the initial step, it acts as a bottleneck later in the reaction sequence. As a result, the product of a fast initial step builds up and starts reverting to reactant, while waiting for the slow step to remove it. With time, the product of the initial step is changing back to reactant as fast as it is forming. In other words, fast initial step reaches equilibrium. As you ll see, this situation allows us to fit the mechanism to the overall rate law. 

Understand elementary steps and molecularity, and be able to construct a valid reaction mechanism with either a slow or a fast initial step ( 16.7) (SP 16.8) (EPs 16.53-16.64)... 

Analyze We are given a mechanism with a fast initial step and asked to write the rate law for the overall reaction. 

When the proposed mechanism for a reaction has a slow initial step— Uke the one shown previously for the reaction between NO2 and CO—the rate law predicted by the mechanism normally contains only reactants involved in the overall reaction. However, when a mechanism begins with a fast initial step, some other subsequent step in the mechanism is the rate-limiting step. In these cases, the rate law predicted by the rate-limiting step may contain reaction intermediates. Since reaction intermediates do not appear in the overall reaction equation, a rate law containing intermediates cannot generally correspond to the experimental rate law. Fortunately, however, we can often express the concentration of intermediates in terms of the concentrations of the reactants of the overall reaction. 

The kinetics of the coupling mechanism include a number of sometimes very fast and competitive side reactions. The following steps, for instance, proceed simultaneously as a separately prepared diazonium salt solution is combined with an initially dissolved coupling component ... 

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

The above classification of polymers according to polymerization mechanism, as shown by the variation of molecular weight with conversion [Figs. 1.2(a) and 1.2(b)], is not without its ambiguities. Certain polymerizations show a linear increase of molecular weight with conversion [Fig. 1.2(c)] when the polymerization mechanism deviates from the normal chain or step pathway. This is observed in certain ionic chain polymerizations, which involve a fast initiation process coupled with the absence of reactions that terminate the propagating reactive centers. Biological syntheses of proteins also show the behavior described by Fig. 1.2(c) because the various... 

The kinetics of the formation of dihydrogen from the reaction of the iron complexes [l.ljferrocenophane, 3,3 -trimethylene[l.l]ferrocenophane, and 2,2 -trimethylene[l.l]ferrocenophane in strong acids have been studied by using tensiometric and spectrometric techniques.The results of these measurements are consistent with the three-step mechanism proposed for the formation of dihydrogen a very fast initial protonation, a slower second protonation, and the elimination of dihydrogen as the rate-determining step. A primary kinetic isotope effect, kn/ D, of between 3 and 5 was observed for [l.ljferrocenophane. The rate... 

If the first step in a mechanism is the slowest step with the highest activation energy, (hen it is ratedetermining, and the overall reaction rate is equal to the rate of the first step because all subsequent steps are so fast that once the first intermediate is formed it results immediately in the formation of products. Once over the initial barrier, the intermediates cascade into products. However, a rate-determining step... 

Both mechanisms with initiation equilibrium (Equations 4 and 5) provide unconstrained explanation for the temperature dependence of the overall rate constant, because addition equihbria usually shift to the side of dissociation with increasing temperature. Both also lead to the rate law reported, and for both mechanisms, it is possible that the second step, the conversion of the primary adduct to NO2, is actually composed of a sequence of fast reactions, and even more intermediates are formed. During the course of the concerted reaction model (3) only one additional intermediate appears to be plausible, the N2O4 isomer ONOONO. 

Recently, typical step-reaction polymerizations, as in polyesters, polyethers, and polyamides, have been forced into chain-reaction mechanisms by designing complex chain ends that react fast with the monomer only. Under the proper conditions, the step reaction can be suppressed almost completely. Such chain-growth polycondensation may even yield living polymers with narrow molar-mass distribution. A link to the initial literature is given in the General References for this section. 

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